Clocking a gas meter

What is going on

Clocking the meter tells you how much gas the furnace really burns, which feeds the BTU input calculation in the heating performance test. Run the furnace at 100% firing rate until it reaches steady state, usually about 15 minutes, then time one revolution of the test dial. Meters that are shared with other appliances give junk numbers, so isolate the furnace first.

Likely causes

• Heating performance test needs an actual input figure
• Suspected over-fire or under-fire after a gas conversion or altitude change
• Complaints of weak heat or flame problems where manifold pressure alone is not conclusive

Check in this order

1. Turn off all other gas appliances in the home, including water heater pilots that may cycle on, and check the meter dial is not moving
2. Turn on the furnace and confirm it runs at 100% firing rate, not low stage
3. Wait about 15 minutes of steady operation so the burners and heat exchanger settle
4. Time one full revolution of the smallest dial with a stopwatch. The one cubic foot dial is used in the example
5. Find the seconds for one revolution in the gas rate table and read CFH from the matching dial column
6. Example 40 seconds on the 1 cu ft dial gives 90 CFH
7. Multiply CFH by the gas heating value to get BTU/h and compare with the rating plate

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)