UKC

I've got a problem with an old Glow-worm boiler (Swiftflow, but can't remember the exact model).

Had a problem 6 months ago where the DHW would only work if the heating was on. Running the hot taps with heating off meant only cold water. An engineer replaced the diverter valve and a flow switch, which then meant we got DHW on demand, rather than as a by-product of having the heating on with a leaky diverter valve.

All was fine for a few months, and now a new problem when having a shower, or just running the hot tap for more than about 5/10 minutes. Intermittently there will be a big groaning noise. Starts loud (can hear throughout the whole house) and stops after about 3-4 seconds. That's the cue that the water is going to be cold in a couple of seconds so jump out the shower! If you leave the tap/shower running (not a power shower btw) then it'll heat up again in about 30 seconds. And then groan again 5/10 minutes after. I can't be more specific with the times I'm afraid, but they do seem to be random-ish.

The noise is very loud, but unsure exactly where it's coming from. The boiler is in the attic so I can't just flit between boiler and taps too easily. I'm 80% sure it's coming from the boiler, and as the water goes cold it's definitely boiler related. It doesn't sound like a fan etc, or normal banging pipes. It's not a moaning noise, more like a very exagerated groan!

Any ideas what it might be before I start making phone calls?

In reply to NottsRich:

Could be a few things but sounds like it could be a choked plate heat exchanger. Have you had to reset the boiler at all?

In reply to Jonny S:

It cut out when the pre-pay gas meter ran out a few weeks ago. Topped up the meter and re-lit the pilot. It was making the noise before that though...

In reply to NottsRich:

I'm guessing from what you've said that it is a combi boiler? Are you sure it has a pilot?
If the plate heat exchanger is choked then the boiler could be getting up to temperature and kettling, meaning it can't get rid of the heat. This could be your noise. The boiler might be cutting off and on giving you intermittent hot water. That would be my first port of call.
Do you have a model or even better the Gas Council number? This will be on the boiler.

In reply to Jonny S:

+1
I'm not an engineer, but I had exactly those symptoms and it turned out to be a gunged up secondary heat exchanger. Took it off and flushed it, and all was well again.

In reply to NottsRich:

Have you checked the pressure relief valve, see if it's been discharging water.

Sounds like I'd be flushing the heat exchanger too.

In reply to Jonny S:
Yep, a combi boiler. It is a "Glow-worm Swiftflow 75/80" (That's what the sticker on the inside panel says, with instructions on re-lighting pilot etc). I was sure it had a pilot, until you asked. Would you not expect it to? I wish it didn't, it seems to use a lot of gas just idling!

From what you said, my understanding is this:

Running the boiler for more than a few minutes means that the heat exchanger is getting hotter and hotter. Because it's clogged up, heat isn't being transferred into the water, so it gets so hot that it cuts out. Then as the cool water runs through it, it cools down and then relights as the flow switch is calling for hot water again. And the cycle repeats. Is that right?

If so, what is causing the noise? What is kettling?

Would the same thing be caused by a low mains water pressure/flow rate?

Thanks!

Hooo, was it a simple job to do? I'm fairly handy with spanners on a car/bike, but I've not fiddled with a boiler before.

Kikoman, I'm spotting a theme here!

In reply to NottsRich:

Do you get any hot water. It may be a faulty D.H.W. NTC not modulating the gas rate, causing the over heat to cut out until the boiler cools down enough to reignite. Also unqualified D.I.Y. repairs carry heavy penalties if you mess up.

In reply to NottsRich:
Just a thought from memory I think your boiler old as it is has a cross flow main heat exchanger, no secondary hot water one. Pump stops in H.W. mode.
P.S You have a flow switch but no diverter valve

In reply to NottsRich:

I think you are after a technician. An engineer would be designing the boiler, not fixing it.

In reply to knighty:

Poor show old boy!

If a gentleman is to issue a dogmatic statement, he must, as a rule, be confident in his manner. You don't "think" you "know".

Pip pip...

In reply to NottsRich:
Cleaning the heat exchanger was the first time I'd done anything more than fiddle with a boiler, and I found it straightforward. Previous experience being rebuilding engines with only a Haynes manual.
However, it very much depends on your boiler. You absolutely must not touch anything to do with the gas supply or burner. On my boiler I could remove the HE without doing this, but this is not the case with all boilers. If Mr Moac is right and it doesn't have a secondary HE, then you can't do it - it's a job for a qualified technician. You don't need an engineer though
Get hold of a service manual and study it. If you can work out how to get the HE out without disturbing any gas bits then give it a go. If you can't work it out from there, then I don't think you should attempt the job. Bear in mind that you will need to re-pressurise the system if it's not vented. This will all be in the manual.
Once out, it's just a case of putting a hose in each hole in turn and blasting water until it comes out clean.

In reply to NottsRich:

Kettling is water boiling inside the boiler - like a kettle. It makes the horrible groaning noise. It means the water is overheating, which means that the heat is not getting out to where it should be going.

A word about plumbers though... I diagnosed the clogged HE myself, by using the manual and some internet research. Two plumbers failed to diagnose it, they fiddled around with temp sensors and suggested replacing the PCB before I got rid of them. They wouldn't listen to my suggestion, which turned out to be correct.

In reply to Hooo:


And whatever you do, don't get a British Gas 'Engineer' in.
He'll just condemn it on the spot and advise you to get them to install a new one at huge cost.
Also, whilst he's there, he'll be telling you that your electrics are faulty and that they can do you a re-wire for 6 or 7 Thousand Pounds.
They're not Engineers, Technician is stretching it a bit. They are salesmen!

In reply to NottsRich:

Sounds like you've sorted it, well done!!

In reply to NottsRich:
Never worked on it but looking at the manufacturers instructions Mr Moac is correct so it's doubtful you've had a diverter valve replaced.
If it's the DHW side of the heat exchanger then you could try and have it flushed out but the part itself is obsolete. They stopped making that boiler in 1999.... Wouldn't advise going in about it as you're going to have to remove the fan, inner case and burner which will disturb certain seals that can affect combustion. Against the law I'm afraid unless you're gas registered. Not to mention the water seals you'll have to replace... Can of worms springs to mind! Good luck.

I would pay no heed to Lusk, British Gas would hopefully give you an honest diagnosis, however there are cowboys in every organisation that slip through the net.

In reply to Jonny S:

Thanks for the advice, I'll certainly take it on board. If gas/water seals need to be broken to remove the exchanger then I'll not do it, so thanks for saving me taking it to bits to discover that.

And thanks to everyone else as well, some useful info there.