Check gas input by clocking the gas meter

What is going on

Time one revolution of the meter's smallest dial with the furnace at steady firing, then work out the input in BTU/hr and compare it with the nameplate.

Check in this order

1. Turn off all other gas appliances in the home.
2. Run the furnace at 100% firing rate (two-stage or modulating units) and let it reach steady state, about 15 minutes.
3. Time one full revolution of the smallest meter dial with a stopwatch. Note the dial size (for example 1 cu ft).
4. Find the seconds in the gas rate table for that dial size and read the CFH value.
5. Check the CFH with the formula (3600 x dial size) / seconds = CFH.
6. Get the local heating value of the gas from the utility (average is about 1025 BTU per cu ft).
7. Calculate input CFH x BTU content = BTU/hr input.
8. If the input is not within 5% of the nameplate rating, adjust the gas valve pressure regulator or resize the orifices.
9. Turning the regulator clockwise raises pressure and input. Turning it counterclockwise lowers them.
10. Make sure airflow is within limits so the unit does not short cycle, and check temperature rise after any change.

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)