Over Heat Thermostat on Worcester 240BF

This is an interesting problem. The boiler is a Worcester 240 Balanced Flue.
Went away for a few days and switched off the power - not the gas.
On return the pilot was out. It would spark up and light whilst holding in the button - up to 2 mins - but always went out as the button reached its outward limit. Checking the obvious, the thermocouple was changed but no result. Next check was the Overheat themostat. With a resistance meter it was found to have a circuit at the gas valve. (Both ends removed from the valve for this check). So thought the OH stat must be good. However, by shorting the two connections on the gas valve the pilot would stay on - albeit not protected should it go into an overheat situation. A new OH stat has been ordered but can anyone throw light onto this strange one.
Why does it work with a piece of wire across the connections but not with the stat that shows closed circuit when tested.

The thermocouple only produces about 10-13 mV on load!

You have not been correctly measuring your o/h stat resistance!

If it is not to reduce the voltage to the gas valve it needs to have a very low resistance of say no more than 0.2 ohms!

Yours may be an ohm or two.

You should really check whats going on with a voltmeter! You are clearly loosing too many volts on the o/h stat.

Tony

Thank you Agile for that information. That certainly makes sense and makes me think that it could be dirty connections at the gas valve. All wire joints to the O/H stat and terminals are soldered, so that only leaves the clamped joints. (However from my electronics knowledge I am aware that dry joints in soldering will lead to a resistance - especially important with very small signal voltages). When the replacement O/H stat is replaced I will try and make a better assessment of the resistance of the old one with my Fluke meter - before and after cleaning the ends that make contact. I'll report back if results show anything interesting.

I do wonder if it is possible to overtighten these connections and partially short the inner core carrying the signal.

Almost certainly the problem is on the opening contacts on the o/h stat itself.

If you were able to take it apart and clean them with a diamond spatula then it would solve the problem. You need Danny's P.O. Tool 001.

Tony

Had this very fault albeit with a vitiation device not very long ago...showed a closed circuit but when connected was not working, dirty contacts as suggested, replaced and bobs your uncle.

superdupergasman said:

Had this very fault albeit with a vitiation device not very long ago...showed a closed circuit but when connected was not working, dirty contacts as suggested, replaced and bobs your uncle.

Click to expand...

Its not sufficient to take a reading on a high ohms range!

You have to actually measure on a low ohms range and see exactly what the resistance is after the decimal point! Needs to be below 0.2 ohms in t/c circuits.

Tony