Check duct static pressure

What is going on

Adds the return and supply static readings to get total external static pressure, then compares it with the blower tables.

Likely causes

• Too much static can cause insufficient airflow, high temperature rise and limit tripping.
• Too little static can overload the motor.
• Closed dampers or registers, and undersized or poorly laid out ductwork, are common causes.

Check in this order

1. With clean filters installed, use a draft gauge (inclined manometer) to measure return static at the furnace inlet. This is a negative reading.
2. Measure supply duct static. This is a positive reading.
3. Add the two readings together for total external static pressure. Include any coil or electronic air cleaner in the total.
4. Compare the total with the allowable minimum and maximum in the blower tables.
5. If the total is outside the limits, check for closed dampers or registers and for ductwork problems.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)