We've got a baxi 105e combi which works fine (no problems with central heating and hot water heats very hot) EXCEPT, maybe a couple of times a week, it doesnt fire up when a hot tap /shower is turned on.
The worst offender is the shower and hot tap at the top of the house. Then the shower on the first floor. And then occasionally the kitchen tap.
If the showers dont work, we can usually get it firing by opening a hot water tap on the same floor. And on the occasions that that hasn't worked, or it won't fire for even the kitchen tap, then turning on the heating seems to get things started. We've had 3 plumbers look at aNd offer the following opinions...

1. heat exchanger is faulty. Then the plumber stopped returning calls so we didn't get any further!

2. second plumber said he thought the pressure in the mains wasn't enough to fire the boiler up at times of high use in the rest of the street. He said we should get a new boiler. Or fit a pump.

3. third plumber said def not the flow valve (other posts on here made me suggest the diaphram) because the pins are coming out smoothly and also that and the heat exchanger have been replaced recently (before we moved in). He didn't check if they were blocked though. He then broke one of the micro switches, and said the other one was already broken so replaced both of them. For a while, it seemed like the problem was sorted but a week later it started happening again. He then said the mains doesn't have enough pressure for this type of boiler and we should replace it. He measured the pressure at 10 lts per min (although this is flow rate i think?). He said the boiler needs more than that to work properly. i read the manual as saying is can work at 2.5lt/min so i'm confused by this. I don't know what the mains 'pressure' actually is, i could buy a gauge to measure it. Could this actually be the problem and would a pump / accumulator help?

So we're back to square one and no wiser. And reluctant to replace the boiler at massive cost when surely there's someone out there who can diagnose the problem?!

Apologies for such a long post. Any pointers would be much appreciated! We're in surrey if anyone knows a plumber who might tackle this?

We're near addlestone / byfleet. Thanks, that would be great.

Thanks, much appreciated!

Ok, thanks for checking. It's seems crazy that we can't find an rgi who is prepared to actually diagnose the problem. Looking at further forum posts, it seems fairly straightforward to replace the diaphragm so I guess I'll give it ago for starters.