Boiler intermittent fault

[fixitflav]

I have an old Ideal SE50 which has an intermittent fault.

The burner sometimes fails to light, though the fan is running. I turn the control-stat down to minimum then back up, and it then fires, each time it's happened so far. It seems to be more likely to happen when calling for HW (it's a W-plan) though I can't see why that would make a difference.

I'm inclined to suspect dirty contacts in the control-stat. Can anybody please comment or think of any other explanation?

 

[CountryFan]

Sounds like a control fault

 

[fixitflav]

Sounds like a control fault

Within the PCB?
As a stopgap, and to see if it happens on CH, I've turned the cylinder-stat down to zero and switched the immersion on.

 

[CountryFan]

Within the PCB?
As a stopgap, and to see if it happens on CH, I've turned the cylinder-stat down to zero and switched the immersion on.

No, either motorised valve, cylinder stat or programmer

 

[fixitflav]

No, either motorised valve, cylinder stat or programmer

Thanks for the reply, but I don't think it's any of them. It's a W-plan motorised valve, no microswitches. Power is getting to the boiler or the fan wouldn't run.

 

[oliverwilliams18185]

That test told you plenty. Moving the dial makes it fire, so the stat still works somewhat. Dirty contacts make total sense for an older boiler.

Try this before spending money on parts. Kill the power. Pop the cover and check those contacts. If they look pitted or black, clean them with fine emery paper. Takes five minutes and costs zero. I fixed two old stats this way.

Check the cylinder stat, too. On a W-plan setup, hot water runs right through it. If this only happens during a hot water call, that stat might be dying. They fail without making a sound.

 

[steven lee]

I would lean the same way on the control stat, but I would check the cylinder stat first if it really is worse on hot water. On a W-plan the hot water call runs through that stat, so a tired one can drop out while the heating side still behaves. Check the connections are tight as well, with the power off, because a loose terminal can act exactly like a dirty contact and it costs nothing to rule out.

 

[Razor900]

Sounds a lot more like an aps fault to me if the fan isn't proving.

Best to get someone in that knows old boilers

 

[fixitflav]

Thanks gents for the suggestions. I read Rogerthedodgers's post in Similar threads below and realised what the problem was - the gas pressure was too low. It was only about 2.5 in wg (it's a long story) so I increased it to the design figure for this boiler model, 3.5 in wg, and that cured the problem. Reverted to HW from the boiler circuit.
Due to its age I'll probably be looking at a new boiler before too long, but at the moment it's working fine.