Combi boiler fires up for hot water but not for heating

Motorised valve not opening.

thermistors

Has someone switched the boiler to summer mode?

Thanks to everyone for the quick replies. Armed with "motorised valve" and "thermistor" as keywords I've been doing some Googling: it sems that Ravenheat boilers are particularly prone to problems with the latter.

I did think to check that, but no. If only it had been that simple!

Would it not need triggering with both at the same time?

i have one of those ravenheap's in my place, rented house so had no choice, it is slightly better than the halstead finest gold it replaced,

Latest problem was similar to yours, hot water but no heating, turned out to be the temp sensor clipped to the pipe for the central heating side (take the front cover off (screws at the top, this is just the outer cover, and not the combustion chamber cove or owt) on the right hand side there's a

what had happened was there was a very slight weep on the pipe to heat exchanger joint, never fixed due to a specially bent spanner being needed to get it fully tight, last lot tried but mangled the case a bit, but plumber said he'd seen this problem due to excess air in the system, so the auto air vent was always popping, letting a trickle of water run back down the pipe,

Anyhoo, water had been dripping down the pipe, which is the one with the sensor on it, and eventually the sensor had corroded enough to stop working,

Common problem, and most plumbing places keep them in stock, think it was about £14 for mine (heating engineer was called out by the landlord, we both worked together to find the fault, i proved it was the thermistor by putting a 100k pot between the wires, call for heat with the pot midway, she fires up, move the pot a bit, and it'll shut down, move it back, re-fire, so he arranged for me to pick up the new thermistor the next day for him, and he'd pop round to fit it (well, he really just popped round to check i had fitted it and put the outer cover back on properly)

A good point. If you're using the timer then yes, the wireless thermostat needs to be set to a triggering level or higher; but if the timer is switched off (and thereby bypassed) just raising the level on the thermostat should surely cause the boiler to fire?

But this morning I'll try playing around with various settings on both and see what happens. Thanks.

Thanks for that, and the detailed description of the fix. I'll take a look today.

Bizarrely, quite a few online accounts of similar trouble with Ravenheats report that hitting the outer case at the top right hand corner can effect a temporary repair. And as at least one disgruntled owner said, even if it doesn't work it's extremely satifying simply to thump the thing.

The fault turned out to be in the receiving unit for the wireless thermostat: the signal was getting there but it wasn't tripping the relay. As far as the boiler was concerned there was no stat connected, so the heating didn't fire up, even on the timer.

Thanks to everyone for all the thoughts and advice.

In article , Bert Coules writes

Honeywell?

Salus:-)?

Actually, Siemens.

Bert

Not sure that I would treat off as "bypassed". I am not familier with this boiler / timer, but those that I have seen usually have settings like "off", CH Once, CH Twice, and CH Continuous / allways on. To defeat the timer, you would normally set it to CH continuous.

In this instance, apparently it is. The only specific bypass and override controls are not on the boiler but on the receiver unit for the wireless thermostat. And it was this unit which proved to have failed, preventing the boiler from "seeing" the stat at all.

In article , Bert Coules writes

Tee hee, Adam & I are clearly aware of common failure modes in those makes, now we know of the Siemens one.

I didn't suggest the Honeywell fault as its failure is loss of pairing between stat and receiver so there's no indication of demand at the receiver which I think you said you had.

Yes, that's right. It confused matters at first.