Fan and Limit Control Adjustment headache!

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Old 02-11-12, 11:11 AM
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Fan and Limit Control Adjustment headache!

Delighted to find this forum! Thanks for being here!
Have said a few bad words trying to figure this out myself. I have a Lennox model G12D-110 gas furnace.
--First problem to surface was that the fan stayed on all the time unless power was shut off to the furnace. Burners would not come on.
--After power switched off and on a few times, fan did not come on when power was switched on. When thermostat called for heat, burners came on, but fan did not come on after more than a minute. I switched power off to the furnace fearing that it'd get too hot.
--Reading about how gas furnaces work, I suspected the Fan and Limit Control unit was faulty, and bought what I understood from a furnace repair shop to be an appropriate replacement unit. It is a Cam-Stat Replacement SPST Combination Fan and Limit Control, with a 7" extension. The literature on it says it can be used to replace all SPST FAL 7 series combination fan and limit controls.
--Old unit was set for fan off at 90 degrees, so I set the fan off pointer (white plastic thing on front) to 90.
--Did not notice if old unit had adjustable "limit" setting on the 7" extension. Left unit at furnace shop, they're closed for a few days now. New unit limit adjuster was set at 200 degrees.
--The third adjustment needed is something called the Fan Differential, a rotating dial that the literature says has a range of 25 to 50 degrees. At first I set it near its counter-clockwise limit.
--When I installed the FAL switch, and took care to wire it up exactly as the previous unit had been wired, turned on power to the furnace, with the thermostat not demanding heat, all was quiet. So far so good.
--Had the thermostat demand heat, gas burners came on. Fan still was not on after about 90 seconds. I worried that this was too long to wait for the fan, so, with the burner going, I played with the rotating fan differential dial, turning it clockwise, and the fan came on. I let the furnace run for a few minutes, then adjusted the thermostat to allow furnace to shut off. Gas stopped, fan kept running, when it was blowing cold air into the house I figured I hadn't adjusted it right, so turned the fan differential knob back counter-clockwise until fan went off.
--Powered up again, and again I feared fan didn't come on soon enough, so at about 90 seconds, I turned the differential knob back clockwise until fan came on. Again fan didn't turn off by the time furnace cooled down.
--I figured one of the other two adjustments weren't right, so played with the fan off plastic dial, but no improvement in functioning.
--Pulled the FAL unit out, adjusted the limit slider from 200 to 175 degrees, put it all back together.
--The story goes on, with no success, more adjustments to the limit slider, to 160, then 225, until finally I'm pretty sure I don't know what the heck I'm doing. Now burners come on, fan comes on, then abruptly goes off for 5 seconds and comes on again two or three more times before it stays on....
--I'm in a hole and decided to stop digging and ask for help. HELP!

Thanks a million!
 
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Old 02-12-12, 12:54 AM
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Ninety degrees is a bit low for the fan off setting, should probably be more like 100 or so as 90 WILL feel cool when blowing on a person. The differential is how far above the low setting the temperature must be in order for the fan to switch on; try setting it to the 25 mark which with a 100 degree fan off setting will have the fan come on at about 125. Understand that these calibration markings are pretty sloppy and could be off by several degrees.
 
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Old 02-12-12, 08:37 AM
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I understand your frustration with the fan on & off several times before finally staying on. Let me start by explaining the fan operation:
The fan is controled by the temperature of the air passing the Fan/Limit.
Shortly after the burners fire there is sufficient heat to bring the fan on.
When the fan comes on, you're now moving cool room air past the fan/limit which can result in it's cooling to the point of shutting the fan off. This is particularly true if the house is unusually cold or the main duct goes thru an unconditioned space.
After a few cycles the burners have now produced enough heat to hold the fan on.

You might be able to reduce the number of on/off cycles by increasing the differential setting. It will take longer for the fan to come on initially. I'd start with a fan off setting of 100[SUP]o[/SUP] & a differential off 40[SUP]o[/SUP].
 
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Old 02-12-12, 10:41 AM
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-Pulled the FAL unit out, adjusted the limit slider from 200 to 175 degrees, put it all back together.
--The story goes on, with no success, more adjustments to the limit slider, to 160, then 225, until finally I'm pretty sure I don't know what the heck I'm doing. Now burners come on, fan comes on, then abruptly goes off for 5 seconds and comes on again two or three more times before it stays on....
--I'm in a hole and decided to stop digging and ask for help. HELP!


How cold is it in your house, the fan off cycle may be due to cold air going across the fan switch until the room is warm enough to keep the blower on. Does your fan/limit have 4 or 6 wires?
 
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Old 02-12-12, 12:07 PM
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Thanks, ray2047, Furd, Grady, and mbk3! I'm about to attempt the adjustment. It's actually at a house across town. I'll post the results when I get back.

Somebody told me the "limit" slider should be set to the limit temp identified on the furnace nameplate. I'll do that first, then try putting the other controls at the levels you suggest. mbk3, the FALC unit has 4 wires: one to the gas valve, and looks like the other 3 go to the fan.
 
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Old 02-12-12, 06:05 PM
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Ok, some hours later: Set limit slider to same temp, 200, as on furnace nameplate. Set fan off temp at 100. Fan differential dial had no numbers, just hi, low, and off. As its range is said in the literature that came with it to be 25 to 50 degrees, I set it first half-way between hi and low.

Tried many different combinations of the fan off and fan differential settings, always got some version of this: Burners come on, then, after 60 sec to 130 seconds, depending on the settings, the blower comes on, then 10 sec later it goes off for 10 sec, on for 15 sec, off for 10, on for 20, off for 5, on for 40, off for 5, then it stays on.

Then when house is warmed to desired temp, burner goes off, fan stays on for 60 sec, then 15 sec later it comes on again for 15 sec, off for 20, on for 4, off for 60, on for 3, off for 2 min, on for 3, then finally stays off.

The first thing I really need to know here is if this seems a safe way to let it perform for a while.

I'm pretty sure I'm going to replace this furnace soon, but I'd prefer to wait a couple of months. My guess is that the FALC unit is NOT the correct replacement for the one that used to work fine in this furnace. Can anyone think of anything else that might cause the strange on-again, off-again fan behavior? It happened when the house was cold, and also when the house was at 72 degrees. The return air duct travels only 6 feet from the house interior to the furnace, and it's insulated. A furnace guy who wants to sell me a new furnace said it's oversized, at 110,000 BTU, he says he'd install an 80k unit.

Any ideas? Thanks a million, guys, again, for taking the time to help me solve this pesky problem!
By the way, Furd, I'm also on the wet side of Washington State, in Olympia.
 
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Old 02-12-12, 09:55 PM
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Several things come to mind that might be causing this somewhat erratic operation but I don't think it is anything that is inherently dangerous.

Your furnace is likely oversized as almost all are, especially if any energy conservation measures (insulation and/or air sealing) have been done since the house was built. The original furnace in my house was rated at 60,000 BTU and I get along just fine with a 40,000 BTU, even during that cold snap we had a few weeks ago and nighttime temperatures in the low 20's.

BTW, I'm in Bothell, about 10 miles NE of Seattle.
 
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Old 02-13-12, 03:21 PM
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I concur with all of Furd's comments but do have a couple of questions:
Are the insertion depths the same on both the old & new controls?
You didn't by any chance drop a shield which was in place by the old control did you?
Often furnace manufacturers put a piece of sheet metal beside or under the fan limit to either deflect the air flow or reduce the radiant heat from the heat exchanger.
 
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