Water going hot-cold-hot-cold. Boiler? Worcester Greenstar 30si

JustAnotherSaver

So for maybe 1-2 months now we thought our shower was on the blink. It'd be nice & hot and then just run cold for a while. Over time this 'while' has got longer & longer.

We had it serviced (the boiler) but as a foreigner. Everything was apparently all ok and within the parameters as stated in the boiler handbook.

The shower still went cold a few times after that but then it seemed to be fine for a couple week so we wondered if it was sorted ... it wasn't.

So we were looking in to getting someone out to fit a new shower until just tonight i decided to run the bath instead as i didn't fancy standing there in cold water with the occasional dash of warm.


The water came out warm, then got a bit warmer. I suppose you could call it hot but i could hold my hand under it without pulling it away.
Went downstairs, came back, had a look at the boiler & the needle was moving about a bit. It settles at 1 bar but it'd gone up and was moving around maybe 1.3-1.5 bar. Normal i figured since it was heating water, right?

Go back to the bath, stick my hand back under the water & it's cold. It may as well have been the cold tap.

Went back to the boiler, it's just a tad under 1 bar (which is where it stayed), went back to the bathroom and basically the water kept alternating between warm & cold, not hot & cold but warm & cold. I'd turned the water dial up on the boiler but it made zero difference.

When i've ran the tap downstairs to do the washing up i don't really remember noticing this hot-cold-hot-cold problem. Not to say it hasn't happened. I'll have to stick my hand under there i guess.




We bought the house in Nov 2013, had paperwork to say the boiler was installed in 2012.
We moved in in March 2015. Boiler serviced late that year & then again a month or so ago - both same guy, both foreigners. To clarify i mean off the books, not a non-British guy.


Looking for advice on what the likely issue & fix is.

Alex1983

I’d run you bath and then watch the boiler, see if the flame light comes on and off while the tap is running. The flame should be on for the whole time the bath is running.

JustAnotherSaver

Thanks i'll take a look tomorrow.

I've just noticed that it's actually a Greenstar 30Si not a 25si. That's what is starred up & underlined on the manual. All this time i've called it a 25si.

I used google images though & most of the pictures don't match my boiler. They're similar but not a perfect match.

This one is though...

JustAnotherSaver

Wife insisted on doing the test tonight. So....

So the boiler was reading at 1bar. Turn the hot tap on. It actually gets quite hot. The flame light lights up green but the pressure needle drops to just at the top of the red zone.

As i had my hand under the running water after 30-60secs my wife said the flame light had gone out, yet it was still warm (but only for a few more seconds) ... it then turned cold.

As it turned cold she said the boiler kept making a noise as though it was trying to fire up but couldn't & would make some clicking noises. She said the pressure needle would start to bounce between around halfway in the red up to 1bar pressure.

So she ran the hot tap and i stayed there watching it. It wouldn't even light up. Kept making some odd noise like a sort of surge noise on-off-on-off. The pressure bar was under 1. More clicking noises.

Then when you turn the hot tap off & the boiler settles ... the pressure settles back at 1 bar.

Alex1983

But it didn’t go to a lockout ( flashing blue lights )? If it’s not go to a lockout it’s a bit strange as after 5 attempts at lightning it would go to a lockout, it could be a restriction in the plate heat exchanger as this wouldn’t give a lockout but the boiler would run for a bit, green burner led and then go out for a short period and then relight again and keep doing that for the whole time you have the tap running. How long did you leave the tap running for? Did the boiler not relight after the flame went out at all?

jrrowleyws

Had a similar issue with our potterton boiler and a new plate heat exchanger fixed it (it was blocked boiler side -I've since replaced the most rusty radiator and everything seems cleaner and the magnetic filter isn't picking up as much stuff).

Cost an hours labour plus the heat exchanger for the replacement.

RedFraggle

It could still be under warranty. Worth calling to check.

rob7475

Get the pressure relief valve (PRV) checked.

JustAnotherSaver

We only ran it for 1 cycle of it being cold (in that it starts out warm, went to cold, came a tad warm again & then we turned the tap off) so a couple of minutes at the most.

If it helps any then I can video it tonight and upload so you can see & hear what's going on?

Problem is getting a plumber who can do the job. A couple years ago we called a couple guys to service the boiler.

Oh we don't do that
We can't do that

That's before I found a guy who said he could (the guy who did it after hours).

rob7475

call worcester bosch - if they won't cover it under warranty, they'll offer a fixed price repair

gingernut352000

Sounds like secondary heat exchanger issue