Heat Store Story: the tale of the vented heat store tank with a solar panel, back boiler and Economy 7 immersion heaters

For a number of reasons our solar panel of some five years decided to stop working. I will not bore you with the details but, among a number of things, this was due to the micro bore pipes which take hot water to our immersion tank getting blocked. Along side this, our plumbing system was originally designed to handle a trickle supply of water from a spring two fields away. Because the spring was only a few feet higher than the location of the header tank the head of water in the house was probably not more than 12 inches above the highest tap in the bathroom. Running the hot water for a bath was therefore a slow job and even slower should someone want hot water downstairs.

To get the solar panel running again and avoid another blockage we would either have to install a water softener or convert to an indirect system whereby the hot water from the panel was confined to a coil and thus kept separate from the water in the immersion tank which was being drawn off for domestic hot water. The other solution was to have a heat store tank. This would have the same effect but by a different method, with the added bonus that all the hot water throughout the house would be at mains pressure and the space taken up by the header tank above the stairwell would be available for storage..

Although I had seen heat store tanks at various NEC exhibitions I had never picked up an explanatory leaflet nor found a sales rep capable of explaining how they worked. This is fairly simple once the basics are grasped. A conventional immersion tank has an electric element like a giant kettle that heats the water.. The hot water in the tank is drawn on when a tap is turned on and is replaced from below by cold water from a header tank. It is the height of that header tanks water which pushes the hot water round the hot water circuit. Unfortunately each batch of hot water as it is heated up will deposit lime scale and other minerals into the tank. Some of the chemicals will corrode the tank and other bits of the pipes. The local water might be treated by adding lime to bring the acidity down but whatever the chemistry there is always a danger that this constant process can corrode the electric element first. Once removed, we discovered that our old tank, fitted 20 years ago, was already leaking.

When the mains water eventually gets pushed through a solar panel from such a system further deposits can be left behind after the high temperatures reached in this ‘roof top boiler’ and so the risk of blockages. With direct systems such as the SolarTwin the softer the water the lower the risk of blockages. This system pumps water directly into the top of a conventional immersion tank and avoids the need for a replacement tank, anti-freeze solutions or a separately powered electric pump as a PV panel is built in for this purpose.

It would seem our local water is rather acid but has lime added to it but it probably varies. The theory goes that for a solar panel that is heating constantly replenished mains water of this type deposits will soon occur if high running temperatures are achieved. The same happens with the constantly heated new water in the tank as well.

As we found that we had no room for a water softener and we were fed up with a slow supply of hot water we opted for a heat store tank. This retains the same water all the time and will only change if it gets so hot the water expands to the point that it returns to its own nearby header tank and the water is thus replenished. Even here the amount of new water entering the tank is small. It is this water, stored in a highly insulated and extra large tank, some five feet tall or more, which heats up and cools as heat is put in and heat taken away. That transfer of heat is brought about by a coiled pipe passing through the heat store tank containing water at mains pressure. The miracle of the system is that the heat store is sufficiently hot and the coil sufficiently long to achieve an instant heating of the mains water passing through the coil which never comes into contact with the column of permanent water in the heat store. Result, hot water at mains pressure at a temperature you can select by a special valve that will mix some mains cold water into the mains pressure hot water when it exceeds the required temperature, usually about 60c but lower temperatures will be ok for hand washing and a or showers. As no cold water enters the heat store it is cooled down much slower than a conventional tank and hence is more efficient. The system makes a shower easier to install because with hot and cold water both at the same pressure all that is needed is a mixer shower valve and you have a power shower depending on what pressure of supply you choose.. Even in the bathroom you can make sure your mixer is thermostatically controlled so you won’t be trapped in a scalding shower ever again. As the meerkat sayeth, ‘shimples!’