What's wrong with Lennox Furnace?

[jstarzy]

Can anyone give me an idea of what is causing the furnace to cycle the gas feed to the burners on for 10 seconds, then off for 10 seconds for the duration of time the thermostat calls for heat?

Thanks.

Jim.

[Toolpusher]

It could be the stat, or the flame sensor, or dirty filter.
Although I wouldn't think a limit switch would cycle that fast.
What kind of t-stat do you have ?
Is it round and gold, with needles to indicate the temps.
Or is it a digital read out ?

Normally if it's not detecting a flame and goes off due to that, the unit will lock out after 3 or 4 trys to light.
So it could be doing what the stat is telling it to do.
Short cycling.

[jstarzy]

Toolpusher...

Thanks for the reply...

The stat calls for heat and releases just fine over a long period of time. All of the on and off is occurring during the duration of time the stat calls for heat. In other words, the stat is not cycling every 10 or 20 seconds like the burners are. Also..because I wanted a new stat, I have had 3 different ones working the unit. The burners exhibet the same 10 seconds on, 10 seconds off with all stats.

Jim.

[mike3]

I would agree the flame rod first place to start. Clean flame rod.

[d d boyer]

Is it a new furnace?? If it is, check to see if it's grounded properly.
Also any codes from the L.E.D's?? and check to see if your getting a constant 24v on the "w" terminal.

[airconman]

clean the flame rod.....make sure the wire is intact all the way to the board.

[rob air]

Some models had upgrade flame sensor kits.
Lennox-parts.com

[aircooled53]

Let the Lennox unit go through this for (3) cycles and then check the fault code (light blinking)and that code is usally on the door.
I would also check flue,draft motor as Lennox has had some problems with draft motors.80MGF models.

The flame sensor can be cleaned by turning off power to unit, then left side of furnace (1) burner you will see a ceramic type steel rod with (1) wire connected to it. Remove wire, remove screw holding sensor, clean with steel wool, re-install and re-connect wire.

[d d boyer]

Yeah, 80mgf's and g24's had flame sensor problems, but that was around '95. Also some of the early mid's had a painted ventor motor, and the paint would flake off from the heat and screw up the pressure switch.

[paddyhvac]

If this is a newwsystem or you recently had electric work done check polarity to furnace

[pulse21]

Can you give me the model number please and what kind of thermostat is it would not happen to be a Honeywell digital 4 wire hook would it?

[jstarzy]

I will have model and serial number tomorrow. Current stat is a new honeywell 8500 digital with 4 wire hook up. However...furnace had the same problem with a Rite-Temp 4 wire digital too.

[paddyhvac]

sounds like polarity to furnce is wrong