Gas combi boiler - intermittent hot water

[Cactussed]

Guys and girls

Know little of this, I need some advice.
The combi boiler runs the heating just fine, so it obviously works.
It also runs the hot water as well. But here's the problem.

Sometimes when running the hot water, it lights up, then turns off, then re-ignites.
This is particularly prevalent when in the shower, which now runs hot/cold/hot/cold.

I am thinking that it is either a bad thermostat in the boiler or a sticky valve / widget which tells the boiler when the hot water tap is running (as the water ignition also seems related to the flow of water).

Does this make sense? Any thoughts or recommendations?

Andrew

[dirkgently]

A wild stab in the dark:- a scaled up dhw heat exchanger

[Cactussed]

In English? (but thank you for the response)

[louiebaby]

I downloaded the instuction manual of ours from the manufacturer, and went through their FAQ at the back. It won't solve everything, but it's a good place to start.

Ours was down to low water levels in the system. I dunno how the thing works, but I do what it says every couple of months, and it seems to be fine.

I'm not an expert. Don't ask me anything technical.

[Cactussed]

Hmmm. Read the instructions. I might actually try that this once!

[dirkgently]

Cactussed said:

In English? (but thank you for the response)

The plate heat exchanger has scale from the heating system in it. This blocks the flow of water from the main heat exchanger, the heat from the burner cannot be transferred to the domestic hot water so the ntc shuts the burner down.

[Ferg]

Unsurprisingly, I'd agree with Dirk.

[Cactussed]

OK, done some reading on how these things work so now understand what you're saying (I think). Looks like the hot water heat exchanger is physically inside the boiler unit. Is this generally a fixable item or a chuck it away and start again job? Or are they model specific (some are, some aren't)...

[Ferg]

Cactussed said:

OK, done some reading on how these things work so now understand what you're saying (I think). Looks like the hot water heat exchanger is physically inside the boiler unit. Is this generally a fixable item or a chuck it away and start again job? Or are they model specific (some are, some aren't)...

Chuck it away unless you have access to ultrasonic cleaning. They are pretty much model specific.

[Cactussed]

Ultrasonic or perhaps some fairly strong de-scaler on a closed loop?
Or am I being overly simplistic?

[Ferg]

The plate heat exchanger is such a fine filter it's almost impossible, by the time this occurs, to clear it with a cleaner.

[Allanv]

We had the same issue a few weeks ago, turned out to be a perished diaphram (sp?) BG changed the part and it has been fine since. The part is made of rubber, might be an idea to have this checked out.

And yes the heating was fine just the hot water was an issue. This was on a Worcester 28CDI.

Edited by Allanv on Tuesday 23 February 20:12

[Cactussed]

Thanks for the replies, everyone.
The diaphram sounds plausible actually.
The heat exchanger build up would be a more constant problem, whereas the diaphram would seem to infor the intermittent problem.

Anyway, time I stopped contemplating and called British Gas...