lennox lp gas furnace won't stay lit

[basshawkyami]

Hi, i have a lennox lp gas . when call for heat, the purge motor comes on, the pilot flame lights, the electric spark clicks, maybe the burner lights, maybe not, the gas valve doesn't open somtimes, when it does it lights for 10 seconds the blows out, and relights and blows out, or if ur lucky it stays on! now that sounds like the flame sensor which i can't find, unless it is the same thing as the sparker, which has a wire attached to it going back to the control. The strange thing is if i go into the atic and just touch any of those 3 wires of the plug attached to the gas valve, the furnace works fine, stop touching those wires and the gas valve tunrns off and pilot flame still lit and the electric ignition starts clicking again, touch the wires and it lights fine and stays on untill you let go!

[hvacwiz]

Sounds like you have whats called a honeywell smart valve (gas valve). If you touch or wiggle the 4 wire plug that goes to the valve then the valve needs replaced, Once done your problems should go away.

[basshawkyami]

changed gas valve still have same problem, you don't have to wiggle the 3 prongs on the plug, just touch the spade connector while touching valve housing, like some sort of grounding problem.

[HayZee518]

I went to a site called www.cozyparts.com and found several lennox valves and burners. Also, some flame detectors are built right into the electrode unit. What you said a bad ground might be your problem with the ignition problem. Hayzee

[basshawkyami]

took out the burner blowed them out, cleaned electrode for the pilot light, still have mostly same problem. now the buners light for 30 sec. blow out and relight, but they relight with flame down inside near the end where the air goes in with a roaring sound. but if you turn off and turn back on holding the gas valve and touching 1 of the wires in plug everything works fine like it would before i did anything! read somewhere about flame current, can someone explain that to me. also put old gas valve back on

[HayZee518]

the flame detector is a miniature dc generator - in technical terms it is a cold junction thermocouple. two dissimilar wires i.e. copper and iron are welded at one end this forms a junction between the two. when heat is applied there is a transfer of electrons from the most active metal to the other one creating a dc current. the shell or outer part you see is one electrode and the inner conductor is the other. at its other end with a meter you can read the output in millivolts - that is a thousandth of a volt which is enough to be detected by the burner control circuit or in the case of a gas valve enough to operate the valve if it is of the millivolt type.

[basshawkyami]

the eletrode is inside of a porcilin housing which on the wire side looks like a spark plug end, the spark plug wire connects there and then connects to control module. the other end, electrode end has a gap between a flat piece of metal that has spark to light pilot. where would i hook up the meter? from what you have told me, i'm assuming that this electrode is also the flame sensor.

[HayZee518]

that's what it seems to me. that electrode is part of the high voltage system to get a spark to the light the gas for a flame, it also seems as it detects the heat for the control circuit. I recall putting in a control module of the white-rodgers flavor into a commercial clothes dryer at a laundromat which has a similar hookup. the spark had a silicone covered brown wire to one electrode, the flame detector had a red push on spade connector with a nylon insulation. the burner valve connected to a 24 volt common and a 24 volt switched off the controller. a 24 volt hot and common also connected to the control.

[basshawkyami]

there is only the 1 wire that plugs on like a spark plug wire with the other end plugged on the control module

[HayZee518]

let me look into this..