Potterton Promax FSB Overheating

[GraRob]

*disclaimer* before anyone mentions it, a Potterton engineer will be getting called out.

So the problem;

Occasionally the boiler was locking out, with the fault code indicating overheat. Takes around 10-15 minutes to reset the boiler.

It usually only happens once, & after reset, works fine for the rest of the night.

I noticed it was ok when DHW only was on, however, 10 minutes after switching on CH it would trip.

Had a look at the motorised valves, & the CH one seemed to be sticking slightly. Great I thought, easy fix.

I then drained & flushed the system, replaced both motorised valves (Drayton 2 port), cleaned out the Magnaclean (which didn't have much in it) filled up with Fernox inhibitor, and tried it.

Worked fine for a bit, then to my great disappointment, it tripped out on overheat, but only when CH is on. DHW is fine.

So, when funds permit, I'll be biting the bullet & getting it looked at, however, in the meantime any thoughts?

I initially thought it might be the heat exchanger blocked, but it's only when the roads are on...

Thanks in advance..

 

[GraRob]

Anyone, please?

 

[ianmcd]

you dont really give much information to go on, but if it doesnt do it with the HW and only does it with the CH I would say it is not a boiler problem , but a circulation problem in the CH circuit, unlikely to be the heat exchanger, does it do it with the CH and HW both on at the same time ?

 

[GraRob]

you dont really give much information to go on, but if it doesnt do it with the HW and only does it with the CH I would say it is not a boiler problem , but a circulation problem in the CH circuit, unlikely to be the heat exchanger, does it do it with the CH and HW both on at the same time ?

Hi, yes, if HW is on and I switch on CH it locks out in overheat in 5-10 minutes.

HW on it's own is ok.

CH on it's own works initially for 5-10 minutes then locks out. Funnily, after the initial lock out, once its reset it seems to work fine?

Also checked pump and it's fine. Water seems to be circulating through the boiler fine too.

I've tried to list all the issues, but its a weird one, hence on here.

I dont want to get a potterton engineer out (v costly) and it isnt the boiler.

Thanks

 

[ianmcd]

With your boiler the System water OH thermostat and the Fan OH thermostat are wired in series so if either operate then you will get the same sequence code, but as it works OK on HW only it doesnt sound like a boiler fault, but sorry I cant think why selecting CH would cause it to lock out on OH, are you sure it is an OH lockout code ?

 

[GraRob]

With your boiler the System water OH thermostat and the Fan OH thermostat are wired in series so if either operate then you will get the same sequence code, but as it works OK on HW only it doesnt sound like a boiler fault, but sorry I cant think why selecting CH would cause it to lock out on OH, are you sure it is an OH lockout code ?

Hi, yes the fault sequence on the 3 front LED's are
On-Off-Flash which is overheat fault according to the fault indication info panel?
Weird I know

As you've said above, the fault finding diagram say check stat for continuity, then check connectors, then check thermostat, then chamber door seal, and lastly it's the ocb swap. If its definately the boiler, I'll need to bite the bullet and pay Potterton.

I'd hate to pay the huge call out charge and it's not the boiler....

Hence trying on here.


Thanks

 

[ianmcd]

Hi, yes the fault sequence on the 3 front LED's are
On-Off-Flash which is overheat fault according to the fault indication info panel?
Weird I know

As you've said above, the fault finding diagram say check stat for continuity, then check connectors, then check thermostat, then chamber door seal, and lastly it's the ocb swap. If its definately the boiler, I'll need to bite the bullet and pay Potterton.

I'd hate to pay the huge call out charge and it's not the boiler....

Hence trying on here.


Thanks

yes that is indeed the OH code , can be a few things as already said but cant understand why it works OK on HW only, sorry I cant help any further, please come back and tell us once you get to the bottom of it

 

[GraRob]

yes that is indeed the OH code , can be a few things as already said but cant understand why it works OK on HW only, sorry I cant help any further, please come back and tell us once you get to the bottom of it

I will, and thanks for info. Much appreciated.

 

[GraRob]

So an update as promised, engineer was in today and replaced the flow and fan thermostat, and the flow thermistor.

Thought it had sorted it, however the boiler again tripped out on over-heat, He also checked wiring.

He's returning with a combustion chamber door seal kit, but I'm not sure why that would affect or influence overheat?

If this doesn't work, it looks like it may be a new PCB

 

[ianmcd]

yes the combustion cover seal can cause that, but I would expect it to do it on HW as well as CH, the PCB doesnt know what the boiler is heating, it just knows that it has a demand to come on, it cant differentiate between a HW and a CH demand

 

[GraRob]

yes the combustion cover seal can cause that

Cheers, struggling with why it could cause overheat trip Ian..

 

[ianmcd]

Cheers, struggling with why it could cause overheat trip Ian..

The OH sensor is outside the combustion chamber, the part where the flames are, if this seal is leaking extremely hot gasses can escape and cause the high limit or fan OH stat to operate, have seen that before but as said I would expect it to happen with HW or CH not just CH

 

[GraRob]

The OH sensor is outside the combustion chamber, the part where the flames are, if this seal is leaking extremely hot gasses can escape and cause the high limit or fan OH stat to operate, have seen that before but as said I would expect it to happen with HW or CH not just CH

Ah, I see, ok, thanks for explanation. Cheers

 

[GraRob]

PS; more for my own information, the CH runs consistently for around 15 minutes, before tripping on overheat. I've turned down the boiler main thermostat as well, but it doesnt make any difference.
No idea why it will run 15 or so minutes before tripping out?

 

[GraRob]

FAO Ianmcd or anyone else that may have an opinion.

In case door seal/PCB swap doesnt work & giving this more thought:

As this only happens with CH, is it feasible the pump might be defective or nor operating efficiently? I ask this as the HW is a relatively small circuit, whilst the CH has a much larger volume to pump & longer pipe runs?

The pump sounds ok and changes speed, but perhaps the efficiency is down, even on fast speed?

Clutching at straws perhaps, as I'm trying save pennies.

Thoughts?