Boiler only lights from cold.

[Mary Pegg]

Got a problematic Potterton Netaheat 10-16 boiler. It will only light when it's been off for half an hour or so. When I start it from cold, the pilot lights after about ten seconds and then the main burners come on about 40 seconds later. This is all as per the manual

However, when the thermostat cuts in (i.e. turns off the gas flow), the boiler will not relight [1] until it's been turned off (at the wall switch) for a while. Even if it's cold (i.e. hours have passed since the main burners were running) it still seems to need turning off at the wall switch for at least five minutes.

Any ideas here? The service bloke seems to think the thermostat is at fault. The neighbour's suggested the mercury vapour switch. I think she might have more Clue than the Corgi-reg'd service engineer...

[1] at this stage the pilot light is off.

[Mary Pegg]

There's a description of how these work here:

down a bit). Unless the noiler is stone cold and has been off for some time, step 5 in the ignitiong sequence never happens.

[Mary Pegg]

OK, just run another test, this time the pilot light came on for a few seconds and then went out again...

[Roger Mills]

Is it actually sparking, or isn't it even doing that?

[Mary Pegg]

It doesn't spark. I.e. no clicky noise.

Now it seems to be going out long before it gets hot. We did think that it might be overheating (due to dicky thermostat) and this is why it needed to cool down, to reset some sort of putative safety cutout.

But this is looking unlikely to me.

[Mary Pegg]

Talked more with an engineer. We think the air switch may be the culprit. This would fit with the fact that the boiler needed to be actually off for some time.

[Ed Sirett]

If things are marginal the extra warmth of the air can make the difference. Try checking and cleaning the tubes and their inlets and failing that replace the air switch (which contrary to BG are still available).

[Mary Pegg]

[snip]

I go a neck-spurt to look at it. He looked at it and fiddled with it and re-seated the relay and now it seems to work.