Boiler needs a 'nudge' to produce heat

[AndyWhite]

I ahve a Halstead Ace High boiler.
Recently it has begun to randomly stop working.
When it happened first I decided to drain the system down and refill it.
As soon as I turned the pressure vent knob it fired up.
Thereafter whenever it does it I just 'nudge'the system by turning the vent valve.
It has begun to do this more frequently.
When in the failed state the flow switches move freely and operate but the CH pump does not spin up and the igniters do not fire.
DHW simply runs through unheated.

Any Ideas?

Many thanks

 

[ChrisR]

Sounds like the internals of the diverter valve are sticking.
If you keep letting water out of the Pressure Relief Valve, it'll start leaking so you'll have to replace it!

Check that the DHW flow switch is operating when you draw HW. That's the horozontally operated microswitch. Its box has a transparent cover but you'll probably need a small mirror to watch it operate on HW flow.

 

[AndyWhite]

The diverter valve works fine. The diaphragm was recently replaced after bursting and it moves freely.
Things have become a little clearer since I posted.

DHW still does not startup the burner despite the diverter valve moving and the switch operating correctly (tested by meter).

I can get heat by forcing the CH microswitch off and on again. The heating comes on and produces heat until the water reaches the upper temp limit when the thermal sensor shuts the burner down again. The pump continues to cycle for a while. It will not restart when the temperature drops.
DHW is achievable by turning the heating on as above.
The thermistors appears to work and shut off the flame at about 1.2Kohms, they also vary nicely with heat (sorry to get geeky but I am an electrical/electronic engineer to trade).

I am leaning towards a dodgy PCB, specifically the 3 minute anti cycle timer that is designed to prevent the system from 'chattering' when near the balance point. I think that the timer is not resetting the system after 3 minutes and therefore it only ever fires once.

Summary (because I tend to talk a lot )

1) No DHW
2) CH only by cycling the CH microswitch
3) All mechanical parts appear to be OK
4) Thermistors work OK

Sound like a dodgy PCB? At £145 plus VAT I'm not happy. I did examine the board but it has surface mount components that are too much for me to fix .

Thanks for your help

Andy

 

[ChrisR]

I've "got" a halstead finest doing much the same.
Haven't investigated yet (changed split diaphragm but apparently there's another fault) but had to call them about another boiler so asked for common causes - obviously the switch would be suspect if it had got wet (which they can if a spindle leaks) but this one hasn't.

They're going all Potterton about their PCB's - "there isn't a reliablility problem". Sure, that's why I've changed and resoldered so many then.

If you get the urge you could try inspect/solder route. At least I have the luxury of trying a new board, & just taking it out again if not the problem!

____________________
Later thought - is it still starting if you tweak the PRV? Does the primary flow switch drop out if you do that?

 

[Paul Barker]

Andy I'm not sure what is going on with your boiler, you haven't been over technical with us. Actually we know how to test thermisters in a bowl of water measuring temperature and checking off against a chart of resistance against temperature. This information is available from manufacturers. Usually however knowing the room temperature resistance is sufficient. A boiler with identical dhw and ch thermisters can be proved by swapping over, seldom would both fail at once.

Your analysis misses out chunks of tests but when we get further down your post we realise those points in the sequence have been proved anyway.

Thye more technical you want to get the better for us to help.

If you are going to just throw a pcb at it, then you do have a few options. You can buy a reconditioned unit. I know many of the advisers here are veheermently against that. If you want to try give cetltd a google search and phone for price, probably £45 plus vat plus pnp. New pcbs from Abcot UK for £85 plus vat plus delivery plus surcharge for not being corgi registered.

You shouldn't be doing this if you aren't corgi registered, but I am not your judge and I'm not going to moralise either way in your case.

Sorry I can't offer you a quick fix, I do hate it when the only option left is pcb. Most pcb's sent back as "faulty" are not.

Like Chris I fix quite a lot of pcb's sorting out dry joints, in the same way as television pcb's suffer from the environment boiler pcb's probably suffer moreso because of the added complication of water. Also like TV's many components are not sufficiently spaced for correct cooling so burn out.