Combi - cold off, boiler fires briefly

DaftyDuck

I've got a Vaillent 937 combi boiler, fairly new (15 months). When the cold water is turned on, boiler stays quiet, but when the cold water is turned off suddenly, the boiler "thinks" about firing up. Heating pump starts, but it doesn't get as far as firing the flame... (at least I don't think it does), but there's a slight smell of gas outside after it's done this, so it thinks about igniting.

I think it used to do this very very occasionally, but it's now doing it more & more, even when a loo cistern has finished filling. Not that I know what I'm talking about, but... I suspect the cold water being turned off sends pressure along the cold pipe, bumps into the boiler, and is briefly detected as the boiler calling for hot. As it used not to do this (maybe - I can't remember clearly), I suspect something might be going wrong(er!).

Suggestions as to what? The boiler has so many advanced settings there may be one for "delay before the boiler fires up when a hot tap is turned on" - under menu 47(b)alpha# or something, but I can't find it/am not sure what I'm looking for.

(Sorry the explanation/description is poor - no sleep for 36 hours!)

Canucklehead

DaftyDuck wrote: »

I've got a Vaillent 937 combi boiler, fairly new (15 months). When the cold water is turned on, boiler stays quiet, but when the cold water is turned off suddenly, the boiler "thinks" about firing up. Heating pump starts, but it doesn't get as far as firing the flame... (at least I don't think it does), but there's a slight smell of gas outside after it's done this, so it thinks about igniting.

I think it used to do this very very occasionally, but it's now doing it more & more, even when a loo cistern has finished filling. Not that I know what I'm talking about, but... I suspect the cold water being turned off sends pressure along the cold pipe, bumps into the boiler, and is briefly detected as the boiler calling for hot. As it used not to do this (maybe - I can't remember clearly), I suspect something might be going wrong(er!).

Suggestions as to what? The boiler has so many advanced settings there may be one for "delay before the boiler fires up when a hot tap is turned on" - under menu 47(b)alpha# or something, but I can't find it/am not sure what I'm looking for.

(Sorry the explanation/description is poor - no sleep for 36 hours!)

Good morning: your Vaillant would still be under warranty...first port of call the installer.

Canucklehead

DaftyDuck

Thanks, Canucklehead. I would/will, but the installer was pretty much defeated by the boiler, didn't understand it, wasn't able to understand the installation manual. He's a good plumber, did an excellent job of getting pipes through weird gaps, and a neat solderer... but he wasn't ... um... well... not sure how to put it politely.

The boiler has been serviced since by a Vaillant engineer; no problems were found, so I'm guessing it was all plugged in the right way round. I can/will call out a plumber (I'm NOT about to dig around inside it myself - too dangerous/complex/potentially expensive), but I'm trying to find out what it may be first. Especially if I get the original chappie back who, as I said, isn't the ... um... well.... Sweet & helpful bloke, though!

macman

So was your 'plumber' even qualified to fit the Vaillant-was he GSR? I would be rather more concerned than you sound to be about an installer (sweet and helpful though he may be) who doesn't understand the boiler or even the manual! How did he manage to commission it in that case? -installing a boiler is not just about connecting up the gas and water to it. Did he provide the correct paperwork and register it?
You don't want a plumber, you need a GSR CH engineer-why don't you get the Vaillant engineer back again, assuming it was only serviced about 3 months ago? Why get someone else to look at it at all when it's still in warranty?

DaftyDuck

macman... Yes, he was fully gas-safe, and did a very careful & studied job. He's part of a medium-sized local company, well reputed. It was a full new install, complete plumbing job for 2 bathrooms as well. He's not used to Vaillant, and maybe not the brightest. As I say, it's since been serviced by a Vaillant service engineer, and no fault found with the installation. That's not my question.

I once had a similar problem with a Worcester oil boiler... my question remains - is this a fault with a common problem?

macman

I don't know, some one else may. But since it's a warranty repair, why worry?-just call Vaillant or your installer in, whichever the warranty is with.

gas4you

I would say it is nothing to do with the boiler, but more likely a dead leg in the domestic HW circuit, especially if he has taken out a cylinder and just capped off the hot water pipe there.

If you cannot find a dead leg, make sure all the hot taps have been run, including any hot fill on washing machines.

Failing that have a small shock arrestor fitted.

Edit:

Try setting d88 to 1. This will make the boiler need a greater flow rate through it before it fires for HW

ziggyman99

I agree with gas4you. The dead leg (full of air) is acting as a shock arrestor and when you turn off the tap the increase in pressure is taken up by the hot water circuit. The rush of water through the boiler is sensed by the flow switch causing the boiler into it's start sequence.

DaftyDuck

Thanks, gas4you. No dead legs in the HW circuit.. (all-new, and good clean, short pipe runs!) but hot fill on WM is an interesting suggestion. I was using it on hot fill occasionally last year, but now powders all claim to clean with a cooler wash, I no longer do. I'll either fill hose, or ensure it's turned off at the connecting tap.

I'll also check out the settings you suggest. Thanks for informed answer. Appreciated! As you'll appreciate, I'd rather not call out a service engineer, only to find it's nothing to do with the boiler.

Edit: Thanks as well, ziggyman99.. If I remember, that's what I had with an oil-fired Worcester some years ago, and it's what I was trying to remember. There, they had connected the new system to the bathroom via a cut-in pipe, and there was an old dead length of pipe that went up under the bath, and was capped off...

keystone

Flow switch issues for one reason or another is my thought too.

Cheers