Potterton Netaheat boiler won't light

[stewie]

Yesterday morning we discovered we had no hot water because the boiler had failed to come on for its programmed morning cycle. Over-riding the controller has no effect. The fan relay operates, but the pilot makes no attempt to light. There are no other signs or sounds of life - just the fan running. The boiler is a Netaheat 16-22 Mk II F

I called my reliable CorgiMan but got no response. Eventually we did get someone else to inspect it, but he was not entirely helpful. Said it was an old unit (true), parts were no longer made and we'd be better off replacing it (which he could do for us of course). I wasn't comfortable with his apparently superficial diagnosis, so said I'd think about it. Shortly afterwards, my usual, reliable heating engineer returned my earlier phone call (from his holiday destination!). He had serviced the boiler last year and was adamant that parts were still available and we shouldn't do anything rash.

We could wait another 10 days for him to return, since we have an immersion heater to provide hot water, but meanwhile I would like to know if there is any diagnosis I can be getting on with. I am an electrical engineer and it seems like the problem is likely to be a controls one, rather than a plumbing issue.

I have the lower front panel off, and am equipped with a wiring diagram. Electrically, this is a fairly simple piece of kit. However, what I do not know for sure is how the electrics are supposed to function with the various plumbing components that I can see: gas valve, pressure switch, etc, and in particular what conditions must be met for the pilot to light.

I don't want to tackle anything that disturbs the integrity of the gas system. I just want to conduct a function check. What would be the recommended sequence of diagnostic tests to pin down the offending component ?

 

[gas4you]

First thing to check is the pipes from the fan to the air pressure switch are clear, then the aps.

As these are all part of the general safety devices of the boiler AND this is a positive pressure boiler, it should be left to a RGI. In your case as you have a working immersion, wait for your usual engineer to return off holiday.

 

[Agile]

When the fan is running you could look in the viewing window and see if you can see a spark at the back or even a pilot light.

It will not fix your boiler but would give us an idea of what might be wrong. The advice will still be to get your engineer.

Tony

 

[twgas]

are you sure the fan is running, they often make a noise when they are in fact seized solid. Can you put your hand by the flue outlet to feel if its blowing or not.

 

[stewie]

thanks guys. Had to take a day off from diagnosis yesterday but back on the job last night.

I can get the boiler to follow a normal start-up sequence by blowing gently in the feed tube to the pressure switch. The fan is operating apparently normally and the boiler will continue to run once started ( if the tube if refitted promptly of course) However, once the boiler has shut off, it fails to re-start again automatically on its next demand cycle.

On the face of it, it looks like a faulty pressure switch, which I'll get my usual guy to replace when he gets back from his hols and does its annual service, albeit a few months early this year. In which case, I'll stop wasting time researching replacement boilers.

However, is it really just the pressure switch that's the problem? Is there a good physical explanation as to why the switch needs human intervention to allow the boiler to start each time, but keeps the unit running fine for hours during "on" cycles?

 

[twgas]

I think there's a brass restrictor in one of the air pressure pipes, could be blocked.

 

[Agile]

Its rarely the switch on your model.

Its more likely to be dirt on the fan or in the tubes as a result of not having a comprehensive full service for several years.

It can also be leaking tube connections as they tend to harden over 20 years.

All those aspects need to be examined by a competent CORGI who is familiar with your model, only a few are.

Tony

 

[stewie]

Thanks for the tips.

The tubes are unrestricted, soft, clear & flexible - look like they've likely been replaced at some point. The boiler was last serviced in October 2007, but don't know how well.

I now know I was exagerrating a bit in my previous post when I said the boiler would run "for hours", once kickstarted with human intervention. In fact today it seems to run anything from 0.5 to 2.5 hours, before it trips out (I have the central heating thermostat cranked well above ambient temperature for these trials, so the demand is still there). The boiler can immediately be restarted by blowing in one of the pressure switch tubes.

Obviously not a tenable situation to have to keep overriding a safety device. I've now just left it switched off, although there's no evidence of any leaks from the main casing and there's plenty of flow from the flue.

I would have thought if it was the fan operating inefficiently because it is dirty, then it wouldn't generate enough differential pressure to sustain operation, once the boiler is lit..?

 

[Agile]

There is a small degree of hysterisis in the operation of the APS.

However, I must stress the seriousness of dealing with a positive case pressure boiler like yours.

You need a competent CORGI !

Tony