Boiler/water pressure issues???

Inforapennyinforapound

Grateful for any useful ideas please.

I have a baxi 105he combi boiler and recently have had problems when drawing hot water(especially in the mixer shower) that the water runs cold,then hot etc.

This happens when hot water is run at the same time as cold water.

British Gas have been and the engineer cannot explain it. The boiler is working perfectly well.

He noticed that when drawing hot water all is ok, but when the cold is turned on(as in showering) the boiler switches off i.e. Stops heating the water so we end up with a cold shower!

He didnt fix the issue and suggests that it is some sort of pressure issue but couldnt expand as he is not a plumber.

Can anyone please help before i call a plumber who may bamboozle me?

Many thanks:)

Alex1983

I'd think hot water diaphragm is worn

Inforapennyinforapound

Alex1983 wrote: »

I'd think hot water diaphragm is worn

Thanks but that was the only item replaced by engineer and it wasnt that

Alex1983

When this happens does the hot water led go out? Does it happen at all the taps or just the shower?

Ruski

British Gas can't measure the standing pressure of the water supply at your house?? Cobblers - he CBA!

Sorry - hate it when people get fobbed off like this.

Russ

Inforapennyinforapound

Alex1983 wrote: »

When this happens does the hot water led go out? Does it happen at all the taps or just the shower?

Havent looked at the led, but the boiler switches off when cold water is used. It is as though the water pressure of the cold mains affects the hot water pressure in the boiler? I am not technical but certainly the problem is connected to pressure(which i dont understand).

It does affect taps also but the shower is a mixer so more noticeable. We have to turn iff the cold in the shower and the hot then starts again and then turn on the cold very gently and not too much!...if that makes sense

Alex1983

Run the hot tap and turn on cold taps and see if the boiler hw light goes out....your boiler only needs 2.5litres of water to flow through per minute for it operate the hot water.
If the led for hw stays on then your problem is with the boiler.

blackshirtuk

Had a similar issue a few years back.

Water was hot then cold then hot. The boiler was heating the water to temperature, but the water would get too hot so the boiler would cut itself off to prevent scalding, then when the temperature was cooler it would fire back up.

A short term fix was to turn down the demanded temp, so the water wouldn't get too hot to trigger the overheat sensor.

Eventually the heat exchanger had to be replaced.

southcoastrgi

Which diaphragm did he replace as there are two

Inforapennyinforapound

Alex1983 wrote: »

Run the hot tap and turn on cold taps and see if the boiler hw light goes out....your boiler only needs 2.5litres of water to flow through per minute for it operate the hot water.
If the led for hw stays on then your problem is with the boiler.

southcoastrgi wrote: »

Which diaphragm did he replace as there are two

Thanks for replies.

The hot water led light on the boiler goes out when cold water is drawn.
I have just tested it using bathroom taps. I turned on hot tap fully and boiler fired up no problem. I then turned on cold tap fully and immediately the boiler shut down. As soon as cold tap was turned off the boiler fired up again.

No doubt its a pressure problem but i am not technical so havent a clue as to why this happens and how to rectify.

I dont know which dispragm was replaced but the engineer checked all he could and stated the boiler was fine but the problem was caused by pressure. He couldnt say how to solve the issue.