Pilot Light won't stay lit
- Thread starter lily2470
- Start date
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Lots:
1) The pilot flame is too small to heat the thermocouple.
2) The thermocouple is duff.
3) Something else - like an overheat stat - needs resetting.
4) The gas valve is faulty.
Let's hope it's not the last one!
I include the following information just in case you're interested because somebody will surely slag me off if I try to tell you how to fix your own boiler ---
Most flame failure devices work the same way. You press the button in to get gas to your pilot jet and light it. The thermocouple heats up and generates a tiny voltage, just enough to energize an equally tiny electromagnet inside the valve. This holds the valve open after you take your finger off the button.
An overheat thermostat may be wired in series with the thermocouple. If it operates it cuts the power to that magnet and the valve shuts, just as if the flame had gone out. If there's a reset button try it - you never know your luck - but you'll also need to know why it popped in the first place.
1) The pilot flame is too small to heat the thermocouple.
2) The thermocouple is duff.
3) Something else - like an overheat stat - needs resetting.
4) The gas valve is faulty.
Let's hope it's not the last one!
I include the following information just in case you're interested because somebody will surely slag me off if I try to tell you how to fix your own boiler ---
Most flame failure devices work the same way. You press the button in to get gas to your pilot jet and light it. The thermocouple heats up and generates a tiny voltage, just enough to energize an equally tiny electromagnet inside the valve. This holds the valve open after you take your finger off the button.
An overheat thermostat may be wired in series with the thermocouple. If it operates it cuts the power to that magnet and the valve shuts, just as if the flame had gone out. If there's a reset button try it - you never know your luck - but you'll also need to know why it popped in the first place.
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Hi folks.
I have a very similar problem.i have an old ideal standard rs 50 boiler.It has gone out before but relighted fine.Recently pilot light went out it would relight but not stay lit.I bought a new thermocouple as many suggested and fitted it.Pilot light did light ok but main flame will not "kick in".You can hear a click and flame goes out after approx 1 min.Any help greatly appreciated as do not fancy buying a modern one if i can help it.
Merseyside area
thanks again.
ps: should there be a seperate immersion so i can at least get hot water,there is a switch by tank but dosent seem to do anything??
I have a very similar problem.i have an old ideal standard rs 50 boiler.It has gone out before but relighted fine.Recently pilot light went out it would relight but not stay lit.I bought a new thermocouple as many suggested and fitted it.Pilot light did light ok but main flame will not "kick in".You can hear a click and flame goes out after approx 1 min.Any help greatly appreciated as do not fancy buying a modern one if i can help it.
Merseyside area
thanks again.ps: should there be a seperate immersion so i can at least get hot water,there is a switch by tank but dosent seem to do anything??
You need a gas safe engineer to sort it i am afraid.....
I've also had this problem, after the gas has been turned of for a couple of weeks.
The obvious cause is the thermocouple, although there could be several other possible causes, which have already been mentioned.
After this had happened to me several times, I checked the old thermocouples and found that they were OK.
The problem was a dirty electrical connection. Cleaning both sides of the connection solved the problem. The connection need to be done up fairly tightly, but not sufficient to strip the thread.
HTH
The obvious cause is the thermocouple, although there could be several other possible causes, which have already been mentioned.
After this had happened to me several times, I checked the old thermocouples and found that they were OK.
The problem was a dirty electrical connection. Cleaning both sides of the connection solved the problem. The connection need to be done up fairly tightly, but not sufficient to strip the thread.
HTH
A DIYer can check the electrical connection of the thermocouple at the gas valve but replacing it or any other gas related work should be done by a gas engineer.
Dirty connections are rare although loose connections are more common and have the same effect.
The efficiency of the thermocouple can be checked electrically with a suitable test meter. Done at the gas valve that could be checked by a competent DIYer with the skills to measure it and understand what to expect. It might be in the FAQ section.
Tony
Dirty connections are rare although loose connections are more common and have the same effect.
The efficiency of the thermocouple can be checked electrically with a suitable test meter. Done at the gas valve that could be checked by a competent DIYer with the skills to measure it and understand what to expect. It might be in the FAQ section.
Tony
I forgot to mention that the actual voltage generated by the thermocouple is very low. Therefore to get the necessary current to hold the gas valve on, the circuit resistance is also very low.
Just for the record, I measured an open circuit voltage of 28 milli-volts, for the thermocouple. The resistance of the thermocouple was 0.2 ohms, and that of the valve also 0.2 ohms.
With such a low circuit resistance a solid connection is essential.
Cheers
Just for the record, I measured an open circuit voltage of 28 milli-volts, for the thermocouple. The resistance of the thermocouple was 0.2 ohms, and that of the valve also 0.2 ohms.
With such a low circuit resistance a solid connection is essential.
Cheers
I forgot to mention that the actual voltage generated by the thermocouple is very low. Therefore to get the necessary current to hold the gas valve on, the circuit resistance is also very low.
Just for the record, I measured an open circuit voltage of 28 milli-volts, for the thermocouple. The resistance of the thermocouple was 0.2 ohms, and that of the valve also 0.2 ohms.
With such a low circuit resistance a solid connection is essential.
Cheers
Just for the record, I measured an open circuit voltage of 28 milli-volts, for the thermocouple. The resistance of the thermocouple was 0.2 ohms, and that of the valve also 0.2 ohms.
With such a low circuit resistance a solid connection is essential.
Cheers
May I belated add my two penn'orth to this now old thread? Who knows it may help others since I have consulted this page many times over the years.
As others have said the thermocouple voltage is only about 28 mV off load and 13 mV when in circuit. I don't know what electronics in the gas valve or circuit board it drives but even small resistances can have a significant effect with such a low driving voltage. I have a stock of some three old thermocouples which have been replaced to cure a failing pilot light at various times in the past in my ancient but simple Halstead Barlo Blenheim boiler. When tested out of the boiler all appear to function perfectly providing the expected 28 mV or so when heated and there is little reason to suspect that they wouldn't work fine when connected up either. I suspect that I replaced them unnecessarily and that in doing so managed to do something else which fixed the problem.
On my boiler the thermocouple is connected to the gas valve in series with the overheat thermostat switch via two spade terminals. If any of the thermocouple connections, the overheat thermostat switch or the spade terminals are dirty these can easily add resistance to the circuit and prevent the pilot lighting. It will be worthwhile checking for resistance and cleaning any terminals or switches in circuit with your pilot thermocouple before assuming that the thermocouple is at fault.
As others have said the thermocouple voltage is only about 28 mV off load and 13 mV when in circuit. I don't know what electronics in the gas valve or circuit board it drives but even small resistances can have a significant effect with such a low driving voltage. I have a stock of some three old thermocouples which have been replaced to cure a failing pilot light at various times in the past in my ancient but simple Halstead Barlo Blenheim boiler. When tested out of the boiler all appear to function perfectly providing the expected 28 mV or so when heated and there is little reason to suspect that they wouldn't work fine when connected up either. I suspect that I replaced them unnecessarily and that in doing so managed to do something else which fixed the problem.
On my boiler the thermocouple is connected to the gas valve in series with the overheat thermostat switch via two spade terminals. If any of the thermocouple connections, the overheat thermostat switch or the spade terminals are dirty these can easily add resistance to the circuit and prevent the pilot lighting. It will be worthwhile checking for resistance and cleaning any terminals or switches in circuit with your pilot thermocouple before assuming that the thermocouple is at fault.
Yes, but all those checks are gas related and mostly located inside the combustion chamber.
This site does not permit giving advice or encouragement for DIY gas work.
The obvious solution is to call a competent gas registered engineer.
Tony
This site does not permit giving advice or encouragement for DIY gas work.
The obvious solution is to call a competent gas registered engineer.
Tony
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