Why Does A Deadleg Make Combi Start Up When Cold Tap Is On?

[CareerChangeChancer]

Can anyone explain to me why a deadleg in the hot or cold pipework causes a combi to start up when a cold tap is turned on?

Presumably the deadleg will be on the cold pipe to the boiler. Curious as to what happens in the pipes and boiler to cause this.

Many thanks.

 

[joe-90]

A combi has a pressure switch. Opening a tap will cause a pressure drop. Even I can work that one out. Not a gas fitter are you?

 

[CareerChangeChancer]

Yes, but opening a cold tap shouldn't affect the combi. Why does the deadleg make the combi start up?

No, Sherlock. I'm not a gas fitter.

 

[DP]

Dead leg (will be on hot water pipe run) will have air trapped in it. This air is at mains pressure. When you run a cold tap, the trapped air expands due to drop in pressure (running tap). When you close the tap, water flows to now compress the air lock which starts the boiler.

 

[ChrisR]

If you can bleed the deadleg, it'll probably stop.

 

[CareerChangeChancer]

If you can bleed the deadleg, it'll probably stop.

Sorry, should have said that its a hypothetical question. Have heard about it happening sometimes, and wanted to know on which pipe the deadleg was likely to be, and the reason for it happening. Think I understand now.

 

[mysteryman]

It is usually where a Fortic cylinder up on staging has been removed and replaced by a combi downstairs. The old hot water takeoff from the cyl is just capped off where it is, and you thus get a vertical length of 22mm pipe full of air but still connected to the hot water circuit. Opening a cold tap then causes a drop in pressure and a momentary flow out of and back into the deadleg. This can initiate a hot water call on the combi, but by the time the boiler responds, the cold tap is shut. Of course, a good professional would not leave a deadleg.

 

[onlyfitidealboilers]

if its a long leg just fit a mini expansion vessel to the cold water pipe.