Gravity Primaries/Pumped Heating Systems are associated with 'Open Vented' arrangements rather than sealed.
With this system, the indirect hot water cylinder is heated as a result of 'gravity' circulation from the boiler to the coil or annular heat exchanger within the cylinder.
Because the cylinder is 'indirect' the water in the coil or annular doesn't mix with the hot water for the taps.
Gravity primary circulation results in a much longer heat-up time for the cylinder and because there is usually no form of hot water control, this often results in dangerously high temperatures at the taps.
This arrangement is not energy efficient.
The way this system usually works is that the time clock will bring both boiler and pump into operation, but the pump will only operate if the room thermostat is 'calling'.
This means that there are times when the boiler will be 'cycling' (ie the boiler will be turning on and off under the control of it's own thermostat)
This is not energy efficient and will add to the system running costs.
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