More boiler problems


My Bosch Worcester Highflow 400 boiler is approx 8 years old. No changes to the system or rads added. For the past year (yes a year) I have had problems it, in that the pilot keeps going out.
Over the year I have had the High Stat changed, The flue replaced and now the gas valve has been replaced. Still the pilot goes out on a regular basis
During the last engineer visit he said there was a lot of air in the system, as he could hear it rushing through the pump. I checked the rads for air and each time only water came out the valve.
I checked again today, and the air is still going through the pump , pretty loud, but again no air in the rads. Would the air not naturally find it way to the upstairs rads ?
Any guidance on what could be causing this problem would be gratefully appreciated.
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I don't know the boiler, but if it is a standard Honeywell gas valve with thermocouple holding in an electromagnetic pilot valve, then check the following.
Is the pilot jet clean. Take it to bits and blow it through or poke with thin copper wire. Make sure all aound thermocouple is clean.
There is a screw on the gas valve near the thermocouple connection. This controls the pilot gas flow. Try increasing it.
You don't mention whether overheat stat trips. If this happens you would need to reset it to relight the pilot. If this is not happening then it is unlikely that air or gas in the water is causing a problem. If the pump is running at a high head the water flow itself can be noisy and sound like air.
You might need a new overheat stat, even if replaced this year you could have a bad one, they can cause a high volts drop in the circuit when heated without actually tripping.
If the thermocouple hasn;t been replaced for some years, get a new one.
If it is the standard Honeywell system then the voltage drops through the overheat stat and wiring connections are very important as the gas valve works with only 8-10mV input. If you have or can borrow a dc mV meter, the thermocouple output should be approx 10 - 13 mV and no more than 2 mV should be dropped across the overheat stat terminals. This simple measurement could pin point the problem.
Is the case around the pilot sealed properly. Draughts caused by air leakage can blow out the pilot or move the flame away from the thermo couple sufficient for the gas valve to shut down the pilot.
Has anything changed around the outside of the flue to increase draughts that might blow out the pilot?
Good luck Dave from Dave
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