We really should be asking what type of furnace it is. While the 20 amp filters would work on a gas or oil furnace (even a wood or coal system), there is a good chance that the OP has an electric furnace that could run as much as 50 amps on a 240 circuit.
While I doubt that tom j has such a device, recommending a 20 amp filter without knowledge of the system would be irresponsible.
Knightrider,
As with so many posts in these forums requesting help tom j expected me to ASSUME exactly what type of forced air furnace he actually has. If you noticed in my post back to him I only sent him a link to the wire in 20 AMP noise filter. I did not suggest that he buy them,how many to buy, or where to put them. I ASSUMED he would know that if his furnace was on a 50 AMP / 240 Volt circuit, he might not want to use this device.
You and others may have noticed in a lot of my posts I request the OP to give me a call and list my phone number. That way I can cut through the ASSUME crap and find out exactly what the entire problem is and make a "responsible" diagnosis of the OP's problem and make a "responsible" suggestion of how to fix it.
BTW Depending on the BTU rating of the electric furnace they can require up to a 240 Volt /120 AMP breaker. Even in the largest one I could still use the 20 AMP noise filter because I would wire it in to the motor circuit. If it were a 220 VOLT motor I would use 2 of them, 1 in each leg because the motor current is what you need to be concerned about.
Also most of these high current furnaces have a branch off circuit breaker at the furnace at a much lower current rating ( 15-20 AMPs ) to provide protection for the motor in case the TOL (Thermal Over Load )fails.
Finally, If you don't know what you are doing with electric, leave it alone and call an electrician!
Till Later,
Bob