Poxy boiler - losing heat - why!

[StairsInTheNight]

Its a combi boiler- has worked ok up to now. But in last couple of weeks its being an arse.

Thank god, heating works fine. But whenever I try and run a hot tap it starts hot...then slowly gets cooler.... turn it off/turn it back on- hot again...then cools.... argh! Anyone know why this might be happening? Do I need a part or do I need a new boiler!?

I just want a hot bath!

Turn the heating off so your boiler isn't making any noise. Get somebody to turn on the hot top (with a reasonable pressure) and leave it running while you stand next to the boiler. You should hear the boiler fire up to heat the hot water (and probably light(s) come on). Listen/watch for a while- does the noise/lights continue?

NotWithoutMyMerkin- thanks- we tried that, it looks like it's working normally!

Bumping for the evening

Have you checked the water doesn't come hot again after running for a while? My hot water often runs warm (from what was in the pipes) then cold, then hot as the boiler gets working

Yup. It just loses heat. Ran a whole bath cold last night. It's obviously something wrong as it used to work fine, guess it was saving it up for winter :-(

turn off the heating at the room stat and/or timer (not at the radiator knobs). Wait for them to go cold.

Turn on the hot bath tap and leave it running. Walk round feeling the radiators. Have any of them warmed up?

Just saw this and will try tomorrow and report back. Thanks!

Ok, did this and the hot tap is running hot now. Radiators have not started warming up.

What does that mean then?

Have you checked the pressure? There will be a little red line that the arrow shouldn't go below. If it does you need to top it up. Check your manual for how to do that, it's usually just a case of turning a lever until it is back at level. Until then it's either heat or water but not both.
Even a modem year old boiler needs checking at least twice a year especially a new install.

if the radiators do'nt go warm when you run the hot tap, it is probably not the diverter valve (which can steal heat and send it to the radiators)

if running the bathtap brings hot water, but smaller flows don't, it might be the flow switch.

or you might have sludge and sediment blocking the heat exchanger, especially if you are in a hard-water area or the radiators are old.

if the radiators work it will not be the pump.

If you get any heat out of it at all, it is not under-pressure (this would stop it running) though it might be interesting to know what the pressure gauge reads, and what it moves to during and after running hot.

or it might be something I haven't thought of.

anyway, I don't see scope for a DIY adjustment, you need to ask around for a recommended local heating engineer. You might find an approved list on the maker's website, if you can't get a local recommendation. yellow pages, letterbox flyers, and recommendamender.com websites are not good sources.

Unfortunately this is the wrong time of year to look for a good engineer who is not rushed off his feet.

DP thought it might be the heat exchanger.

Thanks for that info, agree this is something we will have to get looked at properly.

I showed DP your message and he thought it was really helpful so thank you pigletJohn, really appreciate you taking the time to answer.

And everyone else who answered too! Thank you for helping with my dull boiler query!