Combi Boiler

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Hettster | 13:52 Sat 20th Jun 2015 | How it Works
2 Answers

Should a combi boilers pump start (albeit briefly) when turning on the cold tap?

I am currently having a new shower fitted that will draw water directly from the combi boiler. Since the shower has been installed the motor (pump) within combi boiler starts. I do not recall this happening prior to the shower installation.

The combi boiler is situated beside the kitchen sink, which I why I hear it start when drawing cold water

Thanks in anticipation


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This is most likely to do with fluctuations in pressure in the cold supply. Combis have a pressure sensor which can be quite sensitive. The sudden pressure drop when you release the tap, is possibly causing the combi pressure valve to operate, and try to fire up the boiler.

Commonly, this can happen when alterations are made to the plumbing system. Were there any alteration made to fit the shower?

Was there a shower there before?

If a section of pipework has been capped off as not needed, then this may contain air that has to be bled out, or mains pressure fluctuations caused by water "bouncing" off the air pocket might result.

Such a fluctuation may well cause the combi to try to start up.
Question Author
Just to say many thanks to "the builder" for such a full explanation it's appreciated.

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