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Honeywell gas valve will not hold pilot light supply. I see inside what seems to be a permanent magnet with a spring loaded keep plate. but how does the resistance of the thermocouple affect this?

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  • Keith Harmsworth Nov 09, 2008

    Central heating boiler, not air conditioning

  • Keith Harmsworth Nov 15, 2008

    Thanks, I know why the safety is there, but didn't know why it wasn't working.

    I finally found the fault and have now got it working - there was a break in the coil connection inside the valve.

    The bit I don't understand is how resistance in the thermocouple could have any effect on the magnet. I now realise it is not a permanent magnet.

    The magnetism, which is quite strong, is "generated" by an increase in thermocouple resistance (from the heat of the flame of course) this holds a spring loaded plate captive to release the main gas supply.

    There is a "U" shaped peice of iron with a coil of wire wound up each leg of the U and this is connected to the thermocouple - no external power. magic ?


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The resistance of the thermocouple proves that there is flame present at the pilot it is a safty so that unburned fuel deposits do not occur. If your safty valve is magnetic I dont understand your question.

Posted on Nov 15, 2008

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Pilot light not staying on

A standing pilot is on a gas furnace, unless you have an air conditioning system that uses ammonia as the refrigerant and has a gas flame, as this is improbable I will assume this is in the AC section by accident.
The pilot solenoid is energized by the "thermocouple", a thermocouple creates 30mv electrical power by a temperature differential existing across the tip of the thermocouple.
Inside the tip are 2 dissimilar metals.
Any junction of dissimilar metals creates a voltage potential in relation to temperature. Specific metals have a predictable
and repeatable relationship to temperature gradient and voltage.
By the thermocouple's tip being impinged by the pilot flame, creating a temperature differential across the thermocouple.
Id a thermocouples end is fully impinged it will have no temperature differential across the tip and drop output of any voltage, some try baking a thermocouple into submission, this is counter productive.
1000 MV = 1 volt ,
a AAA battery is 1.5 volts
So 30 MV is a very small amount of power and is easily interrupted by design should the pilot flame not heat the thermocouple meaning no flame exists and the pilot solenoid will close .
When we manually depress & hold the pilot knob in to light the pilot assembly this allows the pilot flame to be established, after 30-60 seconds we can then stop depressing the pilot knob and the pilot flame will remain on by the 30 MV energizing the pilot valves coil holding it open magnetically, like old scrap yards on TV used to show a crane with a huge electro magnet that they power up to grab the metal and kill the power to drop the metal, this is what is happening in a relay or contactor etc.
If the pilot will not stay lit with a new thermo couple, and the thermo couple is properly set in the pilot flame as earlier described, then a gas valve will be required.

Jun 06, 2014 | Heating & Cooling

1 Answer

The pilot light on my daughter's Worcester 240 combi rsf has gone out - how does she re-ignite it?

Hi, I looked up this unit for you and found out it is a hot water system. She needs to get to the main gas valve to light the pilot. This unit has what is called a thermocouple on it that keeps the pilot on, even when the boiler shuts down. Its just a matter of lighting the pilot light. There should be a metal plate with the directions on it at the gas valve to lite it. You will have a knob, more then likely red or black in color on top of the valve. It will say on, off, and pilot. It should be in the on position now. Turn it to pilot. Have some matches with you and a flashlight if you can. You will need to look back into the burner area to where your pilot light will be. The thermocouple is a copper colored tubing that runs right back to the pilot, as it has to get hot from the pilot flame to work. To start, depending on how far back you have to reach with the match, you may need to use a piece of solid cardboard rolled up, a small piece to light it and reach in. Turn the knob to pilot. Now, push down on the knob and hold it down. This lets gas get back to the pilot, so you must keep it down and light it at the same time. Once it is lite, you have to hold it down for at least a minute. Slowly release it, very slow. The flame should then stay on. Once it is on, turn the gas knob to on. With the thermostat set to come on, it will fire up and heat the water. If the pilot will not stay lite after you hold it down for 1 minute, and keeps going out, the thermocouple will need to be replaced. It is the copper line that screws into the gas valve and goes back to the pilot. You can get them at any hardware store for around $6.00. Maybe you can help her install it. It is very simple to do. I believe the pilot just blew out and she will need to light it. Just follow what I have told you, and she will get it lite. Good Luck to you.
A/C, & Heating Contractor

Apr 10, 2011 | Heating & Cooling

1 Answer

Wall mounted 3 grate gas space heater: VIN # 41201. ser. # 1410915. Piolet light won't remain lit to be able to light grates. Also need control knob (if avail) for same model.

your pilot has dust in it. Use a drinking straw and blow air across (not into) the pilot opening. this will remove the small particles of dust that are stopping up the membrane inside the opening. THis will make your pilot flame bluer and hotter and keep the thermocouple hot enought to produce enought magnetism to hold the gas valve open.

Jan 27, 2011 | Desa International Heating & Cooling

2 Answers


The safety feature for your fireplace is probably a millivolt pilot generator. It serves 2 functions. It holds open the pilot safety magnet AND operates the solenoid on the millivolt gas valve. The pilot generator is weak. It will hold open the safety but not work the operator selenoid on the gas valve. Replace it. And party on!

Dec 19, 2010 | Desa International Heating & Cooling

1 Answer

I need to know how to light the pilot light on my gas furnace.

What needs to happen that may not be obvious is that you have to heat the thermal tube long enough for it to expand and open the gas valve. No heat and the system should shut down flow immediately.

The thermal tube is copper in color and runs between the gas valve and the pilot flame.

So, there is an override. usually spring loaded (heavily) and you press some button or know, this manually opens the fuel valve.....You light the pilot, still holding button......Pilot lights, holding, holding...maybe 10 to 15 seconds.

Then the secret is that you SLOWLY let up on the button, observing the pilot....If it starts to go out, push the button all the way in and continue heating the thermal tube. Soon, you should be able to release the button, and the pilot remains lit. You are good to go, and you can open the gas valve the rest of the way, for normal use. (usually past the pilot position.

Nov 06, 2010 | Fedders Heating & Cooling

1 Answer

Pilot light seems to have gone out in the wind and I don't know how to re-ignite it

get long lighter such as kind for BBQ pits and turn gas knob to Pilot and hold it down. Once pilot is lit hold button still for 30 seconds then release and turn valve to on. If pilot keeps going off.. the problem is a bad thermocouple

Aug 26, 2010 | Heating & Cooling

1 Answer

How do you light the pilot light

turn gas valve to pilot position, hold button continuously, light pilot gas at orifice, keep holding button down for 1 more minute as Thermo-coupling heats up. pilot should remain lite.

Oct 18, 2009 | Heating & Cooling

1 Answer

Furnace runs about 1 minute, then total shutdown

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Apr 05, 2009 | Heating & Cooling

2 Answers

I have a dayton furnace with a honeywell vr8440p-2088 gas control. The ignition works fine but the problem is I am getting no gas it seems to light the pilot. I checked the orifices at the end of the pilot...

I had the same problem with the same honeywell vr8440p2088 gas controler.  I found out the nob to turn on the pilot was broken.  If you can pull the knob off it is broken.  The knob has two small teeth on it to keep the pilot and gas open.  good look finding a replacment knob as the part is not made anymore.  But there is a relacement part for it.  it just costs about 250.00.  

Apr 18, 2008 | Honeywell Heating & Cooling

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