Combi boiler cutting out while running hot water

mrochester · 31 May 2015 at 14:51

Hi all

When I run a hot tap the combi boiler fires up and heats the water. However, as the pressure drops, the boiler switches off and stops heating the water. I'm left with just cold water coming from the hot tap. I can force the boiler to start heating the water again by turning the tap on some more, but at this point I'm pouring quite a considerable amount of water down the drain.

Does anyone know why my boiler might be shutting off heating the water despite the fact that it is still running?

I should note that the heating works fine and that the boiler has sufficient pressure in the heating system (1 bar).

Many thanks

M.

Mr^B · 31 May 2015 at 15:05

Has it just started doing this? Or has it always done it?

SexyGreyFox · 31 May 2015 at 15:07

mrochester said:
I should note that the heating works fine and that the boiler has sufficient pressure in the heating system (1 bar).

Many thanks

M.

Mine has to be on at least 1.5

Teknobry · 31 May 2015 at 15:08

is that 1bar at cold?

mrochester · 31 May 2015 at 15:09

Mr^B said:
Has it just started doing this? Or has it always done it?

It started doing it quite a while ago (probably more than a year) but it's never been a problem really as we barely run any hot water. However, as I'm selling the house soon I'd like to get it sorted.

mrochester · 31 May 2015 at 15:09

Teknobry said:
is that 1bar at cold?

Yes it was 1 bar at cold.

Bear · 31 May 2015 at 15:27

Maybe a dodgy diverter valve, does the boiler heat the water ok when the heating is on?

mrochester · 31 May 2015 at 15:38

Bear said:
Maybe a dodgy diverter valve, does the boiler heat the water ok when the heating is on?

As far as I know it behaves the same way regardless of whether the heating is on or not.

RJC · 31 May 2015 at 15:43

When you say pressure are looking at system pressure - this will / should increase as the water heats up or when you close the hot tap to reduce the flow the boiler stops?

Lumeycom · 31 May 2015 at 15:45

I would say your combi is a little old and is not a modulating type so the gas burner is either on or off with no 'throttle' if you will. If you are running the water slowly then its likely that the temprature of the water is exceeding what the boiler will allow and if it cannot modulate then the only thing it can do is shut off to protect the boiler from actually boiling and you lot from scalds from very hot water.

You should be able to find out if its modulating with the help of google quite easily or pop the model in here and I'm sure someone will help you find out.

Bit of a guess really.

Oh while I think of it there is a pressure switch in the boiler too on the cold water feed side so that if there is insufficient flow it will shut the boiler off for the above reasons.

More I think of it the more it sounds like a boiler design "feature"

mrochester · 31 May 2015 at 15:52

Lumeycom said:
I would say your combi is a little old and is not a modulating type so the gas burner is either on or off with no 'throttle' if you will. If you are running the water slowly then its likely that the temprature of the water is exceeding what the boiler will allow and if it cannot modulate then the only thing it can do is shut off to protect the boiler from actually boiling and you lot from scalds from very hot water.

You should be able to find out if its modulating with the help of google quite easily or pop the model in here and I'm sure someone will help you find out.

Bit of a guess really.

Oh while I think of it there is a pressure switch in the boiler too on the cold water feed side so that if there is insufficient flow it will shut the boiler off for the above reasons.

More I think of it the more it sounds like a boiler design "feature"

It's a Baxi Combi 105HE.

Diagro · 31 May 2015 at 16:17

Mine also does this.

Mine does this with the shower on.

If you then mix the water to get a nice temp the boiler will shut off, and to get it back you need to turn the bathroom or kitchen tap on hot untill it fires up.

Bloody annoying when you have soap all over your self and have to get out to get it back lol

Diddums · 31 May 2015 at 16:36

Sounds like a faulty flow switch tbh, although Lumeycom's theory is also very plausible.

Lumeycom · 31 May 2015 at 19:59

Well it's a modulating boiler so my theory is out. However baxi's more than most combi's like blocking the plate heat exchanger.

Possibilities (all of which require a gas type)

Flow switch
Modulating fault/temp sensor (perhaps the dhw temp sensor has failed so it's tripping the overheat)
Partially blocked dhw plate exchanger (blocked boiler side)

mrochester · 31 May 2015 at 20:54

Thanks all for your help. I've got in touch with someone to come and quote to fix the problem.

Lumeycom · 31 May 2015 at 22:21

Let us know the outcome

blueboy2001 · 1 Jun 2015 at 22:02

We had a very similar problem with our Ideal boiler.

We had British Gas out 5 times. I told the first guy I thought it was the plate heat exchanger was blocked. He scoffed at me, said it wouldn't be that and if it was it was a 3 hour job to replace it. Over the next few visits they replaced the diverter valve, thermostat and the pump to no avail.

They then gave up and called Ideal out for a joint visit. Ideal man turned up, cover off, ran the hot water and stuck his hand in to feel the plate. 10 seconds later he said the plate is blocked, I've got one in the van and he had the old one out and the new one ready to go in within 10 minutes of arriving at which point the BG man turned up and looked a bit sheepish. 10 minutes later it was in and working.

mrochester · 31 Aug 2015 at 09:28

Pleased to say my boiler was eventually fixed. Turns out if was a part called a Venturi that was broken. The heat exchanger was also quite dirty so the guy cleaned that too.

FlyingFish · 31 Aug 2015 at 16:47

Dunno if it gave you the run around, but your plumber did well to spot the venturi. Most people miss that, and go around the houses before getting to the venturi :-)

We carry them on the van as stock, cheap and small, easy enough fix.

Mick