I've heard a lot about clocking gas meters over the years, and honestly, in Florida, there isn't too much call for heat. There are even fewer furnaces.

I was pleasantly surprised when I found out how easy it actually is to clock the gas meter. Here is how you do it, step by step:

1. Make sure all gas appliances are off other than the one you are clocking. Even shut off pilot lights, or they can mess with your reading.
2. Ensure the appliance you are checking is running at high fire (max output). On a two-stage or modulating furnace, make sure it is calling for high stage, and let it run a few minutes so the reading settles.
3. Get a stopwatch (your phone has one).
4. Watch the smallest unit dial on the gas meter; it will often be 1/2 cubic ft.
5. Time how long that dial takes to make one full revolution with the stopwatch. For a steadier number, time two or three revolutions and divide the total time by the number of revolutions.
6. Multiply the dial size by 3600 (3600 is the number of seconds in an hr, so if it's a 1/2 cu/ft dial, it would be 1,800).
7. Divide that number by the number of seconds it took. So, let's say it took 22 seconds. That would be 1,800 / 22 = 81.82.
8. Multiply that number by the BTU heat content of 1 cu/ft of gas provided by the utility. 1,000 is the common round number for NG (the U.S. average is closer to 1,040), so use your utility's number when you can get it. Using 1,000, the total BTU/hr would be 81,820.

The complete formula is cubic feet per hour (CFH) = (3600 x dial size) / time (seconds)

Compare your result to the input on the furnace's rating plate. If it's off by more than a couple of percent, check manifold pressure and orifice size before you move on.

This works as written on a standard low-pressure residential meter. If the meter is fed at elevated pressure (a 2 psi system, for example), correct for it: multiply CFH by (meter pressure in psig + 14.73) / 14.73. At 2 psi, that's about 1.14, so the meter would read roughly 14% low without it.

Then, to estimate the ACTUAL heat output in BTUs, multiply the input by the furnace's efficiency. Strictly speaking, AFUE is a seasonal rating that includes off-cycle losses, so the running (steady-state) efficiency is the one that applies here. On a typical 80% furnace, the two are close, so in this case the input is 81,820 BTU/hr, and the output would be about 65,456.

Pretty cool, huh?

—Bryan

Illustration by JD Kelly