Converting Your Geyser to Solar Heating

Converting Your Geyser to Solar Heating

A Brief Discussion on The Basics of Converting Your Geyser to Solar Water Heating

Heating water is often the single biggest consumer of electricity in your home.  When considering going off-grid, heating water is thus a major consideration.  It would be best to find some way of reducing the amount of electrical energy used to heat water before you consider installing expensive alternative power systems such as solar PV panels, because that will significantly reduce your daily electrical energy consumption requirements and thus significantly reduce the size and cost of the alternative power system needed.  To estimate how much energy your electric geyser uses per day, try using our calculator.

One relatively cost effective way of heating water is to convert your existing geyser to a direct solar water heating system. A direct solar water heating system will typically reduce the amount of electrical energy spent heating water by about 50%, on average over the year.

This article provides a brief overview of of how to go about that.

How an Electric Geyser Works

Its probably helpful to first understand how your normal geyser works.  Its a relatively simple device but its operation is often misunderstood.

A geyser is nothing more than a well insulated pressure vessel that holds a specific amount of water.  It can withstand the high pressure (about 4 bar) of the council water system.  Water is heated using an electrical heating element (usually rated at about 3-4kW).  The heating element is controlled by a simple thermostat that is inserted at about one third of the height of the vessel.  The thermostat switches the electric heating element on or off, based on the water temperature it detects at that height.

So lets run through some typical operational scenarios:

  • If the geyser is completely emptied of hot water and is only filled with cold water.  This situation doesn’t happen that often in practice maybe, because most people do not use this much hot water at a time, and thus completely empty the geyser.  But when it does happen, the thermostat will detect that the water temperature at one third of the height of the vessel is cold and thus switch on the heating element.  When the water temperature reaches the set-point temperature (which you can adjust – typically set at 50-60C), it will then switch off the heating element, so that the geyser will not consume any more electricity until the thermostat detects cold water again.  It typically takes about two hours continual heating for the electric element to heat all the water in the geyser from cold, to the set-point temperature.
  • The next scenario is when water in the geyser is all hot and no hot water is used for a long time.  The geyser’s insulation is usually very good, so it will take a long time before the water cools enough for the thermostat to switch the heating element back on again (about 4 or so hours).  When it does switch the electric element on again in these circumstances, it will only be for a short time (about 10 minutes or so) before the water is heated back up above the set-point temperature.  As you can see, the electric element in your geyser thus spends most of its time switched off, contrary to popular opinion.
  • The next scenario is when you open a hot tap and use the hot water stored in the geyser. In this situation, as hot water flows out of the top of the geyser, cold water flows into the bottom of the geyser.  Now you have cold water and hot water in the same container, at the same time.  This is not a problem due to the fact that hot water will float on cold water, without mixing with the cold water, provided that the cold water flows into the bottom of the geyser very gently – SABS certified geysers are specifically tested for this.  This allows you to withdraw at least 160L of hot water out of a 200L geyser, even though towards the end, most of the geyser is filled with cold water!  When hot water is flowing out of the geyser, the thermostat will only switch the electric element on when it detects cold water at one third of the height of the geyser. So if you only withdraw a small amount of hot water, the electric element will not be switched on.

That about covers how your geyser works.  As can be seen, while its a very simple device, some of the things going on can be quite complex.  Understanding this can be helpful when converting your geyser to solar water heating.

Direct Solar Water Heating

SolarGIS-Solar-map-South-Africa-enIn South Africa we are blessed with very high solar insolation. It thus makes a lot of sense to use the sun to heat water because in most places, we get so much sun during the year. In addition, solar water heating panels are very simple devices which collect the heat from the sun quite effectively.  Typical efficiencies of even simple flat plate collectors are about 60-80% or more.

Check out our DIY design for your own flat plate collector.

To convert your existing geyser to solar water heating, you need to install a solar water heating panel, which requires some plumbing changes, as well as adding an electronic control unit.  We will deal with the plumbing and electronics separately.

Plumbing Changes Required

The diagram below defines the various major plumbing components required, as well as shows how they should be plumbed together.

directsolarwaterIn the diagram above, you can see the existing system in your roof, which is basically an existing cold water pipe feeding the bottom of the geyser, the geyser itself, and the existing hot water pipe coming out of the top of the geyser.  There are a few other pipes and components included in a proper normal geyser installation, but for the purposes of this article, you only need to focus on the cold water pipe feeding the bottom of the geyser, as well as the the hot water pipe coming out of the top.

To install a solar heating panel, you need to cut into the cold water pipe feeding the bottom of the geyser, as well as the hot water pipe coming out of the top of the geyser. You will need to shut off the water and drain the geyser before you make the cuts.  There is a special drain valve at the bottom of the geyser – use your hose pipe and clamp it to that valve to get all the water out.

Brass-Compression-Fittings-resize1You should make the cuts as close to the geyser as practical (this is especially important on the hot side).  Once you cut the pipes, you should install T piece connections at those two points. Standard compression fittings, available from most hardware stores, are the easiest way to do this – all you need is a copper pipe cutter and two shifting spanners.

Those two T piece connections then provide you with the connection points to plumb the solar heating panel into the existing system.  In the diagram above, they are represented by the points at which the red and blue lines meet the black lines of the existing pipework.

Then starting from the cold water T piece – you should add a short length of pipe, and then a ball or gate valve (shown in the diagram). Try get valves that use compression fittings to make your life easier.  The purpose of these valves is to allow you to shut off the circuit in the event you need to do maintenance on the solar water heating system.

After that, you should direct the pipework down to a point below, or near the bottom of the geyser.  At that low point, you should install the pump.  You want the pump to be at the lowest point in the circuit.  This helps avoid problems when the system fills with water again.  It ensures that the pump should be full of water after the system re-fills. Most pumps should be installed horizontally, but check the specs of your particular pump.

After the pump, you should install a flap type one-way valve, preferably on a vertical section of the pipe.  After that, the pipe should run up through the roof and then be connected to the bottom of the flat plate collector panel mounted on the roof. Try to avoid excessive corners in the pipe work, and especially ensure that from the pump, the pipe work only goes up or along, not down again, to avoid problems with trapped air pockets when the system fills with water (preferably the pipe work should go straight down to the pump from the T piece, and then after the pump, straight up to the panel on the roof).

That completes the “cold” side of the circuit, represented by the blue lines in the diagram above.

From the diagonally opposite top corner of the flat plate collector, you should connect the pipe that will lead back down through the roof, to the T piece installed on the existing hot water pipe leading out of the top of the geyser, to complete the circuit.  This is the “hot” side of the circuit, shown in red in the diagram above.  Its very important that this pipe has as low a volume as possible.  The volume of this pipe must be very much less than the volume of the flat plate collector, otherwise the system simply will not work.  It should thus be as short as possible, which means the flat plate collector needs to be positioned as close as possible to the geyser below.  For this reason also, we recommend using 15mm pipes for all the pipe work.

Before this  pipe reaches the T piece, you should install a spring type one-way valve, as well as a ball valve (to facilitate maintenance on the solar panel circuit).  You should insulate all exposed pipe work in the circuit, but especially this hot side pipe.

The pipe should preferably go straight down from the flat plate collector,  through the roof, to the T piece connection on the hot side of the geyser.  It should avoid excessive bends, especially any turns back upwards which may cause problems with trapped air pockets when the system re-fills with water.

You should then close the two unused connections of the flat panel collector using suitable compression end pieces (the flat plate collector has a connection at each corner, only two of those have been used in this circuit).

That concludes the plumbing part of the direct solar heating system installation.

All that remains is to turn on the water again, and let the system fill with water.  Make sure the two ball valves are open.

To allow the trapped air to get out, you should slightly loosen the compression end piece fitting on the top of the flat plate collector, just enough to let the air/water hiss/dribble out.  It will take some time for the geyser and the rest of the system to refill, so be patient.  Eventually water will start coming out together with air. Let it flow out until only water comes out, and then re-tighten that fitting.  Check that air is not trapped in the hot side of the circuit (which is quite likely due to the one way valve).  If necessary, loosen the relevant compression fittings a bit to let the air out (use a bucket to catch the water that dribbles out).

After that, try opening a few taps in the house and wait until all the trapped air stops spurting out and the water flows normally.

Check all connections for leaks.

As mentioned earlier, standard brass compression fittings are the easiest way to join all pipes and components.  In the case of threaded components such as the pump, its probably best to use a threaded compression fitting (screw the compression fitting onto the pump and then use the compression fitting to connect to the pipe).  Hemp is probably the most reliable way of sealing threads.

Lastly, please note that as soon as the sun shines on the flat plate collector, the water in there will start heating up, quite rapidly.  Make sure that you get the electronic control system operational before too long, because otherwise the water in the panel will be heated to very high temperatures, because it wont be able to escape.

Mounting the Flat Plate Collector on the Roof

We discussed the plumbing of the system, which assumed that the flat plate collector had already been mounted on the roof, before discussing the actual mounting of the collector, because it was perhaps useful to provide an overall view of how the water would flow through the circuit, and particularly how important it is for the hot part of the circuit to be as short as possible.  This should give you a reasonable idea of where to position the collector on your roof.

Depending on your roof type, there are different ways of mounting the flat plate collector.

hangerboltsIf you have a corrugated or IBR sheet metal type of roof, then its probably best to use Hanger Bolts.  Just drill through the sheet metal and screw the bolts into suitable timber members below (if there aren’t any strong timber beams where you need them, you may need to install some).  Use a high quality silicon sealer to seal the bolts. Also be sure to drill through at a “peak” and not a “valley” of the sheet metal.

If you have a tiled roof, then you can use something like the specialised  Solar Water Panel Tile Mounting brackets that are designed to slip under the tiles, which you then screw onto the timber beams below.

If you have a flat roof, you will have to make some sort of inclined frame to mount the flat plate collector.

The main thing to consider when mounting the panel is that is should face due North, and that it should be inclined to about the same angle as your angle of latitude.  In addition, you should consider shadows that might fall on the panel during the day (both in summer and winter).

You will need to cut holes through the roof for the water pipes.  If you have a sheet metal roof, just use a normal hole saw.  If you have a tiled roof, use a suitably sized masonry bit, or diamond tipped hole saw.  Do not use the hammer function on your drill to drill through cement tiles, because its quite likely that you will break them.

Use high quality silicon sealer to seal the holes.

Electronic Control System

Once the solar collector has been installed on the roof, and the plumbing changes have been completed, then the last thing to do is install the electronic controller unit.

Electronic solar water controllers, such as the Geyserwise Max, are required to switch the circulation pump on or off periodically during the day.  They do this by comparing the temperature of the water in  the geyser to the temperature of the water in the flat plate collector panel.  When the panel temperature is about 7 degrees hotter than the geyser water temperature, the electronic controller switches on the pump, which pumps some colder water out of the bottom of the geyser up into the panel.  At the same time, the hot water currently in the panel, flows into the top of the geyser.  When the controller detects the colder water in the flat plate collector, it switches the pump off again, so that the water can have a chance to heat up.  When the controller later detects that the water in the panel is hotter than the geyser water again, it switches on the pump, so more cold water is pumped out of the bottom of the geyser and simultaneously, the now hot water in the panel flows into the top of geyser.  This process repeats continuously during each day, for as long as the sun is able to heat the  water in the panel to a temperature higher than the water in the geyser.

The other thing the electronic controller does is manage when the electric heating element in the geyser should switch on or off.  This is basically achieved by setting time’s during the day when the electric heating element will become active (typically a two hour period at the end of the day, at dusk, as well as another two hour period just before the family rises in the morning).  During those periods, when the heating element is active, if the water temperature in the geyser is lower than a preset temperature, such as 55C, the controller will then switch on the electric element until the water is heated to that set-point temperature.

So for example, at dusk on a cold overcast day, the water in the geyser may not have been heated to a high enough temperature by the sun.  So when the electric element becomes active at dusk, the controller will detect that the geyser water temperature is too low and thus switch on the electric heating element to heat the water to the set-point temperature.  Conversely, if the day was a nice hot sunny day, and the water in the geyser has been heated to a high temperature, then when the heating element becomes active at the end of the day, the controller will detect that the water is already very hot, and thus it will not switch on the electric heating element, even though it is active.

So you can see, the electronic controller is the “brains” of the system.  It manages when the circulation pump should switch on or off, as well as when the electric heating element should switch on or off.

The diagram below shows the basic components of that system, and how the various wires are connected:

directsolarwater_electStarting at the bottom left of the diagram, you can see the thick red line which represents the existing power cable for the geyser.  This should be disconnected from the geyser and re-routed to the controller.   The controller board should be screwed to a rafter close by.

Make sure you switch off the power to the geyser at your DB board before hand.

The existing thermostat in the geyser is removed and replaced with the Geyserwise temperature probe.  The temperature probe wire (shown in blue on the diagram) should be plugged into the correct socket on the controller.

Use heavy duty 2.5mm Norse cable to connect the power cable leading from the controller to the geyser (the other thick red line in the diagram).

Make sure that everything is properly earthed.

The flat plate collector panel temperature sensor wire (shown in blue in the diagram), should be plugged into the correct socket on the controller.  The sensor itself should be firmly secured to the copper pipe leading out of the top of the flat plate collector using two or three cable ties.  The sensor must be clamped to the top of the pipe right where that pipe comes out of the flat plate collector.  The sensor must be well insulated so that it is measuring the pipe temperature, not the air temperature.   You will need to either drill a hole in the roof for this wire, or carefully thread the thin cable under the tiles.

Use a suitable twin core flex (at least 2.5mm) to connect the 12V pump to the 12VDC pump terminal on the Geyserwise board.  This is represented by the green line in the diagram above.  Please note that there are two terminals for a pump on the Geyserwise board.  One for 12V DC  pumps, and the other for 220V AC pumps.  Make sure you use the correct terminal.

We recommend using a 12V pump because if you want to install an optional 12V battery back-up to the Geyserwise board, to ensure the system remains operational during load shedding for example, then the battery will be able to drive the pump.

The last thing to connect is the remote key pad, which should be installed on a wall inside the house below the geyser.

You can then switch the power back on and program the controller using the keypad.  All you need to set up is the times during the day when the electric heating element should be active, as well as what set-point temperature you want the water heated to.

And that basically concludes your solar water heating system installation!

Disclaimer:  This article is meant to provide no more than an overview of what is involved in converting a normal geyser to a direct solar water heating system.  This article should thus not be  relied upon as detailed instructions for any such project. Anything you attempt as a result of reading this article is done entirely at your own risk.


60 thoughts on “Converting Your Geyser to Solar Heating

  1. What a descriptive and easy to follow post i was able to install my solar geyser with ease thanks to this post i followed it and everything went surprisingly smooth, guess guys should read instructions. thanks for the advice.

  2. Glad you found it useful Markus!

    Although it was meant more as an overview of the subject than a set of detailed instructions, because some of the systems may have slightly different installation requirements etc.

  3. Why do you have to go through exercise with pipes & fittings, etc?- can’t you only put a PV panel on top which heat the element, or another element to keep costs down?. This way you heat water through whole day and it keeps geyser temperature hot that it does not use electricity but only if really needed?

    1. Most hot water heating panels are quite efficient, converting 80% or so of the sun’s energy is converted into useful heat. While most PV panels are about 17% efficient. So trying to use PV panels is a very expensive option in most cases.

      1. Meant Photo Voltaic / Solar panels! To heat an element directly to help warm the water inside geyser. Sensor connected only for electricity, not needed for solar. With such a set-up, you don’t need to do do any plumbing! Heard there is something in the market- do you know where to find or where to look?

        1. it can be done geyserwise sells an element that can work directly from solar panels but you need more panels than they recommend almost making it not viable

        2. In Reply to Albert (Re Using PV Panels to provide power to the electric element): Not sure, but as I said, I don’t think this is a cost effective solution, you will pay far more for all the solar PV panels needed, than the benefits justify IMO.

          In addition, be aware that DC arcs when switches are opened and closed so you cannot use switches that are designed for AC, they have to be specifically DC rated. The thermostat in your geyser is a switch, so you will need to probably replace that with something that will withstand the DC.

  4. Great article!

    Is there a significant difference between the newer solar geysers and retrofitting an existing geyser with a heat collector in terms of energy savings?

    1. In Reply to Alex: There are differences in types of solar geysers available, the biggest being the ability to introduce antifreeze into a dedicated closed circuit heating loop between the solar panel and a solar geyser specifically designed for solar water heating (which has an internal heat exchanger to transfer the heat from the closed circuit water to your “normal” hot water).

      It all comes down to cost. The idea in the article above is to allow one to convert your existing geyser to solar at low cost.

  5. I too have been looking into 12v geyser elements with a PV panel and or a small wind turbine combination connected directly to the element. Can’t find much on the subject it justs seems very simple and uncomplicated

    1. Visit Geyserwise website. I have done 2 PV installations on 200l geysers with dualwise controler and they cost me about R 3000 each more than normal Solar. 3 x 310 watt PV panels per geyser. Works very well and will outlast normal Solar

  6. good day i have installed a 150 l kwikot geyser with the solar tubes on roof , but struckle to get hot water. i do have a circulation pomp on. can you please help me and explain to me what can be the problem.
    regards mechiel

    1. Hi Mechiel

      Its difficult to troubleshoot someone else’s installation.

      I would check that the controller is working properly (switching the pump on an off as it should).

      Also if the pipe from the collector to the geyser has a volume larger than the collector then the system simply wont work as discussed elsewhere in these comments.

      That’s about all I can think of.

      Try getting up into the roof on a hot day, and see if you can feel whether the system is cycling properly. You should feel the hot water pipe feeding the geyser get hot each time hot water is cycled from the collector into the geyser. If that is not happening, then try work out why.

      Apart from that I can’t really help further.

  7. will the direct system work. no pump or electronics. water from cold supply into solar collector panel and out to geyser inlet. will water in the panel be hot when enters geyser and geyser thermostat will not switch on element as it is receiving hot water.

  8. Good day. I read your article and found it very interesting. As I am not technically orientated, my question to you is about the volume of the hot pipe between collector and geyser vs volume of collector. Please explain the science behind this.

    Thanking you in advance


    1. Hi Johan

      The reason this is crucial is that the system works on a repetitive cycle which I’ll repeat below for clarity:

      Cold water from the bottom of the geyser is pumped into the solar collector and then the pump switches off when the controller detects the cold water in the solar collector.

      The sun heats that water until its temp is higher than the geyser water temp.

      When the electronic controller detects that the solar collector water temp is higher than the geyser, it switches on the pump again.

      More cold water is pumped out of the bottom of the geyser while the hot water in the collector flows out of the collector into the pipe leading to the top of the geyser.

      This cycle repeats as long as the sun is capable of heating the water.

      Now if the volume of the pipe leading from the collector to the geyser is greater than the volume of the collector, the hot water will never reach the geyser, because the pump will switch off before then (as the controller detects that collector temp drop and thus switches off the pump). So all that will happen is the hot water in the pipe will cool down while it waits for the cycle to repeat (even with pipe insulation the water in that pipe cools quite fast because its stretched out over a long distance with lots of surface area for cooling). So next time the cycle repeats, that now cold water in the pipe will flow into the top of the geyser, while the water in the pipe will be replaced with hot water, which will cool again, as the cycle repeats.

      So even though your collector will be heating water, it won’t reach the geyser.

      This is why its absolutely crucial that the volume of that pipe is as small as possible.

      Hope that helps?

  9. I find your article most interesting and informative. I am a diy enthusiast and a good handyman. My question is. If I make up my own solar collector panel is there some sort of formula that I should work with in determining the size and volume of the collector. I also see that you mention using 15mm pipe work all round. Would it make a difference if I use 22mm on the cold side and then reduce to 15mm on the hot side (collector outlet to geyser). Would be using a 150lt geyser. Many thanks. Patrick. Live in the Cape Helderberg area

    1. Hi Patrick

      Sorry about the late reply, not sure why your comment got lost.

      The panel manufacturers should be able to tell you the volume of their collectors, each is made for a specific size geyser/tank.

      Yes you can use 22mm on the cold side, but you wont get any advantage, so you may as well stick to 15mm throughout, the fittings and pipework are cheaper as well.

  10. Good Evening

    I would like to connect a geyser to a solenoid valve as we are testing a solar panel.
    I am unsure of the connection procedure can anyone advise?

    Kind Regards

  11. Hey there.I have a pool solar heater is it possible to connect it to my existing 100 liter electric geyser and how.Please help.

  12. Hi There
    Very interesting and informative article indeed – Thanks !
    Can I ask what the size of the pump is that is typically used ?

    Kind Regards


    1. Its normally a little 25W dc pump specifically designed for solar systems – needs to be able to handle hot water which is very corrosive.

      GeyserWise sell their one which is compatible with their controller.

  13. Hey there .The solar thing really caught me and I’m eager to learn .I have 100 litre electric geyser and solar thermal evacuated tubes.Is it possible that you can guide me through the prosess to connect it please.I also had a 200 litre electric geyser if that is to small.If you can post me a detailed illustration I will really appreciate it.

    Many thanks.

  14. Good day. We have installed a retro-fit solar geyser. I feel the pressure valve is releasing way to many times and then we have a 2m high fountain on our roof. Wasting to much water (and now it is nearly always wet at our back door).
    Is this normal?

    1. Hi Mariane

      No it is definitely not normal.

      It sounds like the solar collector panel is too large for your geyser, or else are using vacuum tubes that are over specified for the size of the geyser? But it is difficult to be sure what exactly the problem is from your description.

      In any event the water is getting too hot and thus causing the safety valve to open. This needs to be corrected urgently because it is not a safe situation. I would suggest that you ask a reputable installer to have a look.

      1. Thank you for your advice. I will have this checked out urgently. I am so glad somebody shared my fears as I also saw the safety hazards.

  15. hi
    If the cold water feed to the solar collector comes from the main household cold water pipe and goes both to the geyser and the solar collector then how does the pump circulate water in the geyser ?will the hot water from the collector pump into the geyser in the reverse direction ?
    please explain how the re-circulation works .


    1. Hi
      The pump takes cold water out of the bottom of the geyser, and pumps it into the solar panel. Its a loop. So water flows out the bottom of the geyser into the panel, while the hot water in the panel flows simultaneously back into the top of the geyser.

      The one-way valves prevent any reverse flow when the pump is not running, so water in that loop can only flow in one direction.

        1. Basically yes, the cold inlet usually does have a special sort of nozzle to ensure the cold water flowing into the geyser does so very gently to avoid causing the cold water and hot water already in the geyser to mix. But apart from that, its just a hole – if you understand what I’m saying.

  16. One would think that the non return valve from the solar panel to the geyser is not necessary as there is already a non return valve in circuit at the pump and it would have to be a high temperature all metal type. But I have proved this is not the case as the hot water in the the geyser travels back up to the solar panel at night causing a 10 degree loss of the geyser temperature.

    1. Hi Eddie,
      I have just installed a solar water heater retrofit (kwikot vacuum tube collector) and noticed the same drop in temperature. The geyser dropped 10° in 5 hours from 5pm to 10pm with no draw-off.
      Is the non-return valve the solution to this problem?
      Any advise would be appreciated.

  17. Hey.
    I convert my old electric geyser successfully-thanks for your assistance .With the winter months ahead I decide to put a blanket around it.Very disappointed I found out that since that the water just gets mild.Please help or advice.
    Many thanks.

  18. Hi
    Thanks for all the great advice.
    Im presently installing a 20 tube collector on my 45 degree roof with a 200 liter solar tank inside the roof and higher than than the collector with the cold and hot water collector pipes rising 15 and 16 degrees respectively.
    To increase the amount of hotter and to avoid overheating Im thinking of pre feeding the solar tank with and electrical tank. When the solar tank get too hot a circ pump and non return valve circulates water from the solar tank to the pre feeding electric tank. Is this thinking ok?

    1. Hi Ken

      Its difficult providing detailed advice without seeing the installation, but here are some thoughts which may or may not be useful:

      1) From what you describe, you have the potential for a functional thermo-syphon system in which the system regulates itself without the need for any external control. But thermosyphon operation is notoriously finnicky because the flows are so gentle. For example I don’t think you should have any one-way valves in the loop which might disrupt the natural rise of hot water and sinking of cold. This point is unrelated to your question, but I thought it would be worth mentioning. If I remember correctly, it was advised that the bottom of the tank should be about 300mm above the top of of the collector and the pipes to and from should be as short and straight as possible (we were never involved in any thermosyphon systems, but this was what one of the OEM’s recommended). The colder water in the tank needs to sink naturally into the collector, while the hotter water in the collector needs to rise naturally into the tank.
      2) Vacuum tubes tend to be sometimes overly efficient in our climate, so you may have an issue with water getting too hot. But this depends a lot on specifics, ie your location, the orientation of the system etc.
      3) I really doubt whether a pre-heater of any type is necessary.
      4) The main question in my mind is will the thermosyphon mechanism work without the need for a pump or not? If it does, that would be the way to go IMO (a more simple solution with less moving parts and no external control needed – the system manages itself passively). The separate question is will the vacuum tubes be too efficient. But you can reduce the amount of tubes, in most cases, to suit the situation, so that should probably be manageable.

      As I said, this all depends heavily on the actual specifics, so I would suggest contacting your local professional installers and asking them to have a look at what you are trying to do.

  19. Hi, I need some advice/clarification. I had to replace my geyser and decided to retrofit a solar manifold to it. I have done the installationa as per the instructions from the seller. During a discussion with a representative for a quote on a solar energy system he indicated that the system is actually the wrong way around. As per the instruction from the supplier, the feed to the manifold is taken from the “hot” supply/feed to the house (12 solar pump fitted closest to here) and routed through the manifold and then pumped into from the cold “inlet” side of the geyser. I have checked various blogs about this and all seem to do it the other way around, like your explanation also. When contacting the supplier he was adamant that this is the correct way that their system should be done as they are doing it according to European standards and have had several problems doing it the other way around.
    I can send you a schematic/instruction from them to maybe better understand if you can maybe advise how to upload a scanned document?
    Your comments will be highly appreciated.

    1. Must admit this does sound strange to me. They may have their reasons, but it’s not the way I would think is correct. Without more info, it’s difficult to be definite though.

  20. Thank you for a great article.
    Please could you explain to me:
    Cold water gets drawn from the bottom of the geyser by the pump. The pump and the geyser shares the same supply line. what would prevent the pump from drawing cold water from the supply instead of the geyser?

    1. It all depends if you have a hot water tap open. If there are no hot water tap’s open then no cold water can flow into the bottom of the geyser from the supply (it has no where to go after that, if you see what I mean). So the only thing that is possible when the pump runs in that situation is that it forms a local loop, drawing water out of the bottom of the geyser which eventually makes it’s way back into the top of the geyser.

      Hope you can see what I’m trying to say.

      However, if a hot water tap is open, then cold water will flow into the geyser from the supply, and then if the pump runs at the same time the cold water supply will probably at least partially supply the pump loop as well (depending on flow rates).

      Hope that helps?

  21. 2 questions.
    i have a north south roof thus needs 2 panels. one for the morning and one for afternoon.
    can the controller handle that?

    volume of warm side vs collector.
    the distance of pipe from roof to geyser is set. how can you increase the volume of the collector?
    will a bigger collector not make room for overheating?

    1. I think this will be too complicated to get working in practice.

      You would need to design the control system to pump water from one collector at a time, so that you are not mixing cold water (from the panel which was in shade) with hot water on each cycle. At midday, this will probably cause over heating problems.

      If you made the volume of the collector big enough to compensate for the volume of the hot water feeding pipe, that would work I think.

      Overall I think the complexity would make this an impractical solution. Much easier, for example, to build a frame for the panel so that it faced north on the ridge of the roof and move the geyser closer, if necessary.

  22. Hi, great article! Answers many of my questions. A few more though:

    1. When the hot water demand is high (e.g. two showers running full blast), what prevents high volumes of cold water bypassing the geyser through the collector system, delivering lukewarm water to the household?
    2. What happens when the heat and pressure builds up too high in the collector circuit? I understand that the water can even super-heat in a very effective system. Pressure relieve valve maybe?
    3. I understand the argument behind installing a 12V pump with battery backup for power outages. What powers the controller during those times? Can it also be fed from the 12V?
    4. If the solar circuit goes very cold, e.g. during winter – what switches the element on? Would it still be switched in the conventional 60deg thermostat style?
    5. Pump switches on at a given delta T, let’s say 7deg. When does it switch off again? Where must these limits be set, and where must the conventional thermostat be set, to ensure that the solar circuit fires before the element is powered?

    Thanks! Stefan

    1. Hi Stefan

      Herewith some answers:

      1) The spring type non-return valve acts as “resistor”to flow down that path. So water flows out of the geyser normally instead.

      2) If you get the proper collector for your size geyser, especially of it is a normal flat plate collector, it’s actually quite hard to overheat the system. But of course the system MUST have a proper temperature/pressure relief valve installed (which is included in most SABS approved geysers).
      3) The Geyserwise Max has a power input for 12V DC
      4) The Geyserwise Max controls the electric element
      5) Latest versions of the Geyserwise Max can be adjusted for on/off switching of the pump

  23. Nice. What will happen in a hot day, when no hot water is used? Will the temperature in your geyser just keep increasing? I am not sure what the efficiency of this system is, but some of the commercial systems can reach vey high temperatures, which you might not want in your geyser?

  24. This article is very informative. Please can you suggest some installers who can add solar panels and convert my existing electric geysers to solar cum electric geysers. I have three 200 L geysers in my house.

  25. Hi, Do you have a diagram showing the plumbing for adding a flat plate solar water heater to an existing hot water geyser, when the panel is mounted below the geyser? That is when the system can thermosyphon without the need for a pump.

    1. Hi,
      I want to retrofit a high pressure solar water collector (vucuum tubes) to my existing 150l geyser. I already bypassed the conventional 3 kW element as I have a 5kW heatpump retrofit installed.
      Al above is done within SANS standards.

      The new collector does not have a controller, but only air release valve and circulating pump.
      By saying all above, will a 18 tube collector be overrated for a 150l geyser in terms of overheating?

      Will it be advisable to install an electronic temp controller to switch off the circulating pump when high temp is reached?

  26. Hi, firstly thank you for the informative write-up. I have recently converted my 200L geyser to a evacuated vacuum tube system (20). Geyserwise Max controller and set point is 55 degrees.
    I have noticed a ten degree drop overnight though as mentioned by some other posters – and this is after a hot day with the initial temperature at dusk around 61 degrees. My question is, is this caused by migration of hot water back up to the tube collector overnight and would an additional non-return valve as proposed by some, be the ideal solve for this?

  27. A request not a comment
    I am having trouble with a “professional” who installed the collector panel facing southeast by east as it was the easiest place. Have made a great fuss and quoted consumer protection act and so on.. Problem is that the best place, north-east (can’t easily face true north) is a good 3 metres from the geyser itself, already installed. . Is there a maximum length that the pipes to the collector must be – not “ideal” as the damage has been done – but can I oblige him to install with say 3 metres pipes?. He will doubtless try to bullshit me considering what he has done.

    1. Two reasons, to prevent any flow “backwards” through the system.

      To provide “resistance” (via the non-return spring), to stop the hot water in the geyser being bypassed, via collector, when you draw hot water from the system.

    I have an old gravity fed vertical geyser (open system).

    I assume this would be alot easier to add a solar collector. ( take cold water from the bottom of the geyser and hot water back to the top.

    Depending on the height difference Colector to geyser, the heat convection may even circulate the water ? else i can add a small pump with solar panel, when the sun is out , pump works and water gets free heating, no non return valves needed … ?

    Appreciate any advice. ?

  29. Why are two one way valves needed?

    On my existing solar heating geyser the one way valves keeps getting “gritted” up and allowing the hot water to circulate back into the cold air. We can lose 15% of heat in one night. It has been replaced once but the fault has re-occurred. We have installed a filter in our mains feed into the house. Hopefully that will sort the problem out.

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