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Craig, Electrical Technician
Category: Appliance
Satisfied Customers: 4529
Experience:  33 years experience electrical troubleshooting industrial equipment and control systems.
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Hi, My boiler will not fire up. The pilot light is on. I noticed

Customer Question

My boiler will not fire up. The pilot light is on. I noticed that when the circulator turns on the pilot light becomes much smaller. Is this some safety mechanism?
Submitted: 7 years ago.
Category: Appliance
Expert:  Craig replied 7 years ago.
Is the thermocouple directly in the pilot flame? You mentioned the pilot getting smaller. That's not normal Cehck all fitting connection on the pilot gas tube from gas vavle. You may want to clean orifice at the pilot flame too. If main buner does not come on and thermocouple is in a good flame, change thermocouple.

Edited by Craig on 2/1/2010 at 9:45 PM EST
Customer: replied 7 years ago.
Yes, the thermocouple is directly in the pilot flame. However once the thermostat calls for heat and the circulator turns on the pilot light immediately turns lower so that it is no longer touching the thermocouple. This is not definetely not due to a leaky fitting, but a direct response to the call for heat. If the main burner not igniting is due to a bad thermocouple is there any way to check this before I go out to buy a new one?
Expert:  Craig replied 7 years ago.
The circulator pump starting and running should not affect this pilot flame, you're talking about circulator pump for the water system right?
You can test the thermocouple with a volt meter. It's putting out DC millivolts while in the flame. Does in screw into the gas valve, or two wires?
This is a gas valve problem. It's dropping the safety valve inside the main gas valve partially closed and this is what drops the pilot flame. Change the gas valve.

Edited by Craig on 2/1/2010 at 11:04 PM EST
Customer: replied 7 years ago.
yes, a circulator pump for a wate system. sorry, should have never mentioned the circulator pump. just wanted you to know that it was turning on.

okay, i'm a bit lost here. just to make sure I understand, the problem is not with the thermocouple but the gas valve? Or is it the thermocouple which is not working and causing the gas valve to drop the safety valve?
Expert:  Craig replied 7 years ago.
Yes, I believe that the pilot flame is dropping due to the main gas valve's pilot valve. The thermocouple failure would just not let the main burner start but the pilot would still be looking normal and would not change. The signal from the thermocouple would not allow the main gas valve to open, and the main burner would not light.
Customer: replied 7 years ago.
Is it possible that there is a valve that effects the gas to both the pilot and the main burner? The reason I ask this is that when the pilot light dims, I smell gas as if the main burner is receiving a bit of gas but not enough to ignite. And if this is the case then it would reasonable to assume that it is the thermocouple that needs to be replaced. Yes? This is what I am hoping at least since changing the gas valve is more than I am willing to do myself but the thermocouple i can handle.
Expert:  Craig replied 7 years ago.
The one thing that you could try before changing the gas valve is clean the pilot flame orifice. Right where the gas comes out of the pilot tube for the pilot flame. I've seen this get a bit dirty and lower the pilot flame size.
  • Remember, if the pilot flame dims, then the thermocouple is not going to operate the main valve.
  • If the pilot flame is dim, it is fed from the main valve.
  • Clean and check the pilot flame orifice first
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Customer: replied 7 years ago.
I changed the thermocouple and it is working.