The nameplate tells you what the furnace is supposed to burn. The gas meter tells you what it is actually burning. Two minutes with a stopwatch settles it.
The setup
Turn off every other gas appliance in the house — water heater, range, fireplace, dryer — so the furnace is the only load. Let the furnace reach steady state at the firing rate you want to check.
The measurement
Find the smallest test dial on the meter (often ½, 1 or 2 cubic feet per revolution). Time one or more full revolutions. Then:
Cubic feet per hour = (3,600 ÷ seconds per revolution) × cubic feet per revolution
Input BTU/h = cubic feet per hour × heating value of the gas
Get the heating value right
Natural gas is commonly figured near 1,000 BTU per cubic foot, but the actual heating value varies by supplier and season. Use the value from the local utility for an accurate number, and note which value you used in the report.
Reading the result
Compare your measured input with the nameplate input (adjusted for altitude where that applies — not much of an issue here at sea level). Overfiring stresses the heat exchanger and can drive up CO and stack temperature; underfiring can cause condensation and poor heat. Set manifold pressure to the manufacturer’s spec and clock it again.
Why it belongs on every fall visit
Gas valves drift, orifices get swapped, and somebody may have turned the manifold pressure up to fix a comfort complaint years ago. Combustion readings mean much more when you know the input is right. Clock the meter first, then analyze.