Help!! Firebird oil boiler no central heating...

[Steveasub]

Hello all,
my first post so please go easy on me...
I have a Firebird Combi 90 boiler, installed about 7 years ago.
Poor me...
Ok the latest issues are that that the central heating pump will not work, and the DHW (Domestic Hot Water?) pump will not STOP working!.
The hot water works just fine.
Here's what I know...
There is constant power to the DHW.
There is NO power supplied to the CH pump under any normal operating conditions - though I have tested it, and when it has power, it pumps water into the radiators as it should.
There is power (240v) to the room stat line in, and when the stat operates it supplies power to the line out and back into the boiler.
The central heating switch is working OK in that it supplies power when operated.
No power is supplied to the flow switch that I can find, though I'm not sure under which conditions it should have power.
When the temperature drops for either hot water supply or CH (when I power up the CH pump) the boiler fires up Ok. Also it cycles OK.

I am thinking that if it was only the DHW pump running constantly then it might be a boiler thermostat pump overrun issue, but with no power being supplied to the CH pump could it be that the relay is knackered?
Although the relay certainly operates when it is supplied with power.
Or is there a PCB hidden somewhere?

Any helpful heating engineers out there with any good ideas?

 

[Agile]

It sounds as if the DHW demand switch is jammed on.

If you disconnect it then the CH should work correctly if this is the cause.

As to why its jammed on then see if its being permanently pushed or test it to see if its failed on.

Tony

 

[Lochy]

I agree with Tony. Also this is quite a common fault on this appliance.

 

[Steveasub]

Thanks Guys.
Yes that identified the problem, in that as soon as the flow switch was disconnected the CH pump did it's thing radiators were hot in seconds.
In my OP I stupidly said that there was no power supplied to the flow switch. but that should have been power is constant to the switch line in, and constant to the switch line out. Can't even read my own notes!
Which makes you even cleverer than normally clever people...
So the questions now...

I'm assuming the mechanical part of the flow switch is the likely culprit - would that be right? Or is the microswitch the usual failure here?
Can I repair the jammed/sticky/broken mechanical switch?

Is there a bog-standard replacement available or is is a bespoke Firebird part?
I would guess the best thing is to simply replace the entire assembly - is that what you'd normally do?
Thanks for the help,
Steve

 

[Agile]

We have to take heed of the important diagnostic information and ignore that which is irrelevant or where the OP is unlikely to know how to test it properly.

We are only normal clever.

You have to be like David937 to be extra clever and post answers which look like an experienced engineer when you are only 14 ! But its hard work persuading him that he needs to take "A" levels and go on to uni.

Tony

 

[kevplumb]

We have to take heed of the important diagnostic information and ignore that which is irrelevant or where the OP is unlikely to know how to test it properly.

We are only normal clever.

You have to be like David937 to be extra clever and post answers which look like an experienced engineer when you are only 14 ! But its hard work persuading him that he needs to take "A" levels and go on to uni.

Tony

you on medication

 

[oilhead]

You are best with the Firebird part. At 7 years old, you may have the old pattern switch which is now replaced by a more standard part, but you have to modify the wiring and fit a relay, all supplied as part of the kit.