Gas boiler runs when cold tap is turned on.

[Heaveho]

Hi, as title. I'm refurbing a flat and the combi is turning itself on when the mixer tap in the kitchen ( brand new and just fitted, but was also doing it with the old identical tap prior to being replaced ) is turned to cold. Same situation with the cold tap in the bathroom. Not happening on the ensuite tap, the bath tap or with the shower turned to cold.

Plumbing is my least favourite subject, but if those in the know say it's a diy fix, I'm happy to be guided by anyone who has experience of it.

[Philvrs]

Needs a mini shock arrestor or non return fitting on the cold supply.
When the tap is turned off it bounces a small pressure wave, just enough to trip the flow switch in the boiler, I m guessing the tap that doesn t do it is the furthest from the boiler pipe wise.

Edit to add
Swapping the taps may have introduced air thats stuck in a dead leg somewhere, first try starting and stopping all taps several times to see if it moves/lessens, if not, fir an arrestor.

Edited by Philvrs on Sunday 15th March 12:56

[Heaveho]

Philvrs said:

Needs a mini shock arrestor or non return fitting on the cold supply.
When the tap is turned off it bounces a small pressure wave, just enough to trip the flow switch in the boiler, I m guessing the tap that doesn t do it is the furthest from the boiler pipe wise.

Edit to add
Swapping the taps may have introduced air thats stuck in a dead leg somewhere, first try starting and stopping all taps several times to see if it moves/lessens, if not, fir an arrestor.

Edited by Philvrs on Sunday 15th March 12:56

Thank you, it was doing it before the tap change, so I'm thinking it's unlikely to be anything recent that's caused it. I reckon it's the 2 taps closest to the boiler that are the issue, that hadn't occurred to me until you mentioned it. The ensuite and the bath taps are the furthest away and aren't a problem.

You mention that it's turning the tap off and the resultant shockwave that trips the flow switch, but as things stand the boiler starts to run while the cold tap is actually still running. I only mention that in case it changes the diagnostic process.

Thanks for the input.

[Philvrs]

Hard to diagnose without seeing it, but i’d still put money on the cold feed dropping the pressure switch in the boiler.
Other thing to try if you haven’t already is turning every cold tap on for a minute to see if any air locks come out, but if its stuck in a dead leg somewhere, then a fitting a shock arrestor is best practice.

[Heaveho]

Philvrs said:

Hard to diagnose without seeing it, but i d still put money on the cold feed dropping the pressure switch in the boiler.
Other thing to try if you haven t already is turning every cold tap on for a minute to see if any air locks come out, but if its stuck in a dead leg somewhere, then a fitting a shock arrestor is best practice.

Cheers, I misguidedly tried turning on every hot tap, thinking I was doing the right thing. I'm a genius, but I hide it well.

[Philvrs]

One other test to try
Turn boiler and cold feed stopcock off, open every cold tap and let it drain out then turn stopcock back on, slowly turn off each tap in order you think till the last on the line, hopefully any air will exit at this point, turn boiler on and cross fingers.

[Heaveho]

Philvrs said:

One other test to try
Turn boiler and cold feed stopcock off, open every cold tap and let it drain out then turn stopcock back on, slowly turn off each tap in order you think till the last on the line, hopefully any air will exit at this point, turn boiler on and cross fingers.

Great, will do, thanks for that.