Clocking a Gas Meter (It’s not that hard)

I've heard a lot made of 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. 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).
#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.
#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. If it is 1,000 (which is common for NG), the total BTU/hr would be 81,820.

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

Then, to get the ACTUAL device output in BTUs, you would multiply for the AFUE efficiency. In this case, if it were an 80% furnace, the input is 81,820 BTU/hr, and the output would be 65,456.

Pretty cool, huh?

—Bryan

Comments

david_macmahon
david_macmahon
12/10/19 at 08:26 PM

Hey Bryan,

I recently started a refrigeration apprenticeship in september. You were one of the people that inspired me to pursue this career path.

I live in Abbotsford BC, Canada and I mainly service high rise residential and small commercial equipment. I am also a journeyman plumber and I have my A ticket in Gas fitting.

I read your blog regarding clocking a gas meter and I thought you might need to add a caveat because your formula will only apply to a 1/2 psi gas meters or lower. In the Vancouver region, were I live, it is common to have 2 psi gas meters on single family homes.

Once you go above a 14″ wc meter pressure you need to apply a pressure correction factor: (actual barometric pressure + meter pressure)÷(standard barometric pressure 14.73 psia)
And a temperature correction factor:
(Actual temperature: 460°F + dry bulb temperature) ÷ (standard temperature: 460°F + 60°F)

If somebody tries to apply your formula to a high pressure gas meter they may wonder why its so under fired.

Anyways, I really appreciate all the podcasts you have been putting out.

Thank you,
David

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and tech tips
made possible by generous support from