Resistor on top of auto gas valve??

[mharris251]

Ive seen this several times but dont understand why. On an old GE furnance (30 yr old) with a combination gas valve.
there is a resistor wired in series with one of the 24 volt wires to the automatic gas valve main burner solinoid just above the spade connector.

The last one I saw was so hot It burned me. Funny thing was - the furnace has been calling for heat for 4 years without gas. Worked fine once I restored gas.

What is the resistor for????
Thanks All\

Mike

[hvac1000]

Was this an old White Rodgers spark valve? If it was the resistors were added to the valve to cure a rapid start and stop loop that the valve would get into.
The spark would lite the pilot flame and when the sensor sensed the flame the gas valve would start to open as it would in normal operation but instead of normally operating the valve would open for a second or two then the valve would drop out of the circuit and the unit would start the spark cycle all over again.

This situation would drive a service man nuts thinking it must be a defective mercury sensor (safety), then if that did not work replace the spark box and finally if that did not work they would replace the gas valve. None of those parts made any difference and service people were going nuts trying to figure out what exactly what component was at fault.

The problem was a voltage feedback causing the gas valve to sense a problem with the mercury sensor/safety thus shutting itself off and starting the cycle over in a endless loop. The cure was to add a resistor to bleed off the feed back voltage. The resistors do get hot to the touch because they use the resistor to absorb the feedback voltage.

They were passing out those resistors like they were free candy at the supply houses for awhile because the White Rodgers company was getting a boat load of warrenty parts back and it was putting a hurt on there bottom line. Later on in time they started to charge for them. The resistors are still available in two styles. One for the plug in safety and the other for the hard wired units.

Part number F67-0924 Descriptive Comments or For Whom Made:
Resistor Assembly for 36C Cycle Pilot Types (Natural Gas Applications Only) non Plug-in Types Only, 3049 Type Mercury Flame Sensors


Part number F67-0918 Description of Item:
Resistor Assembly, For Use With 36 Cycle Pilot Gas Valves With Pressure Switch (Natural Gas Applications Only) Plug-In Types Only, 3098 Type Mercury Flame Sensors

[mharris251]

Thanks for the reply about the resistor.The FAF was not a spark system.But your reply was very helpfull in regards to short cycling and mercury flame switches.
This FAF was a standard general control combination valve( Tcouple) and 24 v. The resistor must be to correct the same sort of short cycling problem.

We will share this with our techs....thanks

Mike