Kenmore Force-Air Furnace GAS blows cold air


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Old 01-14-10, 10:55 PM
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Kenmore Force-Air Furnace GAS blows cold air

I am having sort the following problem. My furnace is temperamental in that it blows cold air between hot cycles, such as when I turn the thermostat on, the heater will turn on with a full flame and the blower will work fine for at least about ten minutes. However, a few minutes later the heat flame will come off but the blower will continue, thus blowing cold air. A few minutes later about (10-15 minutes) the heat will kick in, again. I am under the impression that the fan and limit control is dead but I am not certain because of the following.

When I first turn the heater on, the gauge on the fan and limit control is about 50 (it runs from about 50 to 250), then when it reaches about 150, the flame shuts off. After that it goes to about a 100 within a couple of minutes and will stay there for a long time (about 10 minutes), until it kicks in again, setting on the flame and finally blowing warm air. Through out all this process the blower in on, and the thermostat is set at above the current house heat level. I am under the impression that this temperatures are not hot enought to set the fan-and-limit control off.

Its the temperature of 150 so hot to force the fan and limit control switch to shut the flames.

How can I test it to see if this is the problem? By the way, I have already changed the filter and vacuum the motor.

I have a:
Kenmore 80 power miser
Model # 867.768.080
Digital Thermostat
 
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Old 01-15-10, 08:27 PM
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Fan-Limit

Here's a link to the tech literature on the L4064 fan-limits:
http://customer.honeywell.com/techli...0s/69-0117.pdf

Pay particular attention to page 4 (settings & adjustment).

I suggest a "fan on" setting of about 125-130 & a "fan off" setting of 105-110. You need a minimum of 20 between the two settings. Do not change the factory "limit" setting.
 
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Old 01-24-10, 12:41 PM
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Got a new problem after changing fan-limit control

I finally got around to replace the fan-limit control swicth. There is no longer cold air blowing, and the heat does come on.

However, when I turn the thermostast on the furnace pilot comes up, which is ignition based, and I initially see the flames come on and the I see the dial turning on the fan-limit control to 140 (where the fan starts). Everything starts working fine, with the fan blowing and heat coming out. Within a short period of time (less than a minute), the dial jumps to 120 where it will stay for a little bit (where I enjoy some heat) before jumping down to 100 (at which time the fan and the flame will turn off). I should say that a small pilot ligth remains after that. And it takes a long time for the the full flames to come on again and start this cycle all over. I should also say that the furnace makes a noise while the pilot flame remains on, and I have not figure out the noise. It seem to be coming from the wheel fan, and not the blower.

Is this now a flame problem, or did I get a bad fan-limit control switch.


I don't know what is wrong now, please help.
 
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Old 01-24-10, 01:23 PM
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Pilot/Noise

Why the pilot would be staying on, I don't know unless the thermostat is still calling for heat. If that were the case, the burners should be on.

I don't know what you mean by "wheel fan".
 
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Old 01-25-10, 06:00 PM
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Hss anyone replaced the blower motor (and hooked up the motor using the high speed wires)?

What kind of furnace filter is in the furnace? Too porous or none at all might contribute to the problem. Might try pleated paper media filter.

Hopefully you are not observing this with the blower door open allowing abnormally cool air to enter the blower compartment.

Is part of your house where cold air returns are, practically unheated? That could also draw in too cold of air to the blower compartment, causing your fan switch drop.

You also may have a poor quality weaker burn flame due to that inducer making noise. Although you'd think the pressure switch would shut down the furnace, before even the temp dropped on the fan switch - but who knows on that one, for sure.
 
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Old 01-26-10, 10:38 PM
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Can't figure it out.

As far as I know, no one has changed the motor on the unit. Regarding the filter, I am using a 3M Filtrete filter.

I have tried to figure out what really is causing the noise, but can't pinpoint it, yet. But what really worries me, is that the heat comes on for just a little bit, then it stops within a couple of minutes (Max) once the fan-limit control drops down to the fan off position.

Is it possible that the fan-limit control swicht is bad, I am worried that I screwed the whole thing up when I replaced it. It sure does seem that the furnace is not able to maintain a fixed temperature.

Is there another swicth that might be malfunctioning.
 
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Old 01-27-10, 03:51 PM
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Filter

Pitch the 3M & get a simple glass fiber filter. Very few older systems (such as yours) can tolerate the extra resitance of the pleated filters.
 
 

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