Furnace limit swtch function


Old 10-26-01, 07:12 PM
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Gas fired, forced air...

After replacing pressure switch, and installing new thermostat(conventional), the furnace appears to be working normally. I'm a bit confused about the cycle of events concerning the fan on/limit/fan off switch(es). Today, with outside temps running 65 f (afternoon) to 43 f at 11:00 pm, the furnace maintained 70 f living room temp at, at 70 f thermostat setting... LIMIT is set at 150 f, FAN ON at 125 f, FAN (OFF) at 90 f... (by Sears instruction manual)
The fan does come on at 125 f, the gas turns off, then the switch assembly returns to the 90 f , fan off, setting. I thought the furnace was supposed leave the gas on until the switch rotated to the 150 LIMIT position, turn the gas off, let the fan run until rotating the assembly came down to the 90 f, then turn the fan off, then await the next instant of thermostat implementing the next heating cycle. Is there something wrong with this picture?

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Old 10-26-01, 11:36 PM
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Limit at 150 sounds low,should be around 180---this should shut gas off.Fan on should be around 140--this starts fan and should not shut gas off.90 degrees should shut fan off---then waiting for next call from tstat the entire sequence should start over again.PDF
Old 10-27-01, 12:23 AM
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Originally posted by EEEHHHH
Gas fired, forced air...

Will change the settings to Your recommendation ...

Thank You

Old 10-28-01, 10:51 AM
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Did you replace the fan control/limit switch?? if so, you may of wired it wrong??
Old 10-28-01, 04:23 PM
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Originally posted by Jay11J
Did you replace the fan control/limit switch?? if so, you may of wired it wrong??

Fan control/limit switch and wiring NOT changed ...
Old 10-29-01, 03:51 PM
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You need to set the heat anticipator in the replacement thermostat to match the circuit current draw. They usually come set at 0.2 amp. Measure the amp draw on the r and w terminals adjust yours to the measured setting.
Old 11-03-01, 07:37 AM
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In energy efficient furnaces the heat exchanger design is much different than older models. They accomplish more efficiency by increased surface area of exchange and burner efficiency. This accounts for the settings on you limit switch. When there is a demand for heat in our home the furnace comes on and heats the exchanger. When the exchanger reaches 125*F the fan comes on, minimum operating temp. Once the exchanger reaches 150*F and if the demand is not met, the gas goes off and the fan keeps going. Once the demand for heat is met, the fan stays on and drops the temperature of the exchanger to 90*F. In older models because of the poor design of the heat exchanger the minimum operating temp is 140*F and their high limit has to be higher, such as 180*F. This is not true with energy efficient furnaces. Furthermore, with the older models, the residual heat at the exchanger is allowed to dissipate, not true with efficient models. Purpose for 90*F setting.

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