Heating triggering after running the cold tap

[Glastoun]

For a few days, running the hot water tap in the kitchen wouldn't trigger the boiler, so we were only getting cold water. Before a plumber could check it out it corrected itself.

Now when the *cold* tap is run, as soon as it's switched off the *heating* triggers, the gas briefly tries to ignite and the heating pump runs for a few seconds, as if the heating has been switched on and immediately off again.

The boiler is a Vokera Compact 24.

Over the phone (he's serviced the boiler before) the plumber mentioned that a certain part was probably oversensitive and was detecting a demand when there actually wasn't one. But I don't remember which part he was talking about - can anyone help me out? Want to cost it up before getting someone out for a quote.

 

[Jackrae]

If you turn the cold tap off slowly do you still get the boiler kicking in.

It is possible that hydraulic shock when you turn off the cold tap (pressure pulses in the fluid line) is being detected as actual flow. We used to get this effect in fiscal metering systems in the oil industry.

If so then the hydraulic shock may be being sensed by a hot-water-demand flow-switch within your boiler (I presume a combi) and is starting the fire-up process of the boiler. So you might be looking at a water flow switch see section 6.10 on page 19 of :
http://cdn0.vokera.co.uk/wp-content/uploads/2009/04/compact-se-installation-and-servicing-manual.pdf

 

[gigz]

Also check for dead legs in the pipework.

 

[Glastoun]

If you turn the cold tap off slowly do you still get the boiler kicking in.

It is possible that hydraulic shock when you turn off the cold tap (pressure pulses in the fluid line) is being detected as actual flow. We used to get this effect in fiscal metering systems in the oil industry.

If so then the hydraulic shock may be being sensed by a hot-water-demand flow-switch within your boiler (I presume a combi) and is starting the fire-up process of the boiler. So you might be looking at a water flow switch see section 6.10 on page 19 of :
http://cdn0.vokera.co.uk/wp-content...installation-and-servicing-manual.pdf[/QUOTE]

Yes, if I turn the cold tap off as slow as I can (like, really really slowly) then the boiler doesn't react. Thinking back to last year, I could make the pipes go thunk by turning the cold tap on and off really quickly. So hydraulic shock makes sense.

But is the hydraulic shock the thing that needs fixing, or is it the sensitivity of the boiler?

Dead legs - the owners before us de-plumbed the hot water tank from the loft (there's still an active cold water tank up there, along with the decommissioned hot water tank) and we get regular thunking noises in the pipework going up to the loft. I'd always assumed it was the supply to the cold water tank, but could they have just capped off the hot tank supply in the loft and that could be causing the thunking?

Who'da thunked it.