The water in your C-Warm storage heater expands as it heats up. Provided your pressurised system incorporates an accumulator tank, this expansion will be absorbed as it takes place. In the process, a small amount of hot water may flow back into the cold water line. If hot water backflow cannot be tolerated, fit a non-returm valve and a secondary accumulator tank in the cold water supply line to the heater. (See drawing below)
If your system has no accumulator tank, thermal expansion will cause the temperature and pressure relief valve to discharge a little hot water every time the stored water is heated. The valve includes a hose connector to allow this water to be drained into the bilge.