Re: Further to my last post entitled 'Flushing and treating central heating question'

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 minor 'kettling'.
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.
HTH Dave R

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