I understand how to check, what i dont understand is why it isnt more important to check that the outgoing temp is under the specified max. How would it even be possible that if you had an outgoing temp of 140 degrees to have a return temp of 110 (low end of spec)??? How can you have a return temp any higher than room temp? I opened up some return air flow to bring temp from 157 to 140 outgoing, my return temp is 75, only because I removed a return trunk cover close to a feed trunk so it is getting some warmer air in the return. I understand the reason for checking this is so you do not cook the heat exchanger but why is return air temp even an issue? Bottom line is I now have a temp rise of 65, which is close to spec. Can someone shed some light on this??
measuring temp rise is a way of checking for proper air flow and fan speed. you should first check the manifold pressure and make sure its right. a hi temp rise is usually caused by a dirty a-coil, plugged filter, plugged secondary heat exchanger or a dirty blower wheel. (all causes of low air flow) as far as the hi limit spec....the furnace will kill the burner if the hi limit switch is tripped......usually 3 trips will cause a 3 hr lockout. Possum, hvac guy
