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
In 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.
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.
In 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.
You 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.
If 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:
Starting 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.