Combi boiler intermittent fault

[Wozzaaah]

Ok, we seem to have a very broad selection of tradesmen as members so what better (and friendlier) place to try and solve my problem .

I have an Ideal Mini C28 (yes I know, they're not the best as I've found out but the fitter didn't give me a choice) and every once in a blue moon the DHW circuit fails to start.
The water pressure is fine, certainly not what I'd class as low and the flow sensor lights up ok when a hw tap is turned on but the pin on the 3 way diverter diaphragm fails to extend properly.
When the CH is called, the pin always extends fully and operates the microswitch but it's just that once in maybe 50 times that it doesn't come out fully when just DHW is called and it's always that time when I'm at work and Mrs W jumps in the shower .
If i turn on the CH then turn on a hot tap it always works fine.
Oh and if I leave the tap running, turn the boiler off at the wall for a few seconds then on again it often fires up fine.
Any ideas?

 

[madkayaker]

sound like a duff micro switch or solenoid A work mate has similar when the solenoid would work fine cold but once it heated up it would go open circuit and cut out! Buggers to find intermittent faults

 

[Wozzaaah]

The microswitch is fine MK, it's the diaphragm operated pin which doesn't come out far enough to contact the switch which is the problem. Its actuated by water pressure from within the 3 way diverter but I can't ascertain whay sometimes there's not enough pressure to operate it.

 

[rusty69bus]

being stupid can you not extend the pin a bit

 

[Wozzaaah]

There is a fault Rusty, as much as I appreciate any answers, I'm trying to find out what it is, not bodge it .
As said, the pin fully extends when CH is called.

 

[rusty69bus]

in that case goto diynot.com

 

[rusty69bus]

of course if it was a carb diaphragm i woulb presume it had an air leak around the gasket or a pin hole

 

[rusty69bus]

http://www.diynot.com/pages/pl/pl034.php

 

[madkayaker]

does the system loose pressure? And what pressure are you running it at? And whats the recommended pressure?

 

[stormin]

Sounds like the diaphragm (just behind the pin) is faulty. Shouldn't be more than a fiver for a new one.

 

[Wozzaaah]

Rusty, been over there and had a look earlier, was hoping to find someone on here with a solution.
MK, system running at a steady 1.4 bar.
Like I said, very intermittent!
Here's a couple of pics which might clarify things;

The whole diverter assembly


The flowswitch pin

 

[Wozzaaah]

Sounds like the diaphragm (just behind the pin) is faulty. Shouldn't be more than a fiver for a new one.

Wouldn't that also give the same fault when CH is called?
I suppose it's one of the cheapest things to try first though, thanks .

 

[madkayaker]

is there anything making the pin stick.

 

[Wozzaaah]

No, It operates smoothly and as said, it always works fine when CH is called.

 

[rusty69bus]

ok, how about - theres nothing wrong with that part but the call for dh circuit lime scale ??

 

[M4RKE]

As you know the domestic is totally independent from the heating circuit. So is the fault down to low main water pressure into the boiler ?

 

[Wozzaaah]

I don't think so M4rke (Mark?), there seems to be plenty of pressure there but without putting a gauge on it I wouldn't know exactly how much.
It's annoying that it's so intermittent, if it was ocurring all the time it would probably be easier to diagnose.

 

[M4RKE]

Woz on the micro switch it looks like it has a metal tab that the pin strikes ? Has this been deformed slightly with use over time ? thus needing more travel for switch to make contact.?

 

[gavuk]

Can probably get you a gauge in a few days time if you have somewhwere convenient to mount,longshot, no chance the switch mountings are flexing? had that with a pressure washer the other day where the switch was moving away from the pin,and was switching itself on and off,diaphragm is a good start though.....Gav

 

[Wozzaaah]

Woz on the micro switch it looks like it has a metal tab that the pin strikes ? Has this been deformed slightly with use over time ? thus needing more travel for switch to make contact.?

Not deformed at all Mark, it's dead straight. What I can't ascertain is to why the pin fully extends as it should most of the time but every once in a blue moon it stops short of that tab so doesn't make the switch.

Gav, the switch is firmly mounted and doesn't even rock slightly so it's not that. A gauge would be handy anyway, it's always good to see what water pressure you're actually getting.