If you get a bubbling, whooshing sound when the pump turns off then you may
have air trapped in the system. The pump is forcing it down the pipework
then when the pump goes off the air rises again. You sort this by bleeding
the pump - normally a plastic screw on the front which lets air and water
out when you unscrew it.
If you get the bubbling when the boiler is not firing but just after it has
stopped firing this suggests that the pump may have turned off with the
boiler and water is no longer flowing through the boiler so the remaining
heat from the firing boils the water. If so, you may have strange wiring on
the hot water side. Or may just have a set up like mine which gives very
What I think should happen is that the pump should run continuously while
your programmer has 'hot water' turned on, as long as the gate valve for the
heating coil is open. The boiler fires up then stops again depending on the
water temperature of the water being pumped through the boiler. Once the hot
water cylinder thermostat detects you are up to temperature, the gate valve
should close and cut power to the boiler. At this point some systems run the
pump for a little longer (through a bypass loop) to prevent this 'kettling'.
Others, like my old one, just turn the pump and boiler off and you can get a
bit of noise.
You won't notice this if the central heating is on because the pump is
always on pushing water round the radiators. You will only get this when you
are running hot water only.
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