Original message
| FixitKB
 |  "Fan Limiter for Lennox Gas Furnace"
, posted Tue 4 Jan 11:37  
I have a 22 year old Lennox gas furnace. My symptoms are:
1) burners ignite,
2) after 10-15 secs blower starts,
3) blower runs for about 10 secs then shuts off,
4) burners continue for another 15 secs and then shut off,
5) blower starts and runs for about 5-10 secs.
These steps would repeat over and over and over until the house temperature was met. I would imagine this is real inefficient.
I checked the fan limit switch thinking that the lower limit was too high and the blower was being shut off prematurely. I have a Honeywell limit switch. It has 3 "pin" settings. The 3 are set at:
1) low (blower off) was at 80 degrees,
2) middle (blower on) is at 140 degrees,
3) high (i'm guessing this is burner off??) is set at 200 degrees.
I moved the low pin to 75 degrees. When I crank the thermostat up this did allow the blower to remain on alongwith the burners for an extended period of time. At this point I thought I solved the problem. But when the furnace is just maintaining a temerature (thermostat not being cranked up) it still cycles through steps 1-5 above.
Is this a possible thermostat problem?? I have a standard mercury switch honeywell wall thermostat. I know how thermostats allow the temp to dip below the setting and then the furnace will heat to a couple degrees beyond the thermostat setting. I was wondering if this hysteresis was too narrow in my thermostat.
A also noticed that when the burner and blower are on simultaneously the fan limit switch hovers at the 90 degrees mark. This seemed very low to me. Could my fan limit switch be bad??
Thanks for your time and expertise.
In Search of Knowledge
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Replies:
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| zekeman
| "Re(3):Please help....freezing!!" , posted Tue 4 Jan 17:51
The blower stopping while the burner is running is probably due to a malfunction of the low limit switch. Although I'm puzzled that you say the system works ok on a large difference between the setpoint on the tstat and the room temp and doesn't on maintaining temp.
First, reset the low limit deliberately low to say 50 degrees and retest. If problem goes away you have a low limit switch problem. If not, measure the temperature of the air after the heat exchanger to see if it is normal. If so then I would look for a thermostat problem.
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| | FixitKB
| "Re(4):Please help....freezing!!" , posted Wed 5 Jan 10:52
If the limit switch is bad and I buy a new one, what are the correct settings for the "pins"??
In Search of Knowledge
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| | zekeman
| "Re(3):Please help....freezing!!" , posted Thu 6 Jan 11:56
I would set them on 80, 140, 200 for openers. If the warm air feels too cold after the furnace shuts down (note: the fan keeps blowing until the temp drops below the low limit value), then increase the low limit to say 90.
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| | FixitKB
| "Re(4):Please help....freezing!!" , posted Thu 6 Jan 12:11
I set the low limit to 50 (as you suggested) and retested. Now the furnace sequence is: 1) burners ignite, 2) after 15 secs blower starts up, 3) after a period of time of burners and blower running the burners stop, 4) blowers continue to run until low temp limit is reached and blowers stop. This appears to be standard operating procedure.
So it looks like my fan limit control is bad according to the results I get (normal operation) when I set the low limit to 50 degrees.
One thing I've noticed though while testing the fan limit at 50 degrees is that if my thermostat is set at 70 degrees, lets say the room temp is 68 degrees, the furnace kicks on and does steps 1-4 above. After the blower shuts off the room temp is still not at 70 degrees. I'm thinking my thermostat is broke also. Why else would it stop calling for heat and stop the burners until the setpoint temp was reached (and even a couple of degrees beyond)?? Does this sound like a bad thermostat to you?? I currently have a standard Honeywell mercury switch.
I'm thinking I have a bad limit control and bad thermostat. If thats your opinion also, do you have suggestions on where to purchase these parts at a good price??
Thanks again for your time and expertise.
In Search of Knowledge
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| | FixitKB
| "Re(4):Please help....freezing!!" , posted Wed 5 Jan 10:40
I will try what you suggested to isolate the limit switch.
When I measure the temperature after the heat exchanger what should I expect to see??
In Search of Knowledge
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