Combi Boiler Stops Heating

[neil.mcbride]

Hi, Our combi boiler fires up when we put the hot tap on but after about 30 seconds it then stops again. After another 30 seconds it repeats the process. Unfortunately this means we are unable to use the shower.

A British Gas engineer came round and, just by switching the hot tap on, told us that the problem is not the boiler but the mains pressure into the house and requested to call the water board.

They stated that if the neighbours are unaffected this would be unlikely (which they are not). Also, the radiators still heat okay (although they do not require the boiler to be constantly on).

Anyone have any ideas? I don't want to resort to digging up my drive unless all other avenues have been explored.

Thanks Neil

[Mathew Newton]

The symptoms sound like a potential dodgy diverter valve (e.g. perished diapgragm).

Mathew

[neil.mcbride]

It's a Potterton - around 5-6 years old.

Although the radiators are hot, the boiler still emits a similar pattern when heating just the central heating, i.e. on for a short period, off for a short period. I may be wrong, but I think the boiler used to fire for much longer when heating the radiators before the flame going out.

Would that still point it to the diverter valve?

[neil.mcbride]

One other thing. The cold water pressure from my taps is very high, the hot water tap not so high.

[Richard Faulkner]

In message , [e-mail] writes

This sounds like bollocks, (not a surprise with some BG engineers). I have loads of combis, and almost the merest trickle from the hot tap makes them fire up and stay on. In fact... to get the water hot, the flow has to be fairly low. I use narrow bore shower heads to get a reasonable shower "pressure".

Sounds like the boiler may be cycling too fast, and I would guess that there is something in it which controls this.

[Ed Sirett]

The prime suspect is a scaled up secondary heat exchanger that won't allow enough heat to go from the boiler into the HW.

If the model is a Puma then the CH delay is normal on-off 30seconds - on. Longer than that it'll be the wax pellet actuator. However it should not fire up and down repeatedly until the radiators are hot.

[nrh]

In news: [e-mail], [e-mail] scribed:

That sounds pretty normal to me and certainly doesn't suggest an incoming cold water main pressure problem!

I tend to agree with Ed and point the finger towards the secondary Heat Exchanger which I too had to replace just recently under similar symptoms to yours. I had to buy a kit which included pipes, so it ended up costing me just a shade under £100 inc VAT as a DIY fit. However, if you can just get hold of just the exchanger itself, then it should be somewhat cheaper than that. Also, don't forget to put some sludge removing chemical such as Sentinel X400 Desludger (around £15 inc VAT per litre from a good plumbers merchant) into the system to flush the gunge out, or you will only end up blocking up the new exchanger again in short measure.

Nigel

[Neil]

The second British Gas engineer stated my water pressure was fine and that the heat exchanger would need replacing due to limescale build up.

Before I get the quote, can anyone provide any insight as to what a reasonable quote would be for parts and labour? Previous quotes from British Gas have been halved by me obtaining the parts myself and getting a local plumber/electrician to fit.

[Ed Sirett]

I would expect that the heat exchnager (secondary) to be around 100 and the fitting up to 150. BG will be up to double the independent prices.