We are having trouble with central heating causing pipe to bang when the boiler turns off from heating up. The pipe that is banging is the one between the boiler and the the expansion tank in the loft (the smaller of the two and the water in the tank is also much hotter than expected). When the boiler turns off the banging sounds and some water comes out of a pipe that enters overhead (not from the ball valve).
The pump that circulates the water around the system is running normally, the boiler heating up the water as required.
The boiler is for the hot-water and central heating (with expansion tanks), but the hammer is not apparent when it is heating up the hot-water.
This is probably not water hammer, but the sound of water beginning to boil after the boiler shuts off. Once the pump stops, the circulation ceases and sometimes the heat exchanger contains enough residual energy to raise the stationery water to boiling point.
On many boilers the pump has an over run period that should prevent this from happening. What model of boiler is it?
To prove the point, check the expansion pipe going in to the header tank. Is water being expelled when the noise occurs? If you turn down the boiler thermostat does it cure the fault?
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