Powermax 185 Boiler intermittent fault

[Mark96]

Hi all,
Hope somebody out there can help me with an intermittent fault with my Powermax 185.
Note that the boiler is set to 24 hour mode for both heating and hot water.

Roughly once per day the boiler will not react to the central heating or hot water falling below temperature.
The central heating will just go cold.
When you turn the roomstat you can hear it click but you cannot hear the pump start.
When you run the hot water (or more annoyingly take a shower) the water will run cold.
Basically the burner is not coming on when demanded.
To clear this fault the boiler has to be switched off then back on again, the burner then lights and all will be fine until it happens again.

Note that the boiler is set to 24 hour mode for both heating and hot water.
I have checked that the hot water l.e.d. and central heating l.e.d. are lit when the boiler is in the fault condition. Also note that the burner is not locked out.

Any ideas ?? Thanks.

 

[Agile]

It is often a faulty PCB.

But that model is not really suitable for DIY repairs.

Tony

 

[Mark96]

Tony thanks for your reply,
I take your point about it not being a diy job.
I should mention that i am an electrical/instrument tech for kelloggs so i do work on industrial gas burners for ovens, but of course i am not qualified to work on domestic gas boilers, but if it is an electrical component or pcb then i would feel confident to replace it.

 

[andytw]

Tony thanks for your reply,
I take your point about it not being a diy job.
I should mention that i am an electrical/instrument tech for kelloggs so i do work on industrial gas burners for ovens, but of course i am not qualified to work on domestic gas boilers, but if it is an electrical component or pcb then i would feel confident to replace it.

There is a wiring diagram available and it does sound as though you could deal with that. If you have the programmer set for 24 hours on CH and Hot water then, when it fails, I would first check that you have mains on tag block connections 10 and 14. It is possible that it is the programmer and this check will rule that out.

It would be best just to persue the hot water to start with as the central heating pump will not spin up unless the store is up to temperature as there is a cold start stat in the way.
So..Programmer "DHW On" puts mains on tag block connection 14. From 14 it goes to the boiler stat, from the boiler stat it goes to the limit stat and from the limit stat it goes to the control PCB on pin 8 (that is pin 8 of the PCB not on the tag block).

andytw

 

[Mark96]

Andy thanks for the info, this is exactly the info I was looking for.

I understand what you mean about concentrating on the hot water and ignoring the central heating. I will follow up on your info and wait for the fault to happen again.

Thanks a million and i'll keep you posted.

 

[Mark96]

Just an update for anybody who may have this problem in the future.
I ended up replacing the board with a brand new one and it DID NOT cure the fault.
The fault turned out to be the combustion fan pressure switch which was intermittently not switching over when the combustion fan was starting. The combustion fan pressure switch is part of the interlock circuit for the ignition control PCB and if it does not detect that the fan is running then the start signal will not get to pin 8 on the PCB. The solution was simple, there is an adjustment pot on the pressure switch which i tweaked about half a turn, not had a single failure since (that was 3 months ago).

 

[muggles]

So you've adjusted the air pressure switch away from its specific setting, thereby overriding a safety device, and you think that that is ok?

 

[wannocks]

Or it could mean the correct fan pressure is not being achieved and you have a problem that does need a qualified expert. This range of boilers is well known for problems. I'd advise getting someone trained on this model to check it out.

 

[Mark96]

I think half a turn hardly constitutes overriding a safety device. the interlock circuit is designed so that the pressure switch has to switch from "off" to "on" when the combustion fan starts. And yes i have had a qualified person who checked the differential pressure to the switch with a manometer.

 

[Dan Robinson]

Jesus.

 

[DP]

I think half a turn hardly constitutes overriding a safety device. the interlock circuit is designed so that the pressure switch has to switch from "off" to "on" when the combustion fan starts. And yes i have had a qualified person who checked the differential pressure to the switch with a manometer.

Could you say what the pressure is