Boiler starts up when cold water taps switched on

[Gammy leg]

Hi,

I had new Viessman Vitodens 100 combi boiler and central heating system ( incl rads and all pipework ) installed last December and I've recently become aware that routinely ( but not always ), whenever the cold taps are opened, the boiler kicks in and starts heating the hot water.
Is this normal, and if so, why? Seems a waste of gas if I'm not using the hot water.

If it makes a difference, the boiler is currently set on ECO which as far as I can gather means that it doesn't store a reserve of hot water for quick release.

Thanks

 

[DH6]

There will be a 'dead' leg on your hot water supply somewhere in your property. Opening and closing a cold tap quickly ( usually happens more on 1/4 turn taps ) compresses the air then releases it resulting in the boiler recognising what it thinks is a hot water demand.

To rectify remove the dead leg.

 

[John D v2.0]

As above. But the dead leg could just be a tap that's never used so check there first by trying all the hot taps.

 

[Echo the husky]

I've had this for weeks home here due to a section of unfinished pipework that is part of the cloak room alterations, I finally installed the rest of the pipework yesterday and could bleed the air out with the iso valves.

 

[Gammy leg]

Thanks guys, I do indeed have a 'dead leg' in the hot water system under the kitchen sink. It's a pipe about 2' long with a blanking cap on the end, a left- over from the original system. Never though something so simple could cause this to happen.
I'll take a look at the weekend and modify to eliminate it.

Cheers.

 

[AGAS]

Fit a mini expansion vessel on inlet to boiler if access not available to dead legs

 

[gasbanni]

The dead leg might have been put in to stop water hammer ......It was an old trick not correct now. Against the regs

 

[Gammy leg]

OK,

Finally got round to removing the dead leg in the kitchen and it made no difference. After a quick check, remembered there is another redundant section in the bathroom where the shower will be fitted once the tiling is completed.
Purged this section of air and now all is working perfectly. No unwanted boiler start-ups.

Thanks to all who gave advice.

Gammy