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Spydey 12-16-2011 02:56 AM

Furnace running proper?
Hi there,

I have a vintage Lennox GH5-75M forced air gas furnace and need a little advice as to whether it is running correctly. I recently cleaned up the furnace from all the dust and cut off some wire that goes from the blower motor to the fixed limit switch to the good part that was exposed because the plastic covering was brittle and flaking off. I re-attached the good part to the limit switch. This is a pretty old furnace but seems to be running ok. It has a separate fixed limit switch and a fan off switch which has temp settings on it. It is set to 120F. I noticed the furnace ran a little off before I cleaned it up. The blower, it seems, would cycle on again about a min after the the furnace finishes a cycle. It would stay on for less than a minute before it shuts off. I'm pretty sure only the blower goes on and not the burner. After the cleaning and wire fix, the furnace ran fine for a couple days but tonight I noticed it did the quick short cycle again.

When it runs fine the thermostat would call for heat and the burners light for about a couple minutes before the blower/fan kicks on. The furnace would run until temp is reached and then the burners would shut off but the blower/fan stays on for another 2-3 mins before shutting off. The cycle repeats when thermostat calls for heat again.

I noticed now, the blower/fan would kick on a min or 2 after a cycle finishes and stays on for less than a minute before shutting off. Then the cycle doesn't start again till the thermostat calls for heat again which is normal. It's this short cycle after a normal cycle that has me a little confused. Is it normal for the furnace to do that? Could the thermostat be causing it to do that as it is a very old honeywell thermostat. Probably the original that came with the house which is a 1950s house. Another suggestion I was thinking is maybe the fan off switch is starting to go? I had a problem with the fan/ limit switch on another furnace before but the burner and blower/fan was constantly going through short cycles and not just the blower/fan going on off after the end of a cycle.

Any suggestions?

diyorpay 12-16-2011 03:22 PM

It's probably normal for an older unit. What's happening is that after the burners shut down the blower continues to circulate residual heat and to cool down heat exchanger. When it reaches the shutdown setting (you say 120) it shuts off. The remaining heat rises a little, just enough to start blower again for another, but this time short, cooling run. This is really not recycling.

Sounds like furnace is doing its job.

Spydey 12-16-2011 04:26 PM

Thanks for the advice diyorpay. The blower doesn't turn on for that short period all the time but most of the time. Perhaps at certain times the furnace cools down enough without any residual heat remaining.

biggles 12-16-2011 04:39 PM

did you slide the fan section out and wash out the fins on the squirrel cage...if there covered with dust and dirt the will not catch the air to get it thru the heat EX up into the space...yes on the fan cycles after the space satisfies jusr residual heat off the heat exchanger...keeping in mind the fan is the last thing ON when heat calls :thumbsup: and the last thing to go OFF when it satisfies:wink:..and the fan cycle is thru the limit there not the stat the limit is cycling the fan reacting to leftover heat after the burner

Spydey 12-16-2011 05:36 PM

I can't really get to the fan as the space is pretty tight, but I can see the fins slightly and they don't look covered in dust. The furnace is doing what you described exactly. The fan is the last thing on when heat calls and last thing on when satisfied. Thanks for everyone's input and advice on this matter, much appreciated. Happy holidays!!!



biggles 12-16-2011 05:54 PM

any dust around the motor air inlets should be cleaned off old tooth brush bristle brush works good,,index finger run down the fin blade on that squirrel cage is a yearly thing dust is OK normal on residentials have a good healthy new year

how 12-16-2011 07:57 PM

If you simply cut out some bad wiring and shortened the wire lengths but kept the connections the same, then unless that shortened wire is now more exposed to heat, it's all OK.

The short cycling you've described can usually be sorted out with an adjustment to the fan controls. Nudge the off temp on the fan/limit controls down a bit. (Try a couple of degrees at a time). You want the air temp coming of your heat registers to be just slightly warm to your hand. Constantly move your hand in and out of the heat register air stream when doing this test.

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