Boiler firing on cold water demand.

[Seb101]

I had a new boiler fitted a few weeks ago. Since then there has been an issue where the boiler will fire and run for 30 seconds or so whenever the COLD taps are turned on (however the water still comes out cold, so it is not cross-connected).

I called the plumber back out and he verified the installation and said there is nothing wrong with it. His explanation was that there is likely a 'dead-leg' in the hot water piping full of air and when the cold taps are run, it causes enough of a compression/expansion of the air that it trips the flow sensor on the boiler. He said there is very little he can do except tear the house apart looking for it.

Since then I have removed one dead-leg, which I found by chance because I was working in that bit of ceiling and thought "where on earth does this hot pipe go?!" - turns out it went the entire width of the house and was just capped on the end.

This has not fixed the problem. Are there any other issues that can cause a boiler to run on cold demand? Or any tips on locating lost 'dead-legs'?

Or is there another way of removing trapped air from such areas?

Thanks.

 

[terryplumb]

Was your new combi put in as a direct swap for an existing combi ,or did you have another type of boiler / system other than a combi ? No easy way to find dead legs other than follow the pipework.

 

[Seb101]

Was your new combi put in as a direct swap for an existing combi ,or did you have another type of boiler / system other than a combi ? No easy way to find dead legs other than follow the pipework.

It was a straight swap, combi for combi

 

[Seb101]

So I fully purged the system again, which I realised I had not done after draining it to remove the dead-leg. It's improved it. Now it only does it, oddly, when turning the cold tap in the kitchen off suddenly. So you can run the tap as long as you like and it's fine, then as soon as you shut it off, the boiler clicks on.

I've no idea what to try next.

 

[terryplumb]

You need to find the dead leg ,one place worth looking is where washing machine is .newer ones only have a cold fill ,older ones have a hot as well. See If you have a redundant hot ,not many people would strip it out ,but just turn the isolator valve to off.

 

[charliegolf]

Is it every tap? Or just mixers in the cold position? I had the latter and had the same problem. I felt it was the accuracy of the hot/cold selector. Just a thought.

 

[Hot&Cold]

As what terry ^ mentioned,strip out any dead legs and cap them off.

Which boiler do you now have ? do you have a water meter ?

If the fault continues a mini expansion vessel/shock arrester could be fitted

 

[Seb101]

As what terry ^ mentioned,strip out any dead legs and cap them off.

Which boiler do you now have ? do you have a water meter ?

If the fault continues a mini expansion vessel/shock arrester could be fitted

It's a Viessmann Vitodens 100. Yes I have a meter.

 

[wull67]

I’ve had this before and a non return valve fitted on the cold inlet pipe work to the Combi sorted it, the boiler would fire when the toilet was flushed

 

[Seb101]

I don't think there are more dead legs. I found the hot feed for the washing machine, it still had a valve on it, so just purged it through.

Not fixed. It is consistent now that it only fires when cold is turned off, so I guess the pressure of the cold running is possibly drawing water back through the hot system, then when turned off the re-build of pressure through the hot system looks like flow to the boiler.

Should there be a check-valve on the hot output?

[EDIT] I suppose a check-valve on the hot output would be the same as one on the cold input as wull67 said.

 

[Seb101]

OK OK. Panic over.

I had forgotten to purge the shower in the spare bathroom. It's only maybe an extra 1.5 meters of pipe, but it was clearly enough to cause the problem.

 

[bernardgreen]

It's only maybe an extra 1.5 meters of pipe, but it was clearly enough to cause the problem

The real problem is that a small quantity of water flowing the wrong way through the hot water circuit in the boiler can trigger the boiler into firing up. A better type of flow switch would be the better solution.

 

[Seb101]

The real problem is that a small quantity of water flowing the wrong way through the hot water circuit in the boiler can trigger the boiler into firing up. A better type of flow switch would be the better solution.

Bit of a double-edged sword that one though, because a gripe I had with my old Worcester boiler was that it took a long time to register hot water flow, so you'd have to run the hot tap for about 5 seconds before it would light the boiler, then by the time the hot water makes it to the tap, you'd been waiting for 8-10 seconds. Doesn't sound like a lot, but it feels it when you're waiting to wash your hands.

Seems like Viessmann have got it right, provided your overall system is in a decent state.

 

[John D v2.0]

A nrv wouldn't solve the problem anyway, it might help if you're only using cold, but as soon as you draw water downstream of it (ie any hot), the next time you use cold you'll be in the same state.
The only way a nrv would solve it if if you never use any outlet downstream of it, only the dead leg is downstream.
But then you might as well just cap it off.

 

[bernardgreen]

A nrv wouldn't solve the problem anyway,

Fitted on the cold feed into the boiler an NRV would prevent water flowing backwards through the flow sensor and false triggering the boiler to fire up.