Central heating won't come on

Another_Level

Were all the radiators heating up Ok when the boiler was serviced ?

user could 
Up the boiler pressure to 1.5 bar,
Turn room thermostat & boiler thermostat up to max, just for testing.
Ensure all trv's are fully open.
See & record what temperature is displayed on the boiler lcd during central heating demand.

There may be a fault with the diverter valve, sometimes after several attempts to run central heating The valve frees up 

  

Bendy_House

kuepper said:

Bendy_House said:

Hi Kuepp.

What type of room thermostat do you have - make and model, please? Is there just one heating zone? Ie, only one room 'stat?

 Ok, from the standby position, what happens - if anything - when you just turn up your room stat? Is there any noise at all? Any click? Whirr? Anything? Does the pressure gauge reading change to anything (it should do, to 'temp')? Or does it remain 'deed'?

I don't think it's the thermostat (Danfoss TP5000) now as I increased the temperature setting and the boiler kicked in with a click and a motor sound (the pump?) started in the boiler. The green light on the boiler came on but only for a few seconds then went out and the temperature that it showed  - which had started increasing - dropped down again, so all I can hear now is the motor sound I mentioned. So it seems to be something inside the boiler, it was serviced in July but only used for hot water since until tonight.

Ok, that's good info - it does appear as tho' the room stat is doing its job, and the boiler is getting the message.

The most obvious suspect is as said by others - a sticking diverter valve. This is a wee motor-operated valve that diverts the boiler's water to either your CH or your DHW as required. As I understand it, once it's in one of these two positions, it stays there until told to move to the other. Since your CH has been off for a good while, then it's presumably been sitting in the DHW position only for all that time.

They do get worn and/or clogged up. Funnily enough, it's the last part that I replaced on my own boiler; I was getting a 'restricted flow' message coupled with the rapidly-rising temp since the boiler couldn't shift the heated water - the valve was most likely only partially opening.

How old is your boiler? (Mine is ~15 years...). If you know a friendly GasSafe who'd be happy to fit parts you supply (tho' with no guarantee, of course) you can get refurbished 'hydroblocs' for around £40, and I see no reason to not go that way as - once cleaned, and new rubber seals/springs/piston fitted - it should be pretty much as good as new. A new (non-oe) motor is only around £15. By biding my time, and bidding accordingly, I picked up new OE parts for around £45.

New OE parts are, of course, usually a lot more, so expect around £200 for the hydro and motor, and probably up to 2 hours labour. That gets you firmly into 'fixed price repair' territory, especially since many will also replace any other 'dodgy' parts at the same time.

knightstyle

I had a similar problem with an older Baxi boiler, it was the flow switch which was sticking.  Try giving the pipes near the boiler a sharp tap as soon as the boiler pump starts.

Bendy_House

knightstyle said:

I had a similar problem with an older Baxi boiler, it was the flow switch which was sticking.  Try giving the pipes near the boiler a sharp tap as soon as the boiler pump starts.

Isn't the flow switch controlling the DHW side?

kuepper

Norman_Castle said:

How high is your thermostat set to? Try setting it to the maximum.

It was on 20 when I noticed the problem.  I've just tried it on 30 (the max) and boiler fired up for a few minutes then stopped  for a few minutes then came on for a few seconds then stopped again and it's still stopped 15 mins later. Still making the whirring sound which I assume is the pump/motor but all the radiators never got above slightly warm. 

Beyond self help so have an engineer coming round tomorrow.

biscan25

I was going to say the CH pump, but it appears as if that is working by your description of the sound. They can get stuck and need some "persuasion" to get going again.

Section62

kuepper said:

It was on 20 when I noticed the problem.  I've just tried it on 30 (the max) and boiler fired up for a few minutes then stopped  for a few minutes then came on for a few seconds then stopped again and it's still stopped 15 mins later. Still making the whirring sound which I assume is the pump/motor but all the radiators never got above slightly warm. 

As per my post yesterday, a sticky/partially operating diverter valve after a long period of non-use would be top of my list potential causes for the symptoms you were having.

The valve could be open just enough to allow some hot flow to get to the radiators making them a little bit warm, but not enough flow to fully distribute heat input from the boiler.  Hence the boiler quickly reaches the high limit temperature and cuts out - until the circulation water in the boiler has had time to cool down... then repeat the cycle.

A faulty room thermostat (or pump) would more likely result in no heat getting to the radiators whatsoever (unless by thermosyphon).

Hopefully the engineer will be able to confirm that (or something similarly straightforward) and be able to fix it quickly at a reasonable cost. 

Bendy_House

The CH pump also runs the DHW side, so both would be non-functional if the pump were at fault. Kuepp's DHW seems to be fine.

Are you calling out a local GasSafe, Kuepp? Or a GlowWorm/Vaillant?

When mine went faulty, I called out a local guy I'd used before. When he realised the age of my boiler, he baulked. His concern was that even if he replaced the Hydrobloc, the job would disturb other parts which might then leak - he didn't want to take the risk. So he suggested a 'fixed price repair' from Homeserve. They came out, tallied up all the parts they'd have to replace alongside the hydrobloc, and declared my boiler 'beyond economic'... (And tried to charge me £95 for the privilege.)

Not sure who the best company might be. Do a Google for 'fixed price boiler repairs' and see how they come across?

biscan25

Bendy_House said:

The CH pump also runs the DHW side, so both would be non-functional if the pump were at fault. Kuepp's DHW seems to be fine.

Are you calling out a local GasSafe, Kuepp? Or a GlowWorm/Vaillant?

When mine went faulty, I called out a local guy I'd used before. When he realised the age of my boiler, he baulked. His concern was that even if he replaced the Hydrobloc, the job would disturb other parts which might then leak - he didn't want to take the risk. So he suggested a 'fixed price repair' from Homeserve. They came out, tallied up all the parts they'd have to replace alongside the hydrobloc, and declared my boiler 'beyond economic'... (And tried to charge me £95 for the privilege.)

Not sure who the best company might be. Do a Google for 'fixed price boiler repairs' and see how they come across?

So it does, apologies!! Thinking back when I moved into my house neither worked.

kuepper

Problem solved for free as he took pity on me. Simply the CH water temp setting was too low. Must have been changed by person servicing the boiler a few months back but I'd never noticed. Felt foolish but relieved.