This board simulates the electrical characteristic of a flame for troubleshooting residential gas furnaces.

Detecting that the residential gas furnace is producing a flame (when it is supposed to) is critical to safety. In the event the furnace gets all the way through the startup cycle and the gas valve is open but there is no flame, the gas will continue to flow and build up, and may cause asphyxiation, and if it finds an ignition source may explode. However, the most common problem with gas furnaces is the flame sensing system where there really is a flame but the system fails to detect it.

Sometimes the problem is that the flame rod has become dirty and just needs to be cleaned. Sometimes the problem is with the wire going to the flame rod. This wire is usually bundled with all of the other wires, some of which are carrying 120VAC. This can couple to the flame rod wire and can overwhelm the small signal produced by the flame rod. The problem can be that Ground and Neutral are not connected together. This is required in order for the standard flame sensing system to work. (It’s barbaric but I didn’t design it.) According to the National Electrical Code, Ground and Neutral may only be connected together (and are required to be connected together) at the service entrance to the building and no place else. As a result, an electrical connection problem outside the furnace at the service entrance may cause the flame sensing circuit to malfunction even though there is no problem in the furnace itself. 

Sometimes the problem is with the furnace control board which is expensive to replace.

This board simulates the electrical characteristics of the flame. You connect the Hot wire at the furnace control board instead of the wire from the flame rod. The Ground wire connects to the cabinet ground/combustion burner ground. Do not connect it to the control board signal ground because that might not be the real Ground. If the furnace control board now recognizes a valid flame then the problem is probably not with the furnace control board.

Warning. This board will have the un-isolated Mains on it.

1. Put it in an insulated box. A Single Gang “Old Work” Electrical Box works well. Then put something under the box to protect it. (Screw it down, which I haven’t done.) Feel free to make the wires longer.

2. This device is not for amusement purposes. It is for use by experienced techs only. If you don’t know what you are doing you can kill yourself and the other people in the home.

To use it, turn the switch on as soon as you see the flame. If there is no flame Do Not Turn It On.

Do not use it to make the furnace run with a defective flame sensing system. If, after getting the furnace to run you go off and leave it you risk the possibility that the flame will go out but the gas valve will remain open AND KILL EVERYONE.

If you leave it on all the time modern furnace control boards should refuse to start the turn-on cycle if they detect a flame before the gas valve is open.

Here is the parts list:

D1          Diode 1N4007

R1           2.2M  Resistor, 1/4 W, 5%  

R2           10M   Resistor, 1/4 W, 5%  

SW1        Toggle Switch SPDT, C&K 7101 with PCB Mount Pins     

Plastic Box such as Single Gang “Old Work” Electrical Box

(I removed the paper label with Goo-Gone, the liquid version.)          

I have made every effort to make this safe. However, this is a project that you must use at your own risk. Ok? (Say “Ok”)

If you are interested in learning more about gas furnaces see my article What I Have Learned About Propane Furnaces (The Hard Way) or The Mystery of the Flakey Furnace at www.jmargolin.com/furnace/index.htm .

JM