Heat Differential Question

I’ve been over this before but its been awhile and my brain is frosted.

Manufacturers range - 45 - 75 degrees. Return air reads 64 degress and the supply air reads 107. So the rise is beyond the 30 degree range (its 43 degrees). Now, what does this tell us?

Thank you,


Airflow too high, wrong fan speed, poor duct design, gas pressure low, filter completely missing, among lots of other things.

Whats the source of fuel

Right… oil… nozzle size too small… gas… low manifold pressure, heat pump, low on refrigerant or low outdoor ambient, electric, one or more elements failed, coal… stoker control failing or fuel low.

Wood…the old lady is getting tired of chopping it and bringing it into the house.:wink:

It’s gas and thank you Marc, I’ve got those same resons and then some in a pile of notes somewhere.



Easier to just refer for further evaluation, etc., but it seems that, like me, you want to know WHY!!!

Poor duct design? Is that 107 degrees consistent throughout the system?

And one thing that hasn’t been mentioned is that the hi-low limit may be malfunctioning…the kick-in temp for the fan may be too high because of this. They CAN be calibrated, but usually just get replaced by HVAC techs to avoid the possibility of call-backs.

The key to your problem here is that the return air temperature is already too high.
What is your setpoint temperature, 68? The return is 64!
It’s like trying to figure out temperature drop in an air conditioner in the wintertime when the heat is off in the house!
Forget about all these diagnostic formulas. Just turn it on and see if it runs.

Gas heat you say? It’s fine.

From many gas furnaces Ive expected in Texas in the summertime I get reading this high all of the time.