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Gravity Hot Water Pumped Heating
Gravity hot water is just that, when the clock is in the on position the boiler’s burner (Cast Iron) lights and a large volume of water is heated up. Because hot water is lighter than cold water, the hot water from boiler rises up to the cylinder and the cooler water (heavier) drops back to the boiler. If the room thermostat is calling for heat this simply switches the pump on, circulating the heat to the radiators. It is not possible to have heating without hot water. This system can be used with a Primatic cylinder or an indirect cylinder (often fitted with back boiler fires). Usually no cylinder thermostat is fitted so the only temperature control for the hot water is the boiler thermostat. This type of system can be expensive to run but extremely reliable.
Gravity H/W Wiring Diagram (without Zone valve)
Below is a method of getting more control on a Gravity system. A cylinder stat controls a motorised valve on the return from the cylinder. The valve shuts when the hot water temperature is reached and shuts off the burner on the boiler.
Wiring Diagram (with Zone valve)
Possible Problems
No Hot water - could be that the Ball cock is stuck in the closed position and the tank is empty
No heating- this is probably the pump, could just be stuck if it has not been used recently
Excessive air in the radiators- it is probably the antigravity valve that need replacing on the Heating flow
Upstairs radiators get hot when the heating is not on - the antigravity valve is stuck
this is located just above the boiler on the heating flow pipe
