Combustion and input check

What is going on

Combustion and input check means finding out how much gas the furnace is actually burning. You do it by timing the gas meter with only the furnace running, then converting to input and comparing to the nameplate. It sounds fiddly but it is simple arithmetic, and it tells you quickly if the unit is over or underfired. Overfiring shortens heat exchanger life and can create unsafe combustion.

Likely causes

• Gas input above the nameplate rating
• Other gas appliances left on during the test
• Dial size misread or wrong conversion
• Wrong orifices or high manifold pressure
• Utility heating value assumed instead of read from the bill

Check in this order

1. Make sure all other gas appliances are off, including pilots.
2. With the furnace running, clock the gas meter for one revolution and note the dial size.
3. Find the clocked seconds in the flow table and read the cfh on the same row.
4. Adjust the reading for dial size a 1 cu ft dial divide by 2; a 1/2 cu ft dial divide by 4; a 5 cu ft dial multiply by 10 and divide by 4.
5. Multiply the result by the gas heating value from the utility and compare to the nameplate. It must not exceed the nameplate rating.
6. If input is too high, check manifold pressure and orifice size. Gas adjustments are for a licensed technician.
7. Relight other appliances when done.

More furnace guides

• Abnormal conditions and protection
• Adjust gas valve manifold pressure
• Air for combustion and ventilation in non-direct vent installs
• Airflow adjustment check
• Airflow and temperature rise check
• Bottom return filter rack installation (upflow)