Central heating pump short overrun

16 Jan 2018
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United Kingdom
Hello All

Can someone shead some light on my central heating please.
The pump comes on for a bit after the heating goes off, here's what happens.

I have a standard boiler pumped with HW tank, with a room thermostat for the CH also a programmer
When the room thermostat calls for heat the pump starts to run the boiler kicks in at the same time and the rads get hot.
Then when the room thermostat gets to the temp it turns the boiler and pump off.
Then after about 20 seconds the pump starts again for 5 seconds then off again without the boiler lighting or the thermostat calling.

Is this pumping for 5 seconds normal.

Boiler is a glow worm fuelsaver

Thanks for looking
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If you have a multimeter it's worth checking the voltage on the Orange wire from the 3port actuator, when the thermostat is satisfied there will be no voltage on the Orange, Keep multimeter on and see if you get 240v when the pump starts again.
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With some boilers the pump overrun is operated by temperature rather than on time. If yours is like this, sometimes once the pump has stopped the circulation, there may still some residual heat left in the boiler that increases the temperature sufficiently to trigger the overrun thermostat and start it running again for a short period of time.

I suspect that the Glow Worm Heatsaver works like this based on the detail below from the installation manual showing a combined thermostat and pump overrun.


The higher the boiler thermostat is set, the more likely it will be to happen, so if yours is set high, you could try setting it a bit lower so that there will be less residual heat to dissipate and see what happens then.
Last edited:

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