PulsaCoils are thermal stores, rather than hot water cylinders. Like conventional hot water cylinders, a thermal store is a container filled with hot water but here the similarity ends. Once filled, the water in a thermal store never changes. Instead, the heat stored in it is used to heat the tap water using a heat exchanger. This allows the hot tap water to be delivered at full mains pressure, and is one of the primary benefits of installing a thermal store instead of a conventional hot water cylinder. Common problems - depleted water in the thermal store. Original PulsaCoils have a top-up cistern built into the top of them. This may or may not have a float valve connected to the mains water supply to fill it. When there is NO mains connection, there is usually provision for manual filling by the user by means of a tap on the wall nearby. Not good if you are not active! When the user doesn't realise this, water lost from the thermal store through evaporation and/or leaks can prevent the PulsaCoil from working. If the water level falls too low, the heat exchanger coil ceases to be immersed in stored hot water so when a hot tap is turned on, and the unit will not deliver hot water. The problem is progressive. As the water level reduces, so does the hot water performance. The answer is to check the water level in the top-up cistern and top it up to the waterline embossed into the wall of the cistern.