I have a Rheem 80 plus gas furnace with an automatic igniter in my home that has a problem lighting the gas. The glow plug or igniter has no problem glowing bright orange to light the gas, but the device that controls the gas doesn't seem to let enough gas out to be ignited. Once the igniter is hot, the device allows a shot of gas and there will be a flash of blue but the burners don't light, the ignter turns off, the blower fan turns on and no heat. There is also a seprate fan that turns on before the igniter glows and sometimes will stay on for hours if the burners are not lit. This problem was intermintent, but now it has been 2 days since the burns have actually been lit. What is the cause of the burners not lighting?Thanks!

One thing as a homeowner you can do is clean the flame sensor. For some reason this little part causes many problems with modern day furnaces. Stay away from the item called the ignitor when you are cleaning the flame sensor since they are very easily broken. Also never touch the ignitor in the black areas since that can damage it. Sometimes the ignitor is in a close relationship with the sensor. Below are a few pictures of sensors. Clean with steel wool, scotch brite pad or sand paper.

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Thanks, but I can't find a sensor. Here are some pics of my furnace.

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Looks like you do not have one.

Post exact model number of furnace.

Post exact part number of the Robert Shaw gas valve.

It is possible this is one of the setups that uses the ignitor as the sensor. There has to be some type of safety on the furnace to prevent the gas valve from staying open in the event of a flame failure just like the one you are having.

Furnace: RGDE-07NMAGR

Gas Valve: 7A5 A8B 003

Thanks!

Here is some information. The igniter is the flame sensor on that model furnace. Turn the power off.
Pull the common transformer wire off of the circuit board.
Check with an Ohm meter between one of the burners and the common terminal ON THE CIRCUIT BOARD where the common transformer wire was. You should have low resistance of less than 4 ohms. If more than 4 ohms the flame sense circuit is not there.

This is the possible old number
HS780-34PL-308A OR 695-211 (HSC992-21)

The replacement controler is this I believe.

http://www.icca.invensys.com/uniline/c/c49.pdf

Now I have no idea for sure if your module/circuit board is bad or if the ignitor which glows and proves good for ignition is not sensing the flame. Since the unit shuts off so fast I am inclined to think the modual/circuit board is defective but I am not there to test it. Changing the module from what you have to the new style will not be a piece of cake since the replacement is universal and has many different terminals to choose from so it might be a good idea to have a local HVAC company have a look at your unit and do the module replacement if necessary.

The ignitor part is much cheaper than the module but it glows and as far as i know as long as it glows it is good. This is all the info I have on your unit.

Thank you for our help

Hope you get it running.