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Rick Mather
Rick Mather, HVAC Technician
Category: HVAC
Satisfied Customers: 3634
Experience:  35 years experience, HVAC
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My new furnace, a Ducane 95% efficent, was installed by a local

Customer Question

My new furnace, a Ducane 95% efficent, was installed by a local heating supply retailer. The Delta T readings were 126 duct temp, 72 input air temp, and -54 for a differential on his instrument. His comments were that my duct input and output capacity are ok because up to a 70 differential is allowed. I think the system is starving for return air. Can you enlighten me. Thank you very much. Paul
Submitted: 4 years ago.
Category: HVAC
Expert:  Rick Mather replied 4 years ago.
What size duct on return do you have and what is the drop down to return plenham
Customer: replied 4 years ago.
This is a replacement propane furnace to one that was installed in 1992 before electric heat became so expensive. Conequently there is no return duct and the heated air returns from the whole house of 1400 sq ft down thru an open stairway to the cellar. The return at the furnace is thru the base of the furnace, placed on four cement blocks, leaving an area on each side of 7.5 x 14.5 in. I output plenham is attached to the original duct work of 18 X 20 in. Please explain the use of Delta T in this analysis. Thanks
Expert:  Rick Mather replied 4 years ago.
Delta Temperature is the difference between the leaving air and the return temperatures which really does not always tell the whole story. it would be better to take transverse of duct for the CFM that is going thru unit with manometer. This will tell the exact amount of air going across the heat exchanger
Customer: replied 4 years ago.
Can't you make a judgement by the temps I gave you about duct effiency? I know that manometer readings will tell for sure but it still is all about heat transfer temps? paul
Expert:  Rick Mather replied 4 years ago.
Well with some type of chart that would state what type of temperature difference to expect I cannot make a judgement on your performance on that unit but a 54 degree rise does not sound that bad
Customer: replied 4 years ago.
My installer said the limit was 70.. is this correct for delta T?
Expert:  Rick Mather replied 4 years ago.
Sounds reasonable
Rick Mather, HVAC Technician
Category: HVAC
Satisfied Customers: 3634
Experience: 35 years experience, HVAC
Rick Mather and 4 other HVAC Specialists are ready to help you
Customer: replied 4 years ago.
Rick .. I think I was looking for a more diffinitive discription of Delta T as it relates to duct size and furnace efficiency. Paul
Expert:  Rick Mather replied 4 years ago.
I told you unless the manufacturer has a chart that gives comparison it would be meaningless information. It has to measure against a known value such as a chart.or gragh

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