Boiler trouble again.............

[JammyDodger1]

Am waiting for a pressure relief valve fitting to my combi boiler, but last week the shower started going hot and cold every 20 seconds, and now the hot water tap is going the same. Would the relief valve cause this problem or do I have an additional problem do you think?
thanks

Am going to do a shameless bump to see if anyone can answer this as I can hear dd1 upstairs squealing as the shower changes temperature!

Probably the flow switch which is different.

While running a cold tap, run a hot tap into a bucket, time it to full, and calculate how many litres per minute of hot water you get.

Then try it again with the cold tap off. Does the heat still come and go?

Thank you PigletJohn for replying, I only have one tiny sink that has separate taps, and its too small to get a bucket under, but when I run both taps the hot takes a long time to get warm but when it does it stays warm/hot (I can just keep my finger underneath it) but when I run the hot tap by itself it goes hot and cold within 20 seconds,
many thanks.

have you got bath taps?

Hi its a lever with hot to the right, cold to the left and the water come out of one nozzle if that makes sense, cheers

so turn the bath tap to hot while you fill your bucket.

turn on some other cold tap, e.g. the basin, for test 1.

Hi sorry I am thick some times, right don that, with both taps running I get 6 litres of hot water in a minute, with just the hot tap running the 1st in warm but all the rest are cold up to litre 10 which start getting warm again,

cheers

if 10 litres per minute runs cold, it might be that (being winter) the boiler takes a while to ramp up the power. The temperature should then stabilise, as long as the flow does not change.

the going hot and cold is more typical of insufficient flow, so would be more common at your 6 litres rate, or if someone is running a cold tap or flushing a WC while you are showering. The low flow makes the boiler think no-one is running hot water, so it stops firing. An old or worn flow switch may have trouble recognising the flow.

If a shower with thermostatic control keeps violently going hot and cold, it might be that it has been fitted upside down or the thermostatic valve is not working right. But I understand you mean it happens even on ordinary individual taps.

Thanks so much, will get a flow switch and try that, you're a star!!

you really ought to find a local heating engineer you can trust to diagnose it locally. I am anxious that I might have missed something, thinking about it remotely without touching or looking at your installation.

Will do, thanks for your help and advice, brilliant as usual!