Boiler fires on cold tap...

[tgs166507]

About 12 months ago I had a new baxi platinum combi installed, and it would fire when the cold tap shut off - presumably down to a sensitive pressure switch or something. I installed a mini shock arrestor immediately before the cold feed to the boiler, and this seemingly cured the issue.
Now recently I've had the heating circuit drained a number of times to undertake some works, and now that it's refilled, I have the same issue where the boiler is firing when the cold tap is closed. I've checked the shock arrestor pressure, and it's still fine at c. 3.5bar.
Any ideas what else I could/should try?
Would a double check valve in the cold feed to the boiler help?
Thank you.

 

[kidgreen61]

Usually caused by a deadleg of unused hot water pipe. When you turn on the cold water, the trapped air pushes back the hot water until you turn off the tap, the hot water then recompresses the trapped air in deadleg, and boiler thinks hot tap is on and will momentarily fire.

Solution is to cut out deadleg.

 

[tgs166507]

Interesting, thank you.
I'll have a think. I don't know of any dead legs on the hot water... I can think of one on my heating, and a small one on the cold water, but I'm pretty sure there's not any (that I know about!) On the hot water.
I'll put some thought into it.
Thank you

 

[JimCrow]

About 12 months ago I had a new baxi platinum combi installed, and it would fire when the cold tap shut off - presumably down to a sensitive pressure switch or something. I installed a mini shock arrestor immediately before the cold feed to the boiler, and this seemingly cured the issue.
Now recently I've had the heating circuit drained a number of times to undertake some works, and now that it's refilled, I have the same issue where the boiler is firing when the cold tap is closed. I've checked the shock arrestor pressure, and it's still fine at c. 3.5bar.
Any ideas what else I could/should try?
Would a double check valve in the cold feed to the boiler help?
Thank you.

Yes, fit a check valve before the mini shock arrestor.