Mains fed Worcester 24i pump starts when cold feed used

[Dave2k10]

Hi there,

I've just moved into a repossessed property which has been empty since Jan. In the last week we had the boiler serviced, 4 radiators added to the existing Worcester 24i (not junior) combi boiler, bathroom swapped out and a thermostatic shower added.

Boiler is upstairs on landing, mains feed comes up from the kitchen, thru loft down to boiler cupboard before splitting off to the downstairs main bathroom / separate basin in another room.

This morning I noticed that when the upstairs toilet was flushed, once the cistern finished refilling there was a new sound, "gurgling" for want of a better description coming from the boiler before the boiler then starts up. Sounds like the pump starts, the pilot light kicks in but then doesnt start full ignition as there is no flowing water on the hot feed, so pump runs for a few minutes before turning off. This did not happen before we had the heating sorted. After turning on other taps it seems the cold tap in kitchen and the tap by the upstairs WC do not cause this to happen, but anything connected after the pipework in the boiler cupboard makes it fire up.

To make things confusing, we still get normal operation when a hot tap is opened and it fires up as it should do.

We also found today that one of the replaced radiators has a small leak, and theire is a slight weep from the thermostatic shower. The plumber that put those in is due to fix those problems but doesn't have a clue why the boiler now does that it does, and he didnt seem overly concerned. His explanation was that the change in water pressure is causing the boiler to think hot water is needed.

Does anyone have an idea why this has started now? The boiler is quite old so might just be that after being put thru its paces something has failed, but be nice to get a 2nd opinion.

Cheers,
Dave

 

[rojaca]

not sure what worcester use. they call it the dhw turbine i think which acts as the dhw flow switch. I would imagine your getting a little water hammer when the cistern finishes filling which may be enough to activate the turbine. would either try restricting flow of water to your toilet by shutting down ball valve, fit a flow restrictor or maybe just a single check valve on this leg.

 

[Dave2k10]

I think you might be right about the "water hammer" effect. Did a test with the downstairs tap and closed it slowly from fully open (so pressure didnt suddenly build up) and the boiler didnt do anything. Then did the same and closed it quickly and it fired up.

Will see if a boiler engineer can see if a sensor or something in the boiler is acting up, otherwise might have to look into getting a check valve put in.

Cheers,
Dave