John Boy


OK, the 110 volts feeds through the door interlock switch.In other words the door panel must be in place for the furnace to operate.

Do you have a heating & cooling thermostat? If so turn the fan switch from “auto to on” poistion this will prove that the door switch is properly in place and you got 110 volts going to the furnace. Switch back to auto after doing this little test for power.

Another stupid thing that use to happen is the material (fibreglass)in the exhaust muffler would fall in and block the exhaust gas from venting. You may have to cut to the exhaust muffler out and look at it, to see if that isn’t what happen. Get two 2″ PVC couplings and put the muffler back if it’s good or a straight pipe if it’s not. Remember to glue it. (I don’t know if I should be telling you these things) If you feel like you are going to screw-up you better hire some one that knows what the’re doing.

Lennox recommends that the air flapper valve be replaced every 4 years. When was the last time this has been looked at. There are six 7/16″ bolts cover the air box and purge blower motor. You could remove them and the cover. Turn the thermostat to heat, the first thing that should happen is the primary control will start the purge blower motor. You should here that (purge blower) operating with or without this cover removed. Listen for it. If you don’t hear the purge blower motor operating then the primary control is gone (kaput). It is the control above the access panel for the spark plug and flame sensor rod.

Is the gas turned on to the unit. Note on & off knob on gas valve.

Remove the high tension wire going from the primary control to the spark plug turn up thermostat, you should hear a sparking going on.

Keep fingers clear (8000 Volts). If not spark, primary control is gone. Replace with new one.

Getting back to the heavy plate that you removed housing the purge blower motor. There’s a metal cage below that houses the flapper intake valve. Get a pair of pliers and wiggle and put the metal cage out of the fibreglass insulation. Now you should be looking at the flapper valve. It’s a item that looks like the end of a meat grinder. What does the flapper diaphagm material look like?

Does it have alot of holes worn into it? If that the case, then pulse combustion cannot take place,because this baby moves back and forth 68 times in one second. Get new flapper material, note the spacer in between the material, do not loose. Note the postion of the two plates with the holes in them. They have stars on them, that means they go to the outside when you put them back together. (Stars are outside) That is how I remember. Put everything back the way you found them.

On a call for heat from the thermostat, the purge motor should start, then the spark plug is energized for 3 to 5 seconds. Then you should hear a loud click comming from the gas valve. Does that happen. You could put a 24 volt meter test probes onto the gas valve to see if that happens. No 24 volts, gas valve is gone.

Now here’s another item, before spending alot of money on this pulse furnace you should have it checked out by a Lennox Dealer to perform a pressure test on the heat exchanger. If found leaking Lennox will provide you with a new G51MP series furnace FREE, since they no longer make the pulse heat exchangers. Go to http://www.davelennox.com and enter your zip or postal code for the dealer nearest you. Labor, installation cost extra.

Note my post Lennox Pulse G21 Gas Furnace on Apr. 6,2004 3:53pm.

Let us know how you made out.

John Boy

P.S. I’ve installed about 56 pulse furnaces over the years and have replaced over half of them back in 1999.