Why is it not advisable to put a check valve before a storage heater?

Check valves allow water to flow only in one direction.  While it seems sensible to put a check valve before a storage heater to prevent hot water from going back out of the heater or to prevent draining of the tank, this practice is discouraged for safety reasons.  This is due to the thermal expansion of water when it is being heated inside the storage heater.  As water gets hotter, its volume and pressure increases.  For normal water heater operation, this increase is not very significant.  However, a check valve will prevent the natural expansion of water outside the water heater, thereby increasing pressure on the water heater tank.  Excessive water pressure in the water heater tank is released through the pressure relief valve. An option is to install an expansion tank with the storage heater.