Check airflow with a temperature rise measurement

What is going on

Use this check to confirm airflow over the heat exchanger. Temperature rise should sit near the middle of the range on the furnace nameplate. Too high means airflow is too low. Too low means airflow is too high.

Likely causes

• Restricted duct system or undersized ducts (high rise)
• Blower speed tap set too low (high rise)
• Blower speed tap set too high (low rise)

Check in this order

1. Place a thermometer in the supply duct as close to the furnace as possible, out of direct line from the heat exchanger.
2. Place a thermometer in the return duct as close to the furnace as possible.
3. Run the furnace at least 15 minutes in gas heat mode.
4. When the supply temperature stops rising (about five minutes), subtract return air temperature from supply air temperature. The result is the temperature rise.
5. Compare the result to the approved range on the nameplate. Adjust the rise to the middle of the range as closely as possible. Never let it go above or below the range.
6. If the rise is above the range, raise airflow by increasing blower speed, removing duct restrictions, or adding supply or return duct.
7. If the rise is below the range, use a lower speed tap on the multi-speed blower.

More furnace guides

• Annual and start-of-season inspection
• Blower codes 66, 61 and 60 - what to look for
• Blower does not start after burners light
• Burners go out before the heat call ends / short cycling
• Canada only: derate high altitude by lowering manifold pressure
• Cement PVC vent joints