Baffled by boiler thermocouple/limit thermostat conundrum

[ashtons99]

I have an old Myson Economist boiler which although 28 years old and no doubt not very efficient generally works well. However after a pilot light failure I have reached a confusing situation.

The pilot thermocouple has a couple of spade connectors half way along it's length. Connected to these are 2 wires coming from a limit thermostat attached to the flow pipe above the heat exchanger.

I suspected the thermocouple had simply failed but when I bridged the 2 spade connectors with a wire link the pilot lit and remained lit. When I plugged the two thermostat wires back on the pilot again would not remain lit.

I put the continuity tester on across the two connections and with nothing attached the connectors on the thermocouple link showed a closed circuit. When I connected the tester across the wires to the limit thermostat they also showed a closed circuit.

The pilot will only remain lit if I bridge the two thermocouple connectors which is simply bridging an already closed circuit?

Why is this. I could understand it if it was showing an open circuit across the 2 connectors and the bridging wire was closing them.

What do I need to fix the situation and restore the thermal limiter arrangement.

 

[Dan Robinson]

A professional

 

[underwurlde]

^+1

Did you do your test with the pilot lit? As an aside, flames produce ions which may have confused your meter.

 

[hazetimesfive]

don't listen to these cowboys, basically you need a powerflush.

 

[OLLIE20]

don't listen to these cowboys, basically you need a powerflush.

Don't be daft his condensate traps not blocked you fool

 

[oilhead]

What do I need to fix the situation and restore the thermal limiter arrangement.[/quote]

Legally, a gas safe registered service technician.
To further your education, measure the thermocouple current, and the resistance of your limit thermostat. If you have any knowledge of electrical theory, you will understand the situation you have.
If you don't, then you shouldn't mess.

 

[ashtons99]

I am simply asking where the likely fault lies, I'm not wanting to enter the plumbing magic circle.
Replacing either of the 2 components is not a difficult task but if I can avoid spending £60 on a thermal limiter I possibly don't need through some honest advice I would like to do so.
I thank those of you who have attempted a constructive reply

 

[oilhead]

In simple terms, your thermocouple produces a small electric current, measured in millivolts. The carriage of this current passing through the limit thermostat and its attendant will meet with a certain resistance. You need to measure the current being produced, measure the resistance through the circuit and make sure that it is not reducing the current to a level below that required to hold the thermocouple solenoid in the gas valve.
Continuity measured with a circuit tester will not prove the necessary requirements of the circuit as the current you are using to test is way in excess of the thermocouple current.

 

[Agile]

In simple terms, your thermocouple produces a small electric current, measured in millivolts.

Currents are measured in Amps !

 

[underwurlde]

^ sounds counter intuitive, but think of a thermocouple like a small battery. Yes a battery can produce a current when connected to a circuit, but one would use a volt-meter to measure terminal voltage. Ditto thermocouples hence measure the voltage across it to determine temperature seen at the thermocouple junction.

 

[OLLIE20]

Underwurlde you do know Agile is in the top 100 gas men in the country ?

 

[45yearsagasman]

How dare anyone contradict advice from a top 100 gas man.

 

[underwurlde]

Underwurlde you do know Agile is in the top 100 gas men in the country ?

I have the greatest of respect for Mr. Agile. And yes, Current is measured in Amps. Absolutely no disagreement there!

Andy

 

[oilhead]

Yes , current is measured in amps. Sorry for my terminology. Resistive overheat stats were common problems on the first generation permanent pilot low water content boilers. In the day, make sure the pilot was strong, then change T/C, then overheat stat. If T/C was OK, then there was a least a spare for the future. In the day, we probably did not have the equipment (or the nous) to enter into the finer points of millivolts or resistance, but bits were sensibly priced. There is still the possibility that the thermocouple magnet is weak and this could be the problem. From memory, the Economist had a V4600 valve so that is not replaceable( again from memory). I no longer work with gas, as my 'handle' suggests but reckon I could hold my own against most registered technicians.

 

[ashtons99]

Gents thank you for your informative posts and although I understand the principle I have decided to contact a suitably qualified friend to complete the task. I am confident it is the limiter thermodisc which is at fault.

As an aside I am shocked that the price on the part at £60 when it seems that the small thermostat part is readily available on ebay at £2-£3. the only difference seeming to be the one at my local merchant and online have the heat proof leads with them!!