Boiler Cutting Out

[A1Dan]

I've been searching through the forum, but couldn't find any other examples of quite this problem... apologies if I missed one!

The basic problem is that the main flame in my boiler (a glow worm space saver mk2) tends to cut in and out when it should be on, for CH and/or HW.

Note that, unlike most threads I found on here, there is no problem with the pilot light, which remains lit through thick and thin. I've actually previously had a faulty thermocouple leading to the pilot light cutting out, and it ain't that this time!

I've checked the incoming voltage, and whenever the CH or HW should be calling for heat, I have 230v, so no problem with the timer, room thermostat etc.

I've also checked the voltage coming out of the boiler stat, and when the boiler isn't firing, it is zero. So, correct me if I'm wrong, but I think it can be narrowed down to one of three things:

1. Boiler stat is knackered
2. Thermocouple into boiler stat is knackered
3. Boiler is over-heating

I'm hoping I can rule out no3, as it would presumably come on again after a few minutes once cooled down, and it does not consistently do this - it will sometimes fail to fire after not having been on for several minutes. However, if there's anyway to confirm this, that would be great.

So that leaves 1 and 2. Now, sometimes when the boiler is failing to fire, if I turn up the level on the stat, it will kick in, but again, this isn't always the case. So not sure if that helps much!
I'm not actually clear on whether you can buy and fit the thermocouple separate to the whole stat - it certainly doesn't disconnect obviously, like the one for the pilot light on the gas valve does.
I guess if you can't separate them, then it's a moot point which is faulty, but if you can, the thermocouple will only be 1/10 of the price of the stat (which looks to be £50 or so).

So, any advice... does my understanding of what might be wrong make sense, and what options are there for fixing?

 

[swbjackson]

Your only safe and legal option is to get a gas safe registered engineer in to look at the problem.

 

[A1Dan]

Hi Mike,

I appreciate that, and certainly wouldn't get involved in trying to service the boiler myself. But I'm keen to understand where the issue is likely to lie, as in my experience CH engineers can spend a lot of time faffing around and not addressing the right issue. So if I can talk to whoever I get in while already in full possession of the facts this can only help.

See, what I don't want is some guy coming round and telling me the stat needs replacing, paying for it etc, only to find that the problem is with clogged-up water pipes making the boiler overheat etc.

I kind-of presume that's what this forum is for?

Thanks

Aidan

 

[algas]

Have you checked the pump?

 

[45yearsagasman]

From what I read, you have no understanding of the boiler and its component parts. (Thermocouple into boiler stat?) for example.
Such a simple boiler, so get it looked at by a pro and trust them.

 

[ianmcd]

thats not a thermocouple coming out of the boiler stat its a liquid filled Phial that senses the temp of the boiler and is part of the boiler stat and cant be bought seperately

 

[A1Dan]

I wouldn't say no understanding, but it's limited to what I've learned from forums such as this. Thanks for the clarification on the liquid filled temperature probe - right function, wrong technology!

As I say, I will be getting somebody out to look at it, just need to make sure it's not the same people as needlessly ripped the timer unit out of the wall, taking half the plasterwork with it, last time I had any problems.

I did check the pump, and it's certainly whirring / vibrating when CH or HW is calling - not sure if there's any way to be sure if it is succeeding in pumping adequately though?

I did notice that after several minutes of the boiler failing to fire, and the pump going, the boiler was still very hot to the touch... I'd have thought that if water was circulating adequately, with no flame, that would cool the boiler off relatively quickly. So maybe it is an issue with water flow, meaning the stat is cutting out correctly?

 

[gaspro1981]

Dan you can't just make assumption's mate
You need to check the operation of the boiler thermostat using calibrated differential themometers
No voltage out of boiler stat = stat satisfied
Slight deviance in the accuracy of the stat = shorter or longer recycling times
Boiler over heating = excessive hot water or htg temperature's
There's may not even be a fault or issue with the boiler and that it is likely to be related to something else
Get a recommendation from a friend for a decent heating engineer to take a look at it for you

 

[Dan Robinson]

Don't worry - the OP has a few years to get himself acquainted with all the facts. That way us robbing barsteward CH engineers can waste our time elsewhere not addressing other people's problems.


Getting really fed up with this kind of assumption on the part of homeowners.


You obviously need to replace your thermostat just prior to getting a full Powerflush. There you go.. no faffing at all.

 

[A1Dan]

I suppose the whole concept of a DIY forum with an entire Plumbing and central heating section was always going to get some people's knickers in a twist.
I'll remember not to bother taking an interest in future.

 

[stem]

The basic problem is that the main flame in my boiler (a glow worm space saver mk2) tends to cut in and out when it should be on, for CH and/or HW.

Am I missing something here? Isn't that how it's supposed to work? The boiler heats the water up to the temperature set on the boiler thermostat at which point the burner is extinguished. Then when that water is used and the boiler cools, it comes back on again. If the burner fired continually whenever the heating or hot water were calling for heat it would boil the water pretty quickly.

If the load on the boiler is light, ie a few radiators, the TRV's closing down, or if you have gravity hot water, a minute or two off is normal. However, thermostats do get less accurate as they get older. So it may switch off at 60 degrees and not switch back on until it drops to 50 degrees. You can buy a cheap strap on thermometer for a few £ from a DIY shed, fasten it on to the flow pipe from the boiler and see what happens. If the boiler maintains the set temperature, you will know that it's doing its job.

 

[DP]

I suppose the whole concept of a DIY forum with an entire Plumbing and central heating section was always going to get some people's knickers in a twist.
I'll remember not to bother taking an interest in future.

Plumbing dear fellow. NOT gas. To stereotype every person that looks at your boiler as rip off merchant is a bitter pill for hard working guys offering help here. Me thinks you have been watching too much telly. When did you last see a TV program that showed a hard working heating repair person that came away smelling of roses?

 

[A1Dan]

Sadly, it's bitter experience, not TV. As touched on in my last post, last time I had an issue with the central heating a few years back, the guy who came ronud confidently told me it was the PCB in the timer, destroyed a huge area of plastering around the timer getting it off the wall, and replaced it to no effect.
After a little while googling how y-plan systems work, it took me a few mintues with a multi-meter to work out that the problem was simply down to a loose connection inside the junction box.

I don't mean to imply that all repair men are incompetent at all, just that I am wary after coming across one such example (also had a pretty awful experience with an emergency plumber and a leaking water pipe, but that's another story).

I'm quite sure the majority of professionals would correctly and efficiently sort out the problem. But just as when there is something wrong with my car I like to read up on the details (in forums etc), and discuss with my mechanic, so that I can have a full understanding of what is being done (for future reference if nothing else), I'd like to understand what the likely issues are here.

Basically, I've got an engineering background (though I don't profess to have any gas or plumbing expertise!) and I'm always keen to get stuck into a bit of DIY. Where a pro is required don't like being one of those "oh I don't have a clue about this, whatever you say" type people, I want to learn what's happening.

As I say - I kind of presumed that's what this forum is for!

 

[A1Dan]

The basic problem is that the main flame in my boiler (a glow worm space saver mk2) tends to cut in and out when it should be on, for CH and/or HW.

Am I missing something here? Isn't that how it's supposed to work? The boiler heats the water up to the temperature set on the boiler thermostat at which point the burner is extinguished. Then when that water is used and the boiler cools, it comes back on again. If the burner fired continually whenever the heating or hot water were calling for heat it would boil the water pretty quickly.

Sorry, maybe didn't go into enough detail there... The end result is that the HW and/or CH are never satisfied, so something is going wrong somewhere.
The degree to which this happens / periods for which the boiler flames vary, but any radiators past the first one or two in the loop very rarely get warm, despite the room stat not being satisfied, and whether we have a tank of hot water after it's meant to have been on for an hour or more is a very hti and miss affair.

As I say, thinking about it, I suspect the problem is with the water circulation. Seems very likely that, as you say, the boiler is satisfying it's own temperature requirements, but that heat isn't being shifted up to the water tank / radiators - great if you want a hot boiler, but not so useful otherwise.

Anyway, will see what the pros say.