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Wendy R
furnace burners not lighting

I woke up to a cold house this morning. The furnace was running but only cold air coming out. I checked to see if the pilot light was lit and it is. However, I sat there watching but the actual burners that fire up wouldn't light. I vacuumed everything out that I could reach but they still won't light. The pilot light stays on, but no heat. Any suggestions?

Re: furnace burners not lighting


That sounds like an out of position or underheated thermocouple bulb---that's the little piece of silver-colored metal bulb that is in the pilot flame & has a coil going back to the gas valve.

1) Make sure that the little silver element is being heated by the flame---as it heats up it sends a small voltage back to the gas valve to open the gas valve to send gas to the burners.

2) Make sure the gas knob selector is turned to "ON", and not at "Pilot"

3) Make sure the room thermostat is turned up to 80 degrees (calling for heat).

4) Make sure the metal thermocouple bulb tip is being hit by the pilot flame & that the pilot flame is big enough to heat the little metal tip.

Please post back when above items are checked out.

Wendy R
Re: furnace burners not lighting

Well, I tried that and it still won't work. I am going to go to the hardware store for a new thermocouple. Would that be my next step?

Re: furnace burners not lighting


I wish I could say it's that easy---the trouble is, there are 5 or 6 different brands of gas valves & they all operate slightly differently.

A thermocouple is usually a $10-$20 item, but it may be the gas valve itself, or a dirty pilot light circuit.

If you feel comfortable doing this--remember gas is dangerous---make sure you TURN OFF THE GAS SUPPLY VALVE going into the furnace, or at the main meter box.

The handle for the On/Off switch for the main gas valve is IN LINE with the gas pipe when the valve is ON and the handle is at a RIGHT ANGLE to the gas pipe when the gas is OFF.

Turn the main/supply gas valve OFF (you should verify that the pilot goes out)---if you have a gas stove, verify there is no gas at the stove.

Copy down the brand name of the gas valve (Honeywell, Robertshaw, White-Rogers,etc.) and any model number you see---this info will be on the replacement part you will be buying as a guide to your getting the right part at the store.

Remove the thermocouple and bring it with you to the heating supply parts house----Home Depot and Lowe's carry some of these items,--- it's best to have the old one with you to make sure the screw-in end connection is the same, so it will fit on the gas valve.

Do not overtighten the new thermocouple when you screw it in---finger tight, then use a pliers or wrench to tighten it a 1/4 turn more.

Turn on the main gas valve & use soapy water around the new thermocouple connection to verify there is no gas escaping from the connection----retighten if there is.

Wendy R
Re: furnace burners not lighting

OK, I replaced the thermocouple and it still isn't lighting the burners. Gas valve? I think I will leave that one alone. I am afraid I will blow the house up. Time to call the furnace guy?

Re: furnace burners not lighting


Wow! that was a quick change----yes, definitely, time to call a service person.

Re: furnace burners not lighting

Wendy ,
some more information is needed here.

It sounds as though this is a forced air furnace ?

Is this pilot light always on ---- standing pilot light or does it come on when there is a demand ?
How old is this furnace?

Is the blower turning on and blowing cold air ?

Wendy R
Re: furnace burners not lighting

Hi. Yes, it is a forced air furnace and a standing pilot. Yes, it comes on and blows only cold air. I am not sure how old it is. It was here when we bought the house. I am guessing it is more than 25 years old. It is a Sears furnace.

Re: furnace burners not lighting

Wendy, thanks for the additional information.

I'm suspecting the thermocouple you replaced was likely the type that has a metallic tube that screws onto the valve body. If so, these are only for controlling the pilot light and wouldn't control the burner function, Replacing these types are only needed when there are issues with the pilot light has problems with not remaining on.

Going on this assumption of that type of thermal couple it's more than likely your gas valve is 24 volt ac controlled.
Basically there is a transformer located at the furnace which steps down 120 volt ac line voltage to 24 volts ac for the gas valve and likely is also used by the thermostat.

In a nut shell the way your type of furnace normally operates ---- the standing pilot flame remains lit as the ignition source for the burners to fire.
When your thermostat ( in the living space ) calls for heat 24 volts is supplied from the thermostat to the gas valve solenoids to open gas flow to the burners ---- which get ignited by the standing pilot light.

Once the burners fire up this heats up the heat exchanger at which time there is a device that's called a " Fan/ Limiter " that is reading the temperature inside the plenum where the exchanger is located.
It might look like this .........

The fan limit control provides high limit control of the burners and control of the fan motor according to the furnace plenum temperature. A bi-metal probe extends from the back of the fan limit control. This probe is located inside the heat exchanger and the bi-metal of the probe reacts to temperature.

When your furnace burners come on the furnace heat exchanger heats up the bi-metal probe causing the round dial on the fan limit (shown in the picture) to turn. When the temperature of your furnace gets warm enough and to the preset temperature on the fan limit control, the control clicks on and causes the furnaces blower fan to kick on and circulate air throughout your home. When your thermostat is satisfied the burners kick off and the fan continues to run until your furnace's plenum cools down to the fan OFF set point on your fan limit control. The fan limit control then turns the fan off.

The fan limit control is an important safety device. Let's say that your furnace's burners come on but the blower motor is bad and will not start. Without the fan limit control your furnace could over heat and cause a fire! The fan limit control shuts the gas burners down so the furnace will not over heat and cause a fire.

The circuit which directly runs through this switch controls the burners to the furnace. The temperature control limit switch inside the fan control breaks the circuit to the burners if the temperature exceeds the high limit set point. This is a safety feature all furnaces are equipped with in case the temperature gets too hot inside the exchanger. If this happens the furnace blower contacts remain closed so the furnace blower dissipates the excess heat in the furnace exchanger.

This is my guess as to the problem since the blower turns on immediately but the burners don't fire. It may be the dial is actually stuck in the high limit position or the temperature probe is faulty.

Normally this isn't a DIY type of repair however, if you are going to do this yourself ......

Shutoff the power to the furnace by either turning off the breaker or the emergency shutoff switch.

Make sure to either label which wire goes where or better yet take a photo of the control before disassembly.

Remove the fan/limit control and get an exact replacement ---- the bi metal probe is critical and needs to be exactly the same.

Install the new replacement being careful not to damage the probe.

Hook up the wires in the correct positions.

Set the pointers for the limits according to the original one that was removed.

Turn the power back on and fire up the furnace.

Hopefully this makes sense and helps.:)

Re: furnace burners not lighting

To add ---- since your furnace is old you might have a qualified HVAC tech. inspect your furnace heat exchanger to ensure it isn't cracked.

Cracked heat exchangers are dangerous in that they will spill carbon monoxide into your home.

Don't mean to frighten you but rather error on the side of caution.

If a tech does report your exchanger is defective --- make sure to get at least 2 other opinions. There can be cases where unscrupulous companies will tell you the furnace needs replacing when they may not.

Wendy R
Re: furnace burners not lighting

Thank you very much for all you went through to post a solution for us. I am not tackling anything like that. I have a friend who has some knowledge about furnaces and he came over last night and says it is either one of 2 things, the transformer or the gas valve. But I am also going to print your response out to him in case he didn't think of that. I really appreciate yours and others help on the site. I hope to have heat before the 2 degree temp gets here. Thanks again.


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