Suprima APS failure when case closed and sealed - advice?

[davidr]

Hi

I recently returned from a holiday to find my Potterton Surprima 80L would not light (turned off water and main power whilst away). The boiler had been fine for 2 years and has been serviced so I was somewhat surprised. Spark is ok.

Suspecting the usual dry joints, I replaced the generation 2 PCB for the latest version with the full new harness. Replaced therm and overheat sensors at same time since they're cheap. Same problem though.

Next, I find the improved diags from the multi-coloured LED predicts a failed gas valve, so Corgi engineer diagnoses and replaces this and gets the boiler to light.

However, PCB now reports an Air Pressure Switch failure every 90 to 120 seconds (a rapid flashing red - not a lockout though). Boiler fires ok, runs for a couple of minutes then stops with APS problem. It repeats this sequence until no further calls for heat. No reset needed - this is not a normal lockout situation.

Tested APS and seems to be working ok, very little suction needed to make a contact.

Now - for the interesting part:

Leaving the boiler case door slightly adjar solves the problem - once the door seal is broken the thing works perfectly. Suspecting the APS, I temporarily disconnected the vent (red) pipe from the APS to the case internals and all works fine.

So, not wishing to live with an open boiler case and all that implies in terms of potential for CO to enter the kitchen, is the likley real root cause:

1. Weak vacuum from fan (which is working fine, as is the integrity of the pipe to the APS)
2. Weak diaphram in APS - replace?
3. Lack of Air venting into case causing vacuum to build up in case when sealed?
4. Some other issue I haven't considered?

Before I call out my Corgi man, I'd appreciate your advice to help narrow down options.

How can so much go wrong with a perfect boiler swtiched off for two weeks? No floods here.

Appreciate your advice
Many Thanks

BTW I am not a gas engineer but do understand these things reasonably well and I know when I've reached my limit in terms of simple DIY.

 

[baxpoti]

No.3 sounds favourite!

Was the new gas valve set up correcttly?

 

[davidr]

No - I think he just bolted it on. I was expecting some adjustment of the min/max screws but don't think this has been done.

Gas flames are very strong (high) compared to how was before.

Thanks for the quick response

 

[baxpoti]

No - I think he just bolted it on. I was expecting some adjustment of the min/max screws but don't think this has been done.

Gas flames are very strong (high) compared to how was before.

The 80L is not range rateable, it has a fixed burner pressure of 13.9mbar, check the data plate to confirm pressure setting.

 

[davidr]

Thanks Baxpoti - you are corerct, the data plate shows 13.9 and 10.4 (upper and lower limits)?

Is the pressure likely to be too high - could this be causing my problems? I have no way of testing this

Thanks

 

[Agile]

Thats why we say you need a CORGI who has the equipment to test gas pressures.

Your installer should have checked and set the burner pressure after fitting the new gas valve. They do NOT come preset except on most pre-mix boilers.

I would say call him back at no further charge. Its a CORGI offence not to check the burner pressure after doing any work on a gas appliance.

Failing to do that test was what killed the 14 y.o. girl in Wales last year.

Could also be a partial blockage in the tubes and venturi. Particularly common after holidays as spiders find boilers nice quiet places to make webs.

Tony

 

[baxpoti]

Thanks Baxpoti - you are corerct, the data plate shows 13.9 and 10.4 (upper and lower limits)?

There is no lower limit, the gas pressure on a Suprima 80L is 13.9mbar.

Also with all the wet weather we've had (get your RGI) to check for rain water sitting in the bottom of the fan, partially blocking the tube to the aps

 

[gas4you]

I hope you mean you will call out a different corgi to the one who has not set the gas valve correctly

get him to set this as Baxpoti says and also clean out the fan venturi and aps pipes.

 

[davidr]

My CORGI man returned and confirmed that he had indeed set the gas pressure to the correct value - 13.9 mbar using a nanometer.

The boiler is in fact a Suprima 80 not an 80L as originally stated - the previous owner of the house gave me documentation for an 80L and I always assumed this to be correct.

Anyway, the 80 is range ratable and we've now set the pressure to the lower value. This has cured the rapid cycling and the strong flame but hasn't resolved the APS issue.

Looking behind the fan I can see no water or obstructions. There's no venturi on the 80 and no blockage in any air pipe to the APS or in the flue. Spoke to Potterton tech and they suspect a faulty APS. Boiler works perfectly if APS is bypassed by removing the open-ended case pipe. I'll swap the APS next and see what difference that makes.

I'm still baffled as to why a gas valve and an APS would fail whilst the thing was switched off for two weeks? I admit the PCB was probably ok.

 

[Agile]

You should test the on and off contact resistance of the APS !

Tony

 

[x-man]

if the venturi is fine then the fan is faulty

 

[ollski]

holy cow put the digital manometer in line instead of the aps and see what pressure the fan makes

 

[Agile]

The holy cow was put down last week!

Better to use a "T" piece and measure the pressure at the same time as the APS is in circuit.

Also measure the contact resistance of the APS.

Tony

 

[kevplumb]

are we talking big bucks here if you suspect it change the bloody thing
worst case yer carring a spare that works

 

[x-man]

never know an aps to fail with case on and not off