Any HVAC professionals? - Furnace limit switch question

DizzyEdge

Vax me Trudaddy
OT Supporter
Dec 16, 2007
22,058
Calgary, AB
I have an old Lennox furnace and the limit switch has 3 settings which are configured as

Fan off - 100 F
Fan on - 120 F
Kill switch - something high I forget.

Anyway, my understand is it's designed that once the heat exchanger heats up to 120 F it will start the fan, and once the burners turn off and it cools down it will keep blowing until 100F

Is there any reason to have those set so high?
My thermostat is set to 75F in the winter so wouldn't 80F and 100F make more sense than 100F and 120F?

Thanks
 

Pogi

Official Pointy Boot Guey Ver 1.0
Jan 3, 2001
11,811
North Of Chi-town
I wouldn't touch it....it's set that way for a reason.

If set fan to come on too low it cools down the heat exchanger and the temp coming from your vents won't be as warm.
 

whatever

OT Supporter
Feb 18, 2004
218,574
you need a delta to move heat. it needs to be that high.

are you having high outlet temps? turn the fan speed up then. more CFM
 
TS
TS
DizzyEdge

DizzyEdge

Vax me Trudaddy
OT Supporter
Dec 16, 2007
22,058
Calgary, AB
you need a delta to move heat. it needs to be that high.

are you having high outlet temps? turn the fan speed up then. more CFM

The actual problem I'm having is when the burners turn on and the heat exchanger heats up the fan is turning on at the FAN ON setting (as expected) but once they turn off and things cool down the fan is turning off at that same FAN ON setting rather than going past it and turning off at the lower FAN OFF. I'm assuming this means a faulty limit switch but while I was inspecting it I thought the temps that were set both seemed a bit high so I figured I'd ask.
 

Fanya

OT Supporter
Jun 23, 2005
10,262
I'm sure the people that engineered the system had no idea what they were doing and picked those values out of thin air.
 

Pogi

Official Pointy Boot Guey Ver 1.0
Jan 3, 2001
11,811
North Of Chi-town
The actual problem I'm having is when the burners turn on and the heat exchanger heats up the fan is turning on at the FAN ON setting (as expected) but once they turn off and things cool down the fan is turning off at that same FAN ON setting rather than going past it and turning off at the lower FAN OFF. I'm assuming this means a faulty limit switch but while I was inspecting it I thought that temps that were set both seemed a bit high so I figured I'd ask.

Brother, just call a HVAC guy...you dont want your heat exhanger to crack due to wrong temp and leak carbon monoxide into your home.
 

whatever

OT Supporter
Feb 18, 2004
218,574
The actual problem I'm having is when the burners turn on and the heat exchanger heats up the fan is turning on at the FAN ON setting (as expected) but once they turn off and things cool down the fan is turning off at that same FAN ON setting rather than going past it and turning off at the lower FAN OFF. I'm assuming this means a faulty limit switch but while I was inspecting it I thought the temps that were set both seemed a bit high so I figured I'd ask.

pro territory IMO.
 
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