Potterton boiler: kettling, repeatedly on shower

[jamescridland]

I've a Potterton Promax Combi HE Plus. This was fitted around six years ago by British Gas (and I have British Gas cover).

I live in North London, and have hard water.

The boiler is always reliable at producing hot water; and always reliable at producing central heating - and, with a small baby in the house and irregular sleeping patterns, we're currently running the system on 19C thermostat 24h.

However: when we have a shower, the following cycle occurs:
1. It's lovely and warm for four or five months
2. It starts getting hot then cold then hot then cold. On inspecting the boiler, it's kettling and making a dreadful noise.
3. We call British Gas out. They look at it and invariably think of a part that needs to be changed and then they go away.
4. They come back again. They change a part. They run the hot tap in the kitchen. It doesn't kettle. They go away again.
5. It still kettles on a shower. We call British Gas out again.
6. They take the heat exchanger out, and wash it with hot water, removing some hard water deposits. (They never use anything more than water).
7. It fixes the problem. Go to #1 and wait for four or five months again.

This is a bit tedious. Is the boiler at fault?

We had a bad amount of corrosion in the central heating system, since fixed; is it possible to get a new heat exchanger fitted instead of one which would seem to me to be mostly hard-water damaged and probably has some iron flecks in there?

And how do I find a reputable company in the N14 area to look at this boiler for me?

 

[joe-90]

Clean your shower head or get a new one.

Or better still, just unscrew it. If the fault disappears it is a shower head problem. If the fault continues then it is the boiler or the shower cartridge.

Is the hot tap in the bathroom good? If so could be the thermostatic shower cartridge.

 

[45yearsagasman]

I assume that you are referring to the plate heat exch that they are cleaning.
I don't think BG are permitted to use harsh cleaners as per their policies. It was certainly the case when I was there, although I did carry some special stuff in the van. Do you have a scale reducer fitted to the cold inlet to the boiler? If not, it would sure help the situation. I may also help to get the system cleaned and have a filter on the boiler return. If all else fails call Joe 90

 

[jamescridland]

@45yearsagasman I've never been recommended a scale reducer. Can I buy those separately?

And the thing they're cleaning is a long tin box thing. I'd personally like to simply fit a new one. How would I find a plumber who'll do that? Anything I should look for?

@joe-90 I unscrewed the shower head; it still kettles. The shower cartridge *could* be at fault, given that the shower itself no longer actually turns off properly (and drips quietly). It's a Dolphin bathrooms thermostatic shower - would a typical plumber have the right bits?