There are easy guides to follow for the correct size for your house . A typical 4 bed British house can be supplied with heating and hot water by something 14 the size of a dishwasher . You would find that if too much water is being pulled from the taps , the heating will be paused and more energy sent to the hot water . A full size system 12 the size of a dishwasher would only be needed for something 6 bed plus . Heating a whole tank of water and keeping it hot is very expensive here . You will always either be wasting money heating water you aren ' t using , or you ' ll be running out of water . There ' s no middle ground . Paying to heat only the water you use is the most efficient way . Which is why most houses here with a water tank have electric showers , so the tank is mostly used just for baths etc .