Flat plate solar water heaters are very simple devices, which use solar energy quite effectively to heat water.

This article discusses, at a limited conceptual level, a  simple cost effective DIY design for a flat plate solar water heater.

How do Flat Plate Collectors Work?

Flat plate solar water heaters use the greenhouse effect to trap solar energy and thus heat up some form of metal plate, which in turn heats up water in pipes thermally attached to that plate.

In this case, the greenhouse effect is caused by the radiant properties of glass.  Incoming solar radiation energy has wavelengths that can pass through glass. This energy is absorbed by a metal plate, or sheet, on the other side of the glass (which is normally painted or coated matte black to maximise radiant energy absorption). The greenhouse effect comes about because the heated plate emits its own radiant energy, but at much higher wavelengths, which cannot pass back out through the glass as easily. So the incoming solar radiant energy is effectively trapped behind the glass. Transparent plastic does not posses this property to the same extent that glass does. This greenhouse effect can be used to markedly increase the efficiency of the unit, in terms of the proportion of solar energy that falls on the collector, which actually ends up heating water.

Basic Components of the Design

The basic components used in this design are readily available, mostly low cost materials that can be bought at most local hardware shops.

The diagram below shows a cross-section of the collector’s basic components:

The core of the design is based on lengths of 15mm copper pipes that are firmly attached (using something like fence bailing wire) to a length of corrugated steel roof sheeting. The pipes are attached to every second “valley” in the sheeting. Both the pipes and the sheet will be painted matte black to maximise radiant energy absorption.

The ends of the pipes should stick out past the top and bottom of the sheet to provide space to join them together. This can be done using 22mm pipe sections and T-piece fittings. The ends of those 22mm cross-pipes will extend out through holes drilled in the timber frame, which will then provide four points at which water can be plumbed into, or out of, the unit. (Note that you should allow for expansion/contraction as the pipes heat up and cool down each day.  So the holes drilled through the frame should be slightly larger to allow some movement).

To join the 15mm pipes to the 22mm cross-pipes, one can either use compression fittings (which are more expensive, but easier to use), or alternatively, the type that require soldering, which are much cheaper.

When the assembled unit is mounted properly, and pointed towards the sun, incoming solar radiation will heat up the matte black coated corrugated roof sheeting and attached pipe work. The heat gained will mostly be trapped inside the frame. The air gap between the glass and the corrugated sheet will minimise conductive heat loss, while the glass itself will minimise radiant, as well as convective heat loss. A sheet of polystyrene foam behind the corrugated sheet will minimise heat loss in that direction, as will the timber frame around the sides. This will help maximise the amount of energy available to heat water within the pipes inside the frame. (High temperature paint will probably be necessary for the pipes and sheeting for this reason).

Tempered glass is recommended so that it is strong enough to withstand hail.

What Size Collector Do You Need?

This depends on the amount of solar energy reaching the ground at your location on earth, which can vary quite significantly. Luckily, thanks to the US government, we can easily get that data using the NASA Atmospheric Science Data Center.  Just click on the link and then enter the coordinates of your location on earth. And then select “Insolation on horizontal surface (Average, Min, Max)” and then press the “Submit” button.

NASA’s measurements for incoming solar radiant energy for your selected location (kWh/m2/day) will then be presented to you. It’s probably best to use the average annual figure (this will mean that your corrugated sheet will be adequately sized “on average”, but might not produce quite enough heat during winter). Note that it’s better to go for a slightly smaller corrugated sheet, than too large, because if it’s too large, the water might overheat and cause potentially dangerous problems.

Once we have that figure, you can use the calculator below to estimate the size of corrugated sheet you need. (Please note that the results also depend on an estimate of how efficient the completed unit might be, which is uncertain. Some trial and error may be required):

Once you have determined the dimensions of the corrugated sheet you need, you can attach your pipes to that sheet (for example, by drilling small holes through the sheet and using bailing wire to fasten the pipe securely to the sheet). As previously stated, you need to make the pipes slightly longer than the sheet to allow enough space for the fittings that join them to the cross-pipes that run along the top and bottom of the sheet. After that, you can build your timber frame to fit around the entire assembly. So the dimensions of the frame depend on the size of the sheet and pipe assembly.

Once you know the frame dimensions, you can get a suitably sized sheet of glass and install that into the frame.

The same goes for the polystyrene sheet at the back.

The CAD drawing below gives an idea of what the final assembly might look like.

Disclaimer:  This article is meant to provide no more than a conceptual design of a simple flat plate solar water heater for discussion purposes.  This article should thus not be  relied upon as detailed instructions for any such project. We make no claims about how effective this design might be. Anything you attempt as a result of reading this article is done entirely at your own risk.

DIY Design for a Solar Water Heater

10 thoughts on “DIY Design for a Solar Water Heater

  • May 28, 2019 at 14:41

    Thanks for public-spirited info. Can I ask a question:

    Money being short, can I add this to an existing geyser if I set the thermostat of the geyser to say 40 deg C? Location: Ramsgate KZNatal. Geyser is horizontal inside and near top of roof of attached garage; thus if there is a leakage or even a burst there will be little damage. I am aware that any “Latco” valve may need to be replaced.

  • March 8, 2020 at 14:44

    Good article, but I have one concern – will galvanic corrosion not take place between the copper pipe and the corrugated roof sheeting? Plumbers normally go to great lengths to avoid this kind of contact?

    • March 8, 2020 at 21:01

      Ja that is probably a valid concern. The roof sheeting needs to be matt black colour so hopefully the paint will protect the sheet?

      But bare in mind, this is supposed to be a low cost DIY option, so even if corrosion does occur, it should be relatively cheap and easy to repair by replacing the relevant parts.

  • July 18, 2020 at 18:43

    Excellent ideas, I’m getting ready to launch into it. Any reason why only every second valley for a pipe on the corrugated sheet? Is it related to copper fitting size? Or rather the amount of sheet to absorb solar energy?

    • July 21, 2020 at 08:04

      Hi Ryan

      The answer to your question is a bit of both. It’s also a question of transferring the available energy into the water effectively, the less water in the collector, the faster it will heat up, so adding more riser tubes will increase the volume of water. As well as cost (less piping and connectors needed). Obviously a practical compromise between all the different factors needs to be reached probably via trial and error, after the the theoretical calculations are done.

      I followed the configuration of existing commercially available flat plate collectors which I knew worked.

      But the nice thing about DIY, is you can conduct your own experiments and see what the result is. So feel free to try out your own configurations.

  • August 25, 2020 at 08:28

    Hi there, many thanks for the superbly written article. It’s very informative.
    I have a few questions:
    I’m in Parys, Free State, we’re plagued by the water supply going off plus load shedding twice a day.  I’m seriously looking at designing a system for my house, a little extra info would help.
    The link to the NASSA Insolation calc seems to be broken, can you suggest an alternate site for this value ?
    Do you have any suggestions for preventing damage to the collector from frost?
    Would you recommend an air release valve at the highest point on the collector?
    I would you think a high temperature release valve is essential, would a normal geyser type work OK ?
    Has anyone any experience in automatic valves to switch in extra panels ? In summer a small panel would suffice but in winter two might be necessary. It would be great to switch them in or out automatically.
    My geyser is mounted outside on the south side of the house, it’s about 15m from the panel. Is it necessary to move the geyser inside the roof and closer to the panel?

    • August 31, 2020 at 09:16


      Don’t have time to address all your queries but here are a few answers:

      1) An air release valve is a good idea.
      2) Normal temp/pressure release valve should be OK. Make sure you use some sort of SABS approved valve!
      3) That pipe length to the geyser sounds way too long. Ideally it should be about 1-2m, not much more.

  • August 30, 2020 at 08:59

    Live in Free ßtate temp falls to minus 10 degrees what çhance of bursting? Plus can I store hot water in a 100liter tank? Thanks erñie

    • August 31, 2020 at 09:11


      The geyserwise max has a function where it circulates warm water from the geyser through the exposed collector at night to prevent ice formation. Provided that works (ie the pump has electricity etc), then you should be OK.

      But you will have a very cold geyser in the morning.

      Maybe better to get a proper solar geyser which keeps the heated water separate from the hot water you use – which then allows you to put antifreeze in the heating circuit. The system is essentially the same otherwise, so the above instructions are largely applicable.


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