Myson Apollo 30/50si cutting out

[davidrobertson]

My boiler will run for about 5 minutes during which time the radiators start to heat up, then the flame cuts out and the boiler sounds as though it is still running as the flue fan is still working and warm air is blowing out of the flue (the heat exchanger is now effectively working in reverse-cooling the water that it has just heated!).
The first signs of the problem occurred a few months ago. Then the flame used to cut out after 20 to 30 minutes and the ignition would spark maybe 5 or 6 times and the boiler would fire up again for a while. It has gradually deteriorated to its present state.
The water pump was replaced 2 years ago and appears to be working OK and the radiators are bled.
I replaced the motorised valve recently but there was no resulting improvement.
Any suggestions would be welcome

 

[Slugbabydotcom]

If you run it on hi try running it on lo or vice versa and post back

Could be a knackered stat

 

[davidrobertson]

Just tried running the heating on low, but it didn't help, again the flame went out after about 5 minutes.

 

[Slugbabydotcom]

Limit stat is likely to be causing shutdown then
Problem is deciding if it is a cause or a symptom.
For testing purposes only put a wire link across the 2 electrical connections on the thermocouple. If it runs for say 20 -30 mins without banging and clattering you can be fairly sure that limit stat is the problem.
Otherwise its like to be a symptom of poor water flow through the heat exchanger which would also be indicated by the excessive noise.
Fan or flue problem is also a possibilty. Is the fan excessively noisy ?

Must re-iterate the testing purposes only and dont be tempted to leave it like that as it is a safety device

 

[davidrobertson]

I have had problems with the fan in the past when it was too stiff to start but some penetrating oil in the bearing freed it. It spins freely at the moment and is not excessively noisy.
There are three thermostats on the flow pipe and I don't know which is the limit stat. If I run the boiler until the flame cuts out and then tried connecting across each in turn could I do any damage?

 

[Slugbabydotcom]

This post goes into a bit more detail which you may find helpful.
The limit stat is the one connected to the thermocouple so joining the 2 terminals together that protrude from the thermocouple eliminates the limit stat to prove or disprove its failure.

Dont connect across the hi and low stat as they work on a different principle.

If the fan isnt working properly it could cause flame lift which will deprive the pilot of air.

 

[davidrobertson]

I tried connecting a wire across the limit stat (I assume that this is the same thing as is labelled on the wiring diagram as "overheat cut off device") but it did not solve the problem.
However I found that if I gave the pressure switch a sharp tap with the handle of my screw driver that the flame would spark up, sometimes just momentarily but occasionally for a few minutes. So it seems we have narrowed it down to either the pressure switch being defective or that the flue pressure is incorrect. The latter presumably due to the fan not running to its intended capacity.
Any suggestions as to how to positvely identify the culprit would be welcome.

 

[Slugbabydotcom]

The APS measures the pressure differential created by the fan however if any of the sensing tubes ar blocked this would affect the reading. Its usually the one that goes to the flue. Normally I would take the pipe off from the APS and blow through it this should clear a blocked venturi .
More rarely the diaphragm goes in the APS. To test it blow or suck gently into the APS [with the boiler turned off] till you hear the click of the microswitch nip the tube and hold it for a few minutes. If you hear it click again the APS is the problem.
If the fan is the problem its normally stiff to turn or noisy. The resistance across the terminals is usually 45- 50 ohms.

If flame lift does occur you may be able to see the flames lifting off the burner through the sight glass starving the pilot of air and causing shutdown.

If I had the MI I would be able to tell you where to test the pressure differential and where to test APS operation at the PCB. Maybe you can follow the leads?.
Anyway see how you go with that and post back

 

[davidrobertson]

I think we have cracked it!
I blew into the tube connected to the APS (my boiler only has one tube connected to the APS) and yes there was a faint clicking sound, but I was not convinced that it was the microswitch and not just through friction from the diaphragm mechanism. The switch is attached to the side of the diaphragm housing with a plunger connected to the diaphragm which operates the switch when the pressure in the flue changes. I now believe that the problem lay with friction probably between the plunger and the housing. There was just enough space to insert a cocktail stick between the switch and the plunger allowing me to press the plunger in to the housing thus allowing the switch to operate. When I did this the boiler sprung into life.
There appears to be sufficient vacuum in the flue to hold the plunger clear of the switch thereby allowing the boiler to run. However, when the boiler stops (either by thermostat or timer) it will not re-start until I press the plunger again (after allowing it to cool or resetting the timer). The vacuum does not appear to be able to overcome the friction in the mechanism. Probably a sign of age.
Time to buy a new switch I reckon.

 

[davidrobertson]

Well that's it sorted - new air pressure switch bought and fitted. Central heating boiler working like a dream.
Many thanks to Slugbabydotcom for your invaluable suggestions

 

[Slugbabydotcom]

Glad you got it sorted

Thank you too for having the ability to make my efforts worthwhile

 

[Kav69]

I had a similar problem and by reading your posts, I managed to sort out mine too! So, a big thanks first to 'Slugbabydotcom' for his professional advice and explanation and second thanks to 'davidrobertson' who put this into practice and got the result.

Cheers

Kavous

 

[BingoBongo]

hope you checked the case seals

or your next post may be

some1 dies everytime i put on the heating,is this a diy fix