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Solar Water Heating Basics – What You Need to Heat Water with the Sun

In this post, I'll give you an overview of solar water heating basics so you can decide if this green home option is right for you.

solar water heating panels

In my life B.C. (Before Children), I received my Masters Degree in Mechanical Engineering with an emphasis in renewable energy. After graduation, I worked for a company that operated the world's largest flat plate collector solar water heating (SWH) system and did solar water heating installations.

When we built our current home, we added a solar water system shortly after the home was completed. It's been providing us with hot water since 2006. (We also have passive solar heating in our current home.) Basically, I've had solar water heating in my life for over 20 years now, living in an area where you might least expect to see it (Wisconsin).

Why use solar water heating?

After heating and cooling, water heating tends to be one of areas where people use the most energy. Estimates range from around 15% to up to 40% of energy usage, with the high end coming from old, inefficient electric heaters.

A solar water heater uses solar energy from the sun to heat some or all of your water. At its most basic, this can be done with a dark container left out in the sun.

Friends of ours camped out on their property while building their home, and set up an outdoor shower with 50 gallon barrels painted black on a platform above their shower area. Another variant of this are the solar camp shower bags that can be filled and hung in a sunny location to provide hot water while camping (or during an emergency situation).

In our case, our solar collectors preheat water for our domestic hot water and for our in floor radiant heating. On sunny winter days, the passive and active solar cover the bulk of our heating load.

Heating a swimming pool is a great application for solar water heating, because pool usage is heaviest when the sun is most abundant.

The first book on the list is from a fellow Wisconsinite, one of the first solar peeps I encountered in the area, Bob Ramlow.

Indirect Versus Direct Solar Water Heaters

Solar water heating systems can be classified in several ways. One of the main classifications is Direct or Indirect.

Direct systems heat the water that is being used. There is no heat exchanger. These systems work best in warm weather areas, because they must be drained when temperatures get below freezing. (Direct systems that are drained during cold weather are called, not too surprisingly, Drain Back Systems.) These systems are also prone to hard water buildup if the water running through them is high in minerals. These are also known as open loop systems.

Indirect systems heat a fluid (generally an antifreeze mix) and transfer heat from that fluid through a heat exchanger to the potable water in the storage tank. These are a must for year round operation in cold climates. Because you don't introduce new fluid into the collector, they are less likely to get mineral buildup. The down side is that these will generally be more expensive because they have more parts. These are also known as closed loop systems.

Passive versus Active Solar Water Heaters

Passive solar water heaters rely on natural convection to move the cold water from the bottom of the collector to the top as it is heated.

Active solar water heating systems have circulating pumps that move the fluid around (normally a polypropylene glycerol mix).

Parts of a Solar Water Heating System


All solar hot water heating systems need a way to collect the heat. Options range from a black painted tank or a black tank in an insulated box (batch heaters), to flat plate collectors and insulated tube collectors.

  • Batch heaters combine the collector and storage in one unit.
  • Flat plate collectors have fluid piping connected in parallel (typically copper tubing) with metal fins painted black to increase the absorption area, all inclosed in an insulated box.
  • Insulated tube collectors are individual tubes with fluid pathways and fins inclosed in what is basically a thermos. No air around the absorbers = no air flow = superior insulation. If we were to expand our system, I'd want these collectors, since they have the lowest amount of heat loss in cold climates.
solar water heating equipment

Storage Tank

Where the heated water is stored.

  • Batch heaters store the water right in the panel, tank, or storage tank attached to the top of the panel.
  • Flat plate collectors and insulated tube collectors have a separate storage tank.

Depending on the type of system, it may or may not include the following parts.

Heat Exchanger

In indirect systems, the heat from the solar panels is transferred to the domestic hot water via a heat exchanger. This heat exchanger may be located inside the storage tank or outside the storage tank.

Inside the storage tank generally improves heat transfer, because it maximizes the amount of water surface area in contact with the heat exchanger, but it there is ever a problem with the heat exchanger, the entire tank and heat exchanger combination needs to be replaced.

With an external heat exchanger, they can often be plumbed into a standard electric water heating tank with the heating elements removed, which reduces costs, but they may not be as efficient as an internal exchanger.

Our current system has an internal heat exchanger, our last system had an external heat exchanger.

solar water heating controller


In indirect systems with an AC pump, the controller tells the pump when to turn off and on. This is also commonly referred to as a differential controller, because it looks at the temperature differential between the panels and the water in the storage tank.

When a set temperature difference is reached (when the panels get hot enough), the pump turns on. When the panels cool down, the pump turns off.


Indirect systems may have an AC pump or a DC pump. AC pumps are typically powered by the electrical grid. DC pumps are almost always powered by a solar electric (photovoltaic) panel. As mentioned above, AC pumps are turned off and on by a controller.

DC pumps turn on automatically when the solar electric panel generates enough power to turn on the pump. When sized correctly, the fluid in the panels should be hot enough to effectively transfer heat when the pump has enough power to operate.

At our last home, the solar water heating system used a DC pump and solar electric panel. Our current system uses an AC pump with controller.

The solar installer who worked on our current system indicated that he had seen significantly better performance in our cold temperatures with the AC pumps, because the DC pumps didn't have enough power to move the fluid well when it got cold and slushy.

Temperature and Pressure Gauge

Most systems with inside storage will have one of these gauges near the storage tank so you can easily see at a glace the current conditions of the system, and make sure that it is properly pressurized.

Expansion Tank

Water, even water with antifreeze mixed in, will expand and contract as it goes through different temperature and pressure ranges. To give the excess volume of fluid somewhere to go in closed loop systems, a tank with a bladder that expands and contracts is plumbed into the loop.

Temperature and Pressure Relief Valve

As extra protection, many systems place a temperature and pressure relief valve at the top of the collector(s). If the panels are left sitting in the sun and get dangerously hot, fluid will be vented outside the house to avoid damage inside. There may also be a T&P relief valve on the storage tank.

There are many variations of the equipment, but you always need to collect heat and store heat. You can buy parts and assemble your own, get an entire kit that's ready to install, or hire a contractor to install a system for you.

Things to Look for in a Solar Water Heating Installation

Make sure your solar system is installed correctly

Back in the 1970's push for solar, many contractors installed sub par equipment, and did it poorly. One of my first solar jobs was to repair non-functioning systems in the Orphan Solar Program.

You can't take short cuts or the system either won't work right or will have a very short lifespan. When these puppies leak, they can make a really big mess, so it's not generally a project for an unskilled homeowner (unless you're doing something very simple like seasonal pool heating). Also, improperly mounted panels on the roof can cause major roof damage and even structural failure. Water is heavy, and roof penetrations should always be properly sealed. Whether mounted on the ground or roof, you want to make sure your panels are very secure, so they don't act like sails when a good windstorm kicks up.

solar panel supports

Work with quality equipment

The SRCC (Solar Rating and Certification Corporation) tests panels and systems to verify that they deliver the energy they promise. If your equipment isn't SRCC rated, at least make sure it comes with a warranty. Solar water heating equipment must withstand some very extreme temperature and pressure ranges.

Not all material will stand up to the punishment. Some homeowners have tried to save money by running standard PEX tubing from the collectors to the house. Instead, the tubing ruptured under normal (for solar) operating conditions and had to be replaced.

Look for an installer with references

If you hire an installer, make sure they know what they're doing. The North American Board of Certified Energy Practitioners (NABCEP) offers entry level knowledge assessment, professional certification, and company accreditation programs to renewable energy professionals throughout North America.

There are also a variety of local training programs in some states. Your system will be a significant investment, so make sure you work with someone you trust.

Take advantage rebates and incentives to reduce your system cost

DSIRE is the Database of State Incentives for Renewables and Efficiency. Depending on your location, these incentives may help you to dramatically reduce the cost of your solar water heating system. It pays to take a look.

Given that this article is already getting quite long, I'm going wrap things up here. Hopefully this has given you a basic overview of how solar water heating works.

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Last Updated: Jan 2020

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  1. The idea of solar water heating is a cost saver, hopefully. Big question in my mind is when day is done and showers for 4 start at about 9 pm, how is a 30 gallon tank going to contain enough hot water so all showers are provided with hot water ?? The rate of water useage will be introducing cold water and water temperature will be dropping faster and faster, so that 30 gallons of hot water, after 10 gallons used will not be hot enough for next shower. No sun no hot water using sun. But there could be a solution, using passive or active system would not matter if hot water tank were very well insulated, in ground no least then 6 feet and 100 gallons. With 3 feet of sand all around tank to hole heat and all surfaces are protected from dirt and proper installation done, this could be a system which would last 100 years.

    1. Solar water heating performance is optimized when the system operates under load, i.e., you use the hot water when you have the sun to heat it. For us, this is easier than most, since we homeschool and I work from home. This has always been one of the drawbacks of solar thermal.

      A big storage tank can help overcome this situation, but your costs will ramp up significantly with a bigger storage tank. Once you start talking about burying a huge tank in the ground, you could easily add several thousand dollars to your project costs. That would buy a lot of regular fuel for water heating. In our part of the country, roughly nine months out of 12 any heat lost from a water storage tank to the interior of the home would be a help for heating. So for us, isolating the storage from the house doesn’t make a lot of sense. In other cases, quality insulation will isolate and keep the heat within a storage tank – possibly even better – than burying it in the ground, which would act as a semi-infinite heat sink, absorbing heat from the storage tank rather than isolating it.

      If I had that much money to invest in solar, I’d put it into solar electric over solar thermal, because batteries are a whole lot more compact and easier to work with than thermal storage.

      1. Your type is flat type collectors. Am I right, that in cold areas it is not so efficient? Because, insolation is not good as well as vacum tube collectors?

        1. Yes, you are correct. In cold climates, vacuum tube collectors are generally more efficient. I went with flat plate collectors because I had them available for free from my old employer. If I was starting without a collector, I would prefer the vacuum tube collectors for my climate.

    2. Dear Laurie Neverman,
      I am from Uzbekistan country. And our country enough sunny (about 300 days/year). But solar water heaters are not popular here yet, because people are not informed well. Another case is cost of the system. People prefer Ariston brand(el. heater) boilers or some gas mixed boilers.
      I was interested with solar water heater (SWH) s since March 2015. Almost a year. During my trip to China I have seen a lot of SWH s on the roofs.Then I bought 4 (2 split and 2 compact 200L pressure type). All with heat pipe solar collectors (SC). In split system I have double collectors quantity per L. for 400L tank I have installed 90 heat pipe SC s. Tank has inside double copper coil heat exchangers (HE). So, first circulation goes beetween SC and bottom HE, second sirculation goes beetween heating radiators (fan coils, same radiators with fans, to blow hot air in the room, to get faster effect of heating space). Both are closed sirculations. inside of first sirculation runs glycol (antifreez liquid), inside of second is water.
      I am a little agree with Mr. Geraldc, when hot water inside of tank begins flowing to the bath or some other usage, then tank will be automatically filled with cold water, so, in this case cold will be mixed with hot water and makes to drop the rest hot water temp. inside of tank.
      In this case as he said (if I understand correct), additional storage, which keeps hot water and doesn’t mix inlet cold water. So, heated water by SWH from tank will be automatically flow to storage tank, to fill it again and again.
      Also I have seen a compact type system in China, they took 200 L tank, and inner tank they devided into 3 parts just put walls inside of tank. Beetween each part wall they put holes. Their explanation was to my question why? : – to avoid mixing cold water with hot water. So, the first part, where inlet of cold water, SC tors will heat the water, and heated water will rise and enter second part, then to third. So, the first part most coolest and third part is most hottest inside of tank. So, in this case cold water inlet will not influence tothe whole water volume inside of tank, just to first part of the water.
      About split system, do u have pressure relief valve in your collectors?
      Does it relief overpressure or overheated antifreez in summertime? What do u do in this case? Fill often the circulation system with antifreez? Because, every time when relief valve drops outside some pressure and some super hot antifreez, to cool up system, then u will loose antifreez.
      Best regards,
      Maksud Saidov

      1. Back in the Solar Energy Lab, where I did my graduate work in college, they did some experimenting with baffled tanks to increase stratification, and energy collection improvement were modest at best. In the U.S., a specially constructed tank that met code requirements would be prohibitively expensive.

        The hot water naturally rises to the top of the storage tank. When there is demand to the system, yes, cold water is fed into the tank. This cold water feeds into the bottom of the tank, where the heat exchanger is, to maximize the temperature gradient and improve heat transfer efficiency. Unless the load is constant, the storage tend will again stratify in short order. You could add an additional tank if you wanted to, but every added part represents increased cost, increased space requirements, increased installation labor and an another possible point of failure.

        I do have a pressure relief valve on my collectors, but under normal operating conditions it should not be required. The pressure/temperature combination that would be needed to trigger it would only be likely to occur in the case of catastrophic system failure, such as a pump failure or extended power outage in mid summer. Our system pressure has remained within specifications and glycol/water mix has retained the proper specific gravity for the last 10 years.

    3. Gerald, Your family could all shower together. Preferably at 2 pm. on August the 5th. I don’t have a family, but I did give my cat a bath once. I’ll not do that again,… It took a week and to get the fur off of my tongue.

      1. Dear David, for regular cat licking, may I recommend the PDX Pet Design Licki Your Cat Brush.

        “LICKI is a soft silicone brush you can hold in your mouth to lick your cat — like a mama cat grooms her kittens. LICKI is designed to feel pleasurable to your cat’s sensitive skin, and offers a unique bonding opportunity for human and cat.”

        When you’re done, you can just wash the brush off – much easier than cleaning fur off of your own tongue.

    4. Efficient water solar needs a big tank for storage. If water is getting to 160 degree or higher the size of the storage tank is far to small to store the heated water. So the system as noted needs to be sized correctly to provide an optimal efficiency.

      My system goes about this a bit differently. It is a drainback system. I run into a large holding tank that is insulated on 6 sides. 4 x4 x8 is the outside dimension. I plumb two rolls of 1.5 inch poly pipe into the inside and reduce the size of the pipe as it goes over the top edge of the holding tank.. These pipes sit over the top of the tank so the water in the tank stays at a constant level but cannot over flow top of the holding tank. One loop is plumbed to the solar panels. The second loop is integrated into the domestic water. There is almost no way to ever run out of heated water as the tank is huge and well insulated. My goal is to always keep 120-125 degree water in the holding tank. I should have mentioned.. the loops in the tank are 200 ft so that by itself is enough to supply water for one full shower.

  2. The more passive you can keep your system the fewer things to go wrong. Running it with automatic controls that make it more efficient is good but, if its power becomes unavailable then what do you do? Can you bypass it and still get your hot water/heat? I know some places active is the only option but, if it can to work either way then you got EMP/grid down protection with good efficiency until such time. Also if any of the active components fail you still have some hot water until repaired.

    1. I agree that fewer moving parts means less to go wrong, but in our climate the simpler heaters can’t stand up to the cold. We do have a generator backup for short term, but if there was a long term power outage we’d need to figure out another solution. We also have passive solar space heating and a masonry stove for heat, and the house has enough thermal mass and insulation to maintain comfortable temperatures for some time.

  3. Thanks for sharing these solar water heating basics. You’ve been using solar water heating for over 20 years? Wow, that’s cool! I’ve been looking into solar heating a lot lately, and I find it quite fascinating. I really like that you explained that you can’t take shortcuts when you’re installing a system like this. It totally makes sense that if you don’t install it correctly, the system either won’t work right from the start or will have a very short lifespan. Great post.

  4. I bought a solar water heater w/water tank about 6 years ago. I don’t understand the workings but I thought this was a great from ecology point of view as well as money saver. I live alone, except when my kids and grandkids visit, and haven’t seen that much of a savings dollar wise. One thing I can say is the water is really, really hot. Question, if I lower the temperature to my water heater, will it save me money or will it cost the same no matter the temperature. Thank you for your response.

    1. With most systems, there’s no way to lower the temperature of water going from the solar storage tank to the standard water heater. If you lower the setpoint of your conventional water heater, it may lower your bill slightly, but with such a minimal water heating load, I’m not surprised that you saw only a minimal difference in your water heating bill. These systems make the most difference when they are used in a high demand situation where your demand matches you peak sun hours. If someone promised you big savings, I’m sorry to say they misled you. Payback time in many circumstances, even with tax credits, is often a decade or two. Some systems never pay back the amount invested if hot water use and utility costs are low. If you’d like a more detailed explanation on the physics or the finances, let me know.

    2. if the water is to hot then your tank is to small .. add a second inline tank. The more water in your holding system relative to the size of your panels makes a huge difference in the usability of your water heater. Your only other option is to put shade cloth over one of your panels. Generally you can use a rule of thumb calculation of around 2500 btu heat gain per hour per panel. Every system is different but this is a good place to start. Most 30 gallon gas fired water heaters are 80% efficient and have an input between 30.000 to 35,000 btu so the actually heat gain is around 24,000 btu. to 25,500 btu on firing. This also assumes you are running to give yourself maximum water heat. Two panels will give you somewhere around 5000 btu per hour and if you keep your system near 120 degree it will actually recover in just a couple of hours time to your usable heat. This is why I say having a bigger tank makes a lot more sense.. you without family there will likely Never run out of hot water.

      1. I appreciate the well written, interesting article. We just purchased a home ( in TN) which has ample opportunities for solar (both water & electric) and I am wondering where to get started. Happy to hire a contractor to do some of the heavy lifting, but I need to know enough to know what I know and what I don’t know before I hire them. Any pointers to good sites and/or specific companies?

  5. We had a solar powered water heat system installed just over three year’s ago. It provided an ample amount of hot water for our family. Within the last year, the temperature of the hot water has dropped to about 75 degrees F, even in the hottest part of the summer. Do you have any ideas on what could be the problem and how I can fix it? Thanks.

    1. There are a number of different things that could go wrong, depending on your system type. Your pump may be dead, your controller may be dead, there may be a heat exchanger leak (less likely). If you understand how your system operates, you can go through it part by part and see if each element is working. If you don’t understand it, I’d recommend hiring someone who does, since our panels were heating water up to 85 when it was 50 degrees out and partly cloudy.

  6. Hello Laurie,
    My wife and I also have built our own ICF home with floor radiant heat and live in the country. We currently have a wall mounted high efficiency boiler and run it off of propane. The boiler loop does cycle through a small heat exchanger to heat our domestic water heat and our in floor heating system.
    We have thought about eventually adding thermal solar panels on top of a trellis to the south of our home and have that help supplement our winter heat load. We would have the cost of the panels, storage tanks and circulation pumps but may be able to recycle 50 gallon hot water tanks and add insulation to them ourselves. Have you thought about the use of solar for your floor heat for your home? Oh we are happy with our location and plan on staying here for many more years.
    Thank you,

    1. The solar system we have now heats water for domestic use and space heating, although it provides only a small portion of the space heating needs. To cover the majority of the heating load in our climate, the system just would have been too expensive for our budget, and the storage would have needed to be massive. Sometimes the sun shines very little for weeks on end.

  7. Laurie, Great site, great info. Couple of quick questions. I’m planning on building a supplementary system that in some ways is similar to yours. I have a gas fired indirect hot water system at my house. We live in New England so the weather profile (average temps.) isn’t that far from you (maybe slightly warmer on average, but our February avg. temps have been in the mid-teens for the last decade so I can’t imaging it’s that far removed from you). That said, my south facing roof gets good sun and is easily accessible and a small slope to clean off snow, so I was thinking of getting 3 to 4 150 W solar panels up there, having a small (maybe 400 Ah) AGM battery bank. This should stay at 60% charged as we rarely get more than 2 days in a row overcast. I was looking at the 12 V DC electric element from Missouri Wind and Power as a heating element (bonus, no need to us an inverter and loose more power).
    So all that said, I know this is much less efficient than a direct solar hot exchange system, but this way I’ll be able to keep get hot water even in the dead of winter when nights are 14 hours long and we only get 4 hours of good overhead sun. And hey, it’s all free power right? If I need to throw an extra panel on or an extra battery, no biggie, couple hundred extra for the initial cost but it will even out over a year or two in reduced gas cost? And if I find myself dumping power, I can always have my electrician cousin add a grid tie to the bank and get a few cents back off my electric bill one way or another.
    So on to the questions. First, have you ever worked with this kind of setup? I’m trying to get power numbers our of Missouri W&S with little luck so far, I imagine they are just busy and haven’t responded to my queries, but time will tell. If yes, any idea of how much power these elements use per hour? I’m looking to size my battery bank and I’d like to do more than just guess. Second, I’ve no idea what size storage tank to use. I’m reluctant to replace my existing gas boiler setup as my house is hot water heated and it would be a rather large task to set up the whole thing as electric with a charge controller and inverter from grid, I’d much rather just add this system in as supplementary with a shutoff in case things went south for any reason. I was thinking just a 30 gallon tank to keep the element heating time down during the overnight, but maybe I’m thinking backwards, I’m an amateur at best with hydro-thermal properties and while I did go to college for chemical engineering, it’s been a long time since I put any of the principles into practice in the real world.

    Anyway, any answers you (or anyone on here) may have would be great, even if it’s just observations.

    1. Solar electric panels are so expensive that’s I’ve never seen anyone install a system like this, or even talk about installing one. Generally, your cost for electricity is way higher than your fuel cost, so your payback time for the solar electric system would be hugely shorter if you used it directly for electricity instead of converting it to heat.

      1. I can get panels for about 150 bucks per panel. This whole setup should cost me around 1600 dollars (I add 200 for miscellaneous extra expenses that always seem to happen) I think (450 for the panels, about 400 for the batteries, about 300 for the tank, and maybe 200 for the various other parts and pipes). There’s a few you tube videos on it. This video from Missouri W&S kind of shows the outlines I figure if I can cut my gas bill in 1/2 (the only thing we use gas for is hot water) that’s about 900 bucks per year I save (my gas bill averages about 150 per month). And It will have paid for itself in under two years. That’s a pretty decent ROI for a very small amount of work.

        1. I should probably update this and say, doing the math on the element (400 W, 12V DC), I probably need about 700 for the batteries since I forgot to calculate the cycle going only down to 50%, so that would make it about 1700 instead of 1400. Still, 2 within 2 years ROI though which is nice. This is assuming I need to heat water 2 times a day for about 45 minutes each time.

          1. If you think you can make the math work, go for it. This is not a tech I’m familiar with, so I can’t offer experience. Just make sure your roof penetrations are well sealed – the freeze/thaw cycle and snow load can be brutal to improperly sealed connections – and don’t get electrocuted.

  8. Hello,

    thanks for all of your info.
    I am considering on building a passive, closed loop, solar heating system for my fishtanks (large), which currently use a lot of electricity to heat!
    I am concerned though, will the convection work to COOL the fishtanks in the night/colder days if the system is just left open? will I need to fit some sort of valve/switch to prevent this (would that work?!)

    any thoughs greatly appreciated


    1. You should be able to install a check valve to prevent backwards flow of fluid. I’d hunt around for any local solar installers and see if they have any used panels or odds bits that might be made to work for your situation.

  9. I had no idea that 15 to 40% of energy usage came from water heating! I can definitely see how switching to a solar water heater would be a great way to lower your energy bill and save money. It would probably help you lower your carbon footprint too.

  10. Hi Laurie, great article! I have a question regarding my thermosyphon solar hot water system with flat plate connector. It has been showing yellowish discoloured water (like a sign of corrosion) for the past few weeks. I’ve changed the anode and valve on the tank thinking it might fix it but the water is still of the same colour. A plumber advised me to change the tank but I’m wondering if the flat plate collector’s chrome tube is the one that’s been corroding. Can you advise if it is more likely to have the tank corroding in this situation than the collector? Really appreciate your great help.

    1. I don’t have any corrosion data available for tanks and collectors, nor any experience working with thermosyphon systems since they don’t work well here in Wisconsin. By any chance is it possible to contact the original installer, as they should be more familiar with the equipment that they install.

  11. There are a shit ton of typos in this post. I’m not judging, I just thought you might want to know so you could review it. The content is great as usual. Thanks.

  12. I built a simple flat plate collector and used a 200 liter tank for my storage.(totally passive system) I am having trouble getting the water to circulate without a pump. My tank has a float on the supply. the supply to the collector comes off the bottom of the tank goes through the collector and returns in the top 2 inches of the water level. Am I missing something? The flat plate collector is heating the water well, because I took the line loose which leaves the collector and about burned myself, but it will not push back into the tank.
    BTW I am in a sunny climate in Africa.

    1. Unfortunately, you’re not really “missing” anything. Pump free systems just don’t work all that well. No pump systems often have the storage integrated right into the collector so that heat has somewhere to go without having to move. You might try elevating your storage tank, or using bigger water lines between the tank and collector so that the water moves more easily. Do be careful, as stagnating water in the collector will get dangerously hot. A T&P relief valve at the top of the collector is probably a good idea.

  13. Another thing, The line to the collector leaves the bottom of the tank and goes up about a foot and a half to the bottom of the collector. Then the collector slopes up about a foot itself before the line goes into the top section of the tank.

  14. hi
    I live in the middle of a desert so any one would guess hot water is not a problem. ???? our problems are slightly different so I hope you can give some ideas.

    the biggest issue we have during the hot long summer’s is that virtually all water supplies are boiling. so during the summer we switch off the heaters and let the water get cold in the heaters. so we use our taps hot water line as the cold water line during summer months.

    This is a nice solution as we have heavy use of A.C. IN THE summer and water heaters in the winter. our electricity bills tend to even-out.

    Now I am moving in to a new house. it has a n ingenious design; under ground water tank. it’s great as it helps to keep water cold during long summer’s. unfortunately this means I would have to run our water heaters during the summer too. which could increase our energy consumption significantly.

    now I am trying to setup a solar water heater. we will be using a direct heater. however my installer insists that I use the water tank supplied as a part of the solar heaters system. and remove the current electrical heater from the system. (they will have another electrical cool in the tanks supplied by the contractor.

    My concern is do we really have to use a new water tank? cannot use the existing water tank connected to solar heater? its a 200 l8tr tank

    Much appreciate your support

    1. Even with a direct system (where you heat the water directly without a heat exchanger, you need two ports for the solar in your storage tank – one near the bottom, to pull out cold water, and one near the top to return heated water to the tank. Odds are your current water tank does not have the needed ports, but you can look and ask and talk it over with your contractor.

  15. Here is a complicated question, if I may: I live in a desert environment in Palm Springs, California. Temps average 115 plus throughout July and August. Sunny skies 350 days of the year, and winter temps never below 40 F. I am building a new house and want it to be “net-zero.” So I am looking at trying something very unusual for this region: a subfloor hydronic household heating system in a closed loop. It has to be a closed system using distilled water because local water has an astronomically high mineral content that will damage the PEX subfloor pipes. I was thinking of using a SWH system to supplement a gas water heater (water for domestic use will be heated by a separate inline on-demand system). But the PEX pipe used in subfloor heating has a surprisingly low max temp tolerance. Water circulating through the system is seldom above 85 degrees. I am over-engineering? Given the low water temps required for the subfloor system, might I do better to rely solely on a high-efficiency low-temp gas water heater for the subfloor system? Eliminate the SWH system, at least for the subfloor heating? After reading your post, I have the impression that an SWH system would be TOO hot and would constantly “use” water through expansion “blow off.”

    1. I haven’t worked in that climate, but with those numbers it seems like any heating load would be minimal and you would be prone to overheating because of excess capacity for the majority of the year. I’d talk to a reputable local contractor and get their input. The people in the trenches have the best feeling for how a system will work in their area.

      Also, be aware that on demand heaters generally do not cope well with high mineral content water. We considered using them in our home and ended up not using them for that reason. The tech may have improved since we built, but I’d recommend double checking that again with a local plumber before committing to the design.

  16. Hi Laurie,
    We are wanting to have a solar hot water system installed in our renovation work. But our problem is that we live in Bulgaria and this last winter we saw temperatures drop to -25 .Which system would you recommend .we were thinking of a thermal system.

    1. I’d recommend a closed loop system (heat exchanger in tank with antifreeze going out to collectors). Use an AC pump and evacuated tube collectors if they are an option.

  17. So, I built a copper coil water heater for my pool with 1″ copper, and whenever I feed the pool water into it (via hose from return line), the water ends up cooling the pipes instead of vice versa. What am I doing wrong??

    1. Do your copper coils get hot enough? If not, there’s no way they can transfer heat into your pool water.

      I have not worked with pool heating systems, but for home hot water we generally look for at least a 14-16 degree F temperature differential between the panels and the storage tank before circulating the fluid. Any less, and the panels will cool too quickly and the pump will turn off again.

      You could also look at time in the collector. If the collector isn’t seeing a temp well above pool temp, but does get slightly above pool temp, you could potentially still get some heat by circulating your water extremely slowly, so it has time to sit in the collector and heat up more. It’s nearly impossible to say for sure what’s going on from one sentence, but your heater may be undersized for the volume of water you’re trying to heat, too.

      Check in with a local pool guy with a clue, or find some pool heating forums where people have done what you’re trying to do and hit them up for some troubleshooting assistance.

  18. We had our 2 solar hot water panels removed to have a roof replacement. When the panels were reinstalled we are now experiencing a problem. The circulating pump turns on and temp only goes up to 120 and then the pump turns off. System has all new parts and had been serviced prior to the removal and re installation. They re-used our existing glycol. Said it looked fine. Our understanding is that the differential controller turns on the pump. Both controller and pump are working. Could this be a sensor that wasn’t attached correctly on the roof is the reason why the system keeps shutting off too soon?

    1. Glycol is pretty stable, so it likely was fine. It sounds like either a sensor issue or a differential controller issue. When the differential controller is working correctly, it looks at the difference in temperature between the storage tank and the panels. If your storage tank gets hot and the differential gets less than the setpoint, it would shut off the pump to let the panels heat up more, and then kick on again once the differential was reached. Simply kicking out at a given temperature doesn’t make much sense, unless the controller is broken or something causes a sensor to detach or not register correctly at higher temperatures. Have you tried adjusting the differential on the controller to see if the problem still occurs?

      You probably need to get a tech there to troubleshoot. I can’t poke and prod it over the internet.

  19. Hello Laurie,

    I seem to have the exact same setup as your system. I reside in Moore, Ok. I do believe the last time my system was professionally looked at was in 1989. Unfortunately, the previous owner had passed before he could clue me in on an annual maintenance.Do you take care of any maintenance yourself? For instance, draining the water to flush out calcium. Replacing the antifreeze soulution. If you have any recourses as far simple updates and maintenance on this particular product, I would greatly appreciate the help.

    Torren 2/20/18

    1. Like a regular water heater, your storage tank may get sediment build up in the bottom over time. It’s a good idea to flush it out every couple of years or so, similar to the way you would flush out a regular water heater. (Shown below.)

      To replace the anti-freeze, you’d need to have specialized equipment or hire a professional. The closed loop to the collector is under pressure, so if the anti-freeze is drained and replaced, the system needs to be recharged with a pump hooked up appropriately. During routine maintenance, the quality of the fluid is checked for obvious signs of damage as well as appropriate level of glycol for freeze protection. Propylene glycol tends to be pretty stable, but it can break down over many years of service. If the fluid looks good, they’ll check for any other signs of trouble and that pressure is correct and assume it’s good to go.

      If the fluid is degraded, they’ll need to drain, flush and recharge the system. Without the right equipment, you can’t do this at home. The video below shows how to charge a similar system.

  20. Hi,

    We installed a new heating system and would like you to comment on the practical application of this system versus solar thermal hot water, also would you have a name for this type of system. System consists of solar panels that provide electricity to air to water heat pump with glycol mix. Transfer plates transfer heat to 1000 gal storage tank that then runs through radiant floor. Propane and wood gasification boiler tied in as back up. We live in northern Vermont and are more than happy with the system keeping warm with temps in the -5 to -10 degree range on heat pumps alone considering we have extended periods of cloudy days and shorten winter daylight hours as we are on the 45 degree latitude.

    1. I’m not quite sure what you need. The system works for you, so that’s the most important thing. 1000 gallons is an enormous storage tank. I think most folks would have some concerns about the size. That’s a lot of water to have hanging around inside your house. The system is also more complicated than a thermal system.

      The advantage of using solar electric is that you could use the panels to meet your electric needs when you don’t need them for heating. I have a snappy name for it, although I suppose you could call it a hybrid solar electric heating system.

  21. Laurie,

    Question about heating a 6’x12’ black cargo trailer. I bought property in Southern NM ( plenty of sun, daytime highs in the 50’s & lows in the 30’s) off grid & was considering purchasing a 30 tube panel with a solar powered pump to circulate water into a 275 gallon uninsulated ibc tote inside to heat the interior. I understand that there’s no way to heat up the entire 275 gallons, but partially full would the tank give off enough heat at night if I put the head of the bed by the tank?
    I have the ibc tank to haul water to my other black painted ibc tote with hand pump on property anyway, so why not put it to use instead of hauling & burning propane?

    1. Without knowing the specs on your equipment, it’s impossible to run heating calcs. Check with the manufacturer of whatever collector you want to buy and look at their projected BTU output under your winter conditions. Calculate how much this many BTUs this would put into your storage tank. Whatever you are able to dump in, you’ll probably get between 50-80% of that back out as heat to your living quarters. Whoever you’re buying the collector from should be able to help you with estimates of how much heat their equipment could potentially gather.

        1. I’m sorry, Neal, but this goes beyond the scope of general information provided for free. I could probably dig up software to run the calcs, but if I did that, I’d need to charge you consulting fees for the time involved.

          Your heat will bleed into your living area, even if the tank is insulated. It’ll just bleed slower or faster.

          Talk to whoever wants to sell you the collectors. They should be able and willing to help you with some calculations, since they want to sell equipment. There are also online tools, such as

          The programs require you to plug in your location, then they grab appropriate data sets of solar insolation numbers to calculate potential energy available to the collectors, based on collector parameters. These numbers work in combination with storage tank calculations. Solar thermal collectors are most efficient (gather the most energy) when they are operating under load when the sun is most available. Since you’d likely be dumping heat into a lukewarm tank, this is not going to optimize collector performance, but it’s a big tank, so you’d probably be okay – BUT – there’s the matter of freezing. If your nighttime temps drop below freezing, are you planning to use a drainback system that drains all water out of the collectors at night? Or are you planning to use antifreeze through your panels in combination with a heat exchanger?

          You really need to talk with professionals in your area, or get some training yourself so you have a better idea of how to make a system that fits your needs. It’s not something that I can cover in a quick email answer.

          Best of luck,


  22. Great article, thank you. My question is about supplementing the heat during winter months when the temperature of the solar water is slightly too low to bath or shower. I would still like to use this hot water but as it is not hot enough, to but send it through a gas boiler. Problem is it needs to be a “Thermostatic” gas heater according to my plumber so that the gaskets etc don’t deteriorate. He is suggesting a small electric storage tank of 500watts instead as it would be cheaper to install. I don´t know which would be cheaper to run, can you say?
    Do you have any information or opinions on supplementing the heat of a solar storage water heating system? Thanks in advance!

    1. It depends on your gas and electric rates. In general, gas heaters are cheaper to operate, but the heater itself is more expensive and the installation is slightly more complicated so it will also be more expensive to install. That’s why your plumber is suggesting the small electric tank.

      To install supplementary electric, they cut into the water line to install the tank. To provide heat, they might be able to use an existing outlet to plug it in (easiest option), or they need to run an electric line. Adding an electric line entails flipping a breaker to make sure the circuit isn’t live (or adding a new breaker), and running wire as needed. If your breaker box and water heater are in the utility space, it should be a quick and easy addition.

      To install supplementary gas heating, they still need to cut into the line to install the heater, as with electric. Boilers (at least in our area) are quite expensive. Gas water heaters are less expensive, but still more expensive than electric. Assuming they use a gas water heater, they still need to run an electric line as above, because almost all gas water heaters have electric ignition and controls. In addition, they also need to add a dedicated gas line. Some plumbers are trained to work with gas lines, some aren’t. If they need to call in another contractor, that’s a significant added expense. Even if they can do it themselves, they still need to add both gas and electric lines.

      If your need for supplemental heating is small, the electric tank will get the job done. If you need quite a bit of supplemental heat and plan to be in the home for some time, gas may be well worth the additional investment.

  23. I’m considering a solar hot water system in my Colorado vacation home. The incoming water temperature is very cold year round, probably in the 40s. That means that my current electric hot water heater is working overtime when I’m here. I also have times where there are six users for short periods of time. Occupancy is usually in the winter with short stays in summer and fall. The house is two story and the upper story receives substantial solar gain on sunny days but the lower level remains pretty chilly all year round. It never gets above low 50s without electric heaters.
    I’m thinking about installing a flat-plate collector on my south facing deck and optimize the angle for winter use. I would probably use a per-heat tank in my utility room on the lower level. I’m wondering if I can dump excess heat somehow into that lower level particularly when the house is vacant and hot water is not being used. Any advice would be appreciated

    1. Given that you are there more often in winter than summer, optimizing the mounting angle for winter makes sense.

      As for system specifics, you’ll need to talk to a local contractor who can get in the building and give you some estimates. Typically, solar space heating involves dumping heat into some sort of thermal mass, like a sand bed under the floor, or even radiant heating coils in the floor itself. (Coils in the floor gets tricky because of expansion and contraction.) There are solar space heaters that can mount on walls and floors, too. They aren’t as effective, but are easier to retrofit.

      A big storage tank will act as thermal mass and heat up the area that it is in, but that doesn’t move the heat around the rest of the floor.

  24. Ive a flat panel system – 2 panels on the house roof, solar hot water tank on the ground that has a pump to circulate the water which is effectively a main pressure system. We are on tank water with a pump to get it to the house for the last 4 years and don’t has natural gas.

    During winter our electricity bills double ($55 per week to 125 per week), day to day usage remains the same.

    What I’ve found is at night and sometimes on really cold days (it doesn’t snow but we do get frosts), the solar pump kicks in to circulate warm tank water to the panels and the frost valves release water even at temps of 5 to 8 degrees. So in my mind, the negative benefits of this system in the cold months far out way the positives – tank pump kicks in because the frost valves are draining water, solar pump kicks in to circulate warm water, loosing hot water to circulate into the panels which then the electric got water element also kicks to heat the water in all consuming electricity.

    Looking at options:
    1. drain the panel for the winter – pain as I need to get up onto the roof to bleed the system of air when putting it back into service)
    2. Retro fit evacuated tubes. – Don’t know if these required water to circulate in cold temps and would defeat the purpose if they did.
    3. …maybe mount the panels on the ground for ease of access with option 1. – Not sure how far from the hot water tank/house I could do this without loosing heat pumping it back to the hot water tank.

    Any suggestions?

    1. The cheapest option would be to drain it for winter. A hassle, for sure, but the most affordable.

      Evacuated tube collectors are still likely to freeze in temps below freezing. It’s recommended that they be used with antifreeze and a heat exchanger where temps get below freezing.

      If you ground mount your panels for ease of access and run an insulated pipe below ground back to the house, heat loss should be minimal.

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