Combi fires up momentarily when cold water demand

[michael.erskine]

Hi all,
Our combi boiler (Myson Midas) fires up momentarily whenever we turn a cold tap on or off unless we turn it on or off ever so gently. Other than that the whole system works perfectly. I'm assuming that it is the sudden change in pressure that is triggering this. Any ideas?

Regards,
Michael

 

[ChrisR]

Yes you're right. Might be air in a little used water pipe?

 

[chrishutt]

You're right. It's some kind of pressure wave from shutting off taps triggering hot water function. It's quite common, and harmless.

You probably have a pocket of air in the hot water pipework. This would expand and compress in relation to pressure changes in cold supply (caused by opening and shutting taps). This would allow water to flow backwards and forwards through combi, creating a momentary pressure differential around diaphragm, so operating valves and switches leading to ignition of gas.

Edit 5.49pm - ChrisR beat me to it again - that's the trouble with long winded replies. By the way, congrats ChrisR on 5000 posts. Is there some traditional way of marking this?

 

[ChrisR]

Thanks - "plumber working for nothing" award?
No seriously I learn a lot from these forums. Actually I never touch the stuff, just regurgitate other people's answers....

 

[michael.erskine]

Thanks all,
Whilst I'm reassured that it's harmless, I'd like to fix it since it's burning more gas than necessary (it goes berzerk when I use the garden hose jet nozzle!)

Any pointers where to start?

Regards,
Michael.

 

[ChrisR]

Turn all your taps on full, one at a time to get any loose air out.

Put a single check valve in the CW supply to the combi - then the water can't go backwards.

 

[michael.erskine]

Turn all your taps on full, one at a time to get any loose air out.
Put a single check valve in the CW supply to the combi - then the water can't go backwards.

Excellent I shall try both of these

Many thanks,
Michael