Heating performance test

What is going on

Before you chase a fault code on a furnace that runs but does not heat well, find out whether it is actually putting out what the rating plate says. A heating performance test does that. The input should be within 5% of the BTU shown on the unit rating plate. Airflow has to land inside the temperature rise range on the spec sheet, and external static pressure should be around 0.5 in water column. Do it on a clean furnace with a clean filter, and you will often find the real problem is gas rate or airflow rather than a bad part.

Likely causes

• Gas valve manifold pressure off, so the input is high or low
• Wrong or partly plugged burner orifices
• Local gas heating value different from what the furnace was set up for
• Restricted airflow from a dirty filter or undersized ductwork, throwing off temperature rise
• External static well above 0.5 in water column

Check in this order

1. Confirm the local natural gas heating value (BTU content) with the utility, since it varies by region
2. Turn off other gas appliances, then clock the gas meter to find the gas rate in cubic feet per hour
3. Calculate the BTU input from the gas rate and heating value and compare it to the rating plate. It should be within 5%
4. Measure supply air and return air temperature at steady state and work out the temperature rise. Compare to the range on the spec sheet
5. Measure external static pressure (about 0.5 in water column) using the service manual procedure
6. If input is off, check manifold pressure and orifices before touching anything else. Gas adjustments are for a licensed gas technician

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)