Any HVAC professionals? - Furnace limit switch question

[DizzyEdge]

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

 

[Ebtek]

Mf101s furnace has 4 settings

 

[Pogi]

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.

 

[Pogi]

I believe this setting and the fan speed are related too...

 

[whatever]

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

 

[DizzyEdge]

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]

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]

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.

 

[DizzyEdge]

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.

Yep will be doing that today.

 

[whatever]

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.