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Furnace gas input clocking calculator
Clocking the gas meter tells you what a furnace is really burning. With everything else on the meter off, time the test dial, turn the seconds into cubic feet per hour and multiply by the heating value of the gas. A 1/2 cu ft dial that takes 22 seconds per revolution passes 81.82 cubic feet per hour, which is 81,818 BTU/h at 1,000 BTU per cubic foot. Against an 80,000 BTU/h rating plate that is 2.3 percent high, outside the 2 percent the Carrier instructions allow. Above 2,000 ft, clock against the derated input from the furnace altitude derate calculator. Once the input is right, check the airflow with the furnace temperature rise calculator.
- Turn off every other gas appliance and pilot served by the meter. Call for heat and let the furnace run at least 3 minutes, at high fire on a two-stage or modulating furnace.
- Pick a test dial and note its size. Time one full revolution with a stopwatch, or two or three revolutions for a steadier number. Get the heating value of the gas from your gas supplier.
- Enter the dial, the seconds, the revolutions and the heating value, plus the rating plate input or the derated input at altitude. The result gives the input in BTU/h and whether it is within 2 percent. Adjust as the installation instructions say and clock again.
Example result for the default inputs
1/2 cu ft dial, 22.0 s for 1 revolution, 1,000 BTU/cu ft.
The clocked input is +2.27% from the 80,000 BTU/h you expect, outside the 2 percent window in the Carrier instructions, so the furnace is over-firing. Confirm the heating value and the timing first. Carrier then says to lower the manifold pressure and clock again. At a different heating value or altitude its instructions also have you check the orifice size against their tables, and they say never to redrill an orifice.
Clocked input
81,818 BTU/h
Gas flow
81.82 cu ft/h
Against the expected input
Above the 2 percent window
| Test dial | 1/2 cu ft |
|---|---|
| Time timed | 22.0 s for 1 revolution |
| Seconds per revolution | 22.0 s |
| Gas flow at the meter, 3600 x dial x revolutions / seconds | 81.82 cu ft/h |
| Heating value | 1,000 BTU/cu ft |
| Clocked input, flow x heating value | 81,818 BTU/h |
| Expected input | 80,000 BTU/h |
| Difference | +2.27% |
| Carrier window, +/- 2 percent | 78,400 to 81,600 BTU/h |
| Seconds per revolution at the expected input, 1/2 cu ft dial | 22.5 s |
- The rating plate input applies up to 2,000 ft. Above that, compare the clocked input with the derated input from the installation instructions, not the plate figure.
- The timing is short, and one second of error is 4.5 percent of 22.0 s. Time two or three revolutions (HVAC School), or use a larger dial. Carrier says the 2 or 5 cubic feet dial gives a more accurate reading.
- Turn off every other gas appliance and pilot on the meter, run the furnace at least 3 minutes (Carrier) and at high fire on a two-stage or modulating furnace, then time the dial. Use the heating value your gas supplier gives for your area, on the pressure base your meter reads: at high elevation, ask the supplier which base its figure uses.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the gas input is calculated from the meter
Clocking a gas meter measures the input a furnace is actually burning. With every other gas appliance off, the test dial on the meter turns only for the furnace. Time one or more revolutions, turn the time into cubic feet per hour, and multiply by the heating value of the gas. Compare the result with the input on the rating plate, or with the derated input at altitude. Turned around, the same relation gives the seconds per revolution to look for before you start.
- Gas flow
CFH = 3600 x dial size x revolutions / seconds3600 is the seconds in an hour, and the dial size is the cubic feet one revolution of the test dial passes. This is the HVAC School formula and the relation behind Carrier Table 20. A 1/2 cu ft dial at 22 seconds passes 81.82 cubic feet per hour. Carrier lists 80 for 90 seconds on the 2 cu ft dial.- Clocked input
input, BTU/h = CFH x heating value, BTU per cubic footCarrier's step f multiplies the gas rate by the heating value to obtain the input rate. 81.82 cubic feet per hour at 1,000 BTU per cubic foot is 81,818 BTU/h, the HVAC School result of about 81,820.- Check against the expected input
difference = clocked / expected - 1; within: |difference| <= 2%The Carrier instructions say the input must be within plus or minus 2 percent of the rating plate input, or as adjusted for altitude. For 80,000 BTU/h that window is 78,400 to 81,600 BTU/h. The default reading, 81,818 BTU/h, is 2.3 percent high. The page compares the clocked input with that window, not a rounded percent.- Seconds to look for
seconds per revolution = 3600 x dial size x heating value / inputLennox writes the formula for the 1 cubic foot dial, and its example is 1,000 BTU gas at 45,000 BTUH: 80 seconds for one cubic foot. A 80,000 BTU/h furnace at 1,000 BTU per cubic foot turns the 2 cu ft dial in 90 seconds, and the 2 percent window is 88.3 to 91.8 seconds.- Meter pressure
factor = (meter pressure, psig + 14.73) / 14.73For a meter above the standard low pressure, HVAC School multiplies the flow by this factor. At 2 psig it is 1.14, so the true flow is about 14 percent above the uncorrected reading, which reads about 12 percent low. Leave the field blank on a standard house meter and the factor is 1.
Worked examples from the published sources
Eight cases below run through the same functions the form uses. The first comes from HVAC School, the Carrier row from Table 20 of the 59SC5A instructions and the target from the Lennox example. The 80,000 BTU/h plate and the under-firing furnace are examples chosen for the page.
| Case | Setup | Result |
|---|---|---|
| HVAC School reading | 22 s for 1 revolution of the 1/2 cu ft dial, 1,000 BTU/cu ft | 81,818 BTU/h (81.82 cu ft/h) |
| Same reading against an 80,000 BTU/h plate (plate chosen for the page) | 22 s for 1 revolution of the 1/2 cu ft dial, 1,000 BTU/cu ft, expected 80,000 BTU/h | Above the 2 percent window: 81,818 BTU/h (81.82 cu ft/h), +2.27% |
| Same flow, two revolutions timed | 44 s for 2 revolutions of the 1/2 cu ft dial, 1,000 BTU/cu ft, expected 80,000 BTU/h | Above the 2 percent window: 81,818 BTU/h (81.82 cu ft/h), +2.27% |
| HVAC School reading on a 2 psig meter | 22 s for 1 revolution of the 1/2 cu ft dial, 1,000 BTU/cu ft, 2 psig | 92,927 BTU/h (92.93 cu ft/h) |
| Carrier Table 20 row at the 1,050 BTU/cu ft of its orifice example | 90 s for 1 revolution of the 2 cu ft dial, 1,050 BTU/cu ft | 84,000 BTU/h (80.00 cu ft/h) |
| Under-firing furnace (example chosen for the page) | 50 s for 1 revolution of the 1 cu ft dial, 1,000 BTU/cu ft, expected 80,000 BTU/h | Below the 2 percent window: 72,000 BTU/h (72.00 cu ft/h), -10.00% |
| Lennox example | 45,000 BTU/h, 1,000 BTU/cu ft, 1 cu ft dial | 80.0 seconds per revolution |
| 80,000 BTU/h furnace, 2 cu ft dial | 80,000 BTU/h, 1,000 BTU/cu ft, 2 cu ft dial | 90.0 seconds per revolution |
Seconds per revolution at 1,000 BTU per cubic foot for round inputs. At another heating value the seconds scale with it: at 1,040 every figure is 4 percent longer.
| Input, BTU/h | 1/2 cu ft dial, s | 1 cu ft dial, s | 2 cu ft dial, s | 5 cu ft dial, s |
|---|---|---|---|---|
| 40,000 | 45.0 | 90.0 | 180.0 | 450.0 |
| 60,000 | 30.0 | 60.0 | 120.0 | 300.0 |
| 80,000 | 22.5 | 45.0 | 90.0 | 225.0 |
| 100,000 | 18.0 | 36.0 | 72.0 | 180.0 |
| 120,000 | 15.0 | 30.0 | 60.0 | 150.0 |
Carrier's Table 20 runs the other way, from seconds to cubic feet per hour on the 1, 2 and 5 cubic feet dials. The tests check all 267 of its entries against the formula: 256 match it rounded to the whole cubic foot and the other 11 are within 1.4 percent, truncated or misprinted (51 seconds on the 5 cubic feet dial prints 355 for 352.9).
Standards and data sources
- Carrier 59SC5A gas furnace, Installation, Start-up, Operating and Service and Maintenance Instructions (59SC5A-13SI) - the clocking procedure (all other appliances and pilots off, run the furnace 3 minutes, time one revolution, the 2 or 5 cubic feet dial is more accurate), Table 20 of cubic feet per hour for 10 to 180 seconds on the 1, 2 and 5 cubic feet dials, the 2 percent input tolerance and the note that the plate input applies up to 2,000 ft. The tests check all 267 entries of Table 20 against the formula.
- Lennox conversion and high altitude kit instructions for UHRS/LF unit heaters (507941-01), section E, Means To Verify Gas Rate - the seconds for one cubic foot from the heating value and the input, with the example of 1,000 BTU gas at 45,000 BTUH giving 80 seconds, the advice to get the heating value from the gas utility and the note on temperature and pressure correction factors for the meter.
- HVAC School, Clocking a Gas Meter (It's Not That Hard) - the formula CFH = 3600 x dial size / seconds, the worked example of a 1/2 cu ft dial at 22 seconds, timing two or three revolutions, the US average heating value near 1,040, and the meter pressure correction (psig + 14.73) / 14.73.
Limitations of this calculator
- The heating value is yours to enter. The page has no utility data, and 1,000 BTU per cubic foot is the round number in the examples. Your gas supplier publishes the figure for your area.
- Natural gas through a meter only. The heating value field stops at natural gas figures. For propane, follow the conversion kit instructions for the furnace, as Carrier directs.
- The rating plate input applies up to 2,000 ft in the Carrier instructions. Above that, compare the clocked input with the derated input the installation instructions give, which the furnace altitude derate calculator works out.
- No meter temperature correction is applied. Lennox says to use temperature and pressure correction factors for the meter, and the page applies only the pressure factor, and only when you enter a meter pressure.
- The meter pressure factor is the HVAC School correction to a 14.73 psia base. Your gas supplier may bill on its own pressure base, so ask for its factor on an elevated pressure meter.
- Any other burner on the meter adds to the reading. A pilot, a water heater or a range left on makes the furnace look over-fired.
- Two-stage and modulating furnaces fire at more than one rate. Clock at the rate you are checking, normally high fire, and compare with the input for that rate.
- Clocking checks the gas input only. Manifold pressure, orifice size, flame appearance and combustion are separate checks in the installation instructions. The page makes no claim of code compliance, and the manufacturer's instructions and the authority having jurisdiction decide the setup.
Common gas meter clocking mistakes
- Leaving another appliance or pilot on. Carrier's first step is to turn off all other gas appliances and pilots served by the meter. A water heater that fires during the timing adds its whole input to the dial and the furnace looks over-fired by that much.
- Entering the wrong dial size. The size is printed beside each dial. Taking the 22 second reading on the 1/2 cu ft dial as if it were the 2 cubic feet dial gives 327,273 BTU/h, not 81,818. Read the label on the dial you timed.
- Timing too short. One second of stopwatch error on a 22 second reading is 4.5 percent, more than the 2 percent window: 85,714 BTU/h at one second less. Three revolutions in 66 seconds cut that to 1.5 percent. Carrier's table starts at 10 seconds per revolution and recommends the larger dials.
- Using 1,000 BTU per cubic foot without asking. HVAC School puts the US average heating value closer to 1,040 than to 1,000. That 4 percent is twice the 2 percent window: the default reading is 81,818 BTU/h at 1,000 and 85,091 BTU/h at 1,040. Get the local figure from the gas supplier before you touch the manifold pressure.
- Comparing with the plate input at altitude. The plate input is for installations up to 2,000 ft. Above that the installation instructions reduce it, and a furnace clocked at the plate figure there is over-fired. Work out the derated input first, then clock against it.
- Skipping the pressure factor on an elevated pressure meter. At 2 psig the meter passes more gas per revolution than the dial says. HVAC School's factor is 1.14, so the 81,818 BTU/h reading is really about 92,927 BTU/h.
Common questions about clocking a gas meter
How do you clock a gas meter on a furnace?
Turn off every other gas appliance and pilot on the meter. Run the furnace at least 3 minutes, at high fire on a two-stage unit. Time one or more full revolutions of a test dial with a stopwatch. Carrier says the 2 or 5 cubic feet dial is more accurate, and two or three revolutions of a small dial do the same. Then divide 3600 times the dial size times the revolutions by the seconds to get cubic feet per hour, and multiply by the heating value of the gas to get the input in BTU/h.
What is the formula for clocking a gas meter?
Cubic feet per hour = 3600 x dial size x revolutions / seconds, and input in BTU/h = cubic feet per hour x heating value in BTU per cubic foot. A 1/2 cu ft dial at 22 seconds is 81.82 cubic feet per hour, or 81,818 BTU/h at 1,000 BTU per cubic foot.
How many seconds per revolution should an 80,000 BTU furnace take?
With gas at 1,000 BTU per cubic foot, it takes 22.5 seconds on the 1/2 cubic foot dial, 45 on the 1 cubic foot dial, 90 on the 2 cubic feet dial and 225 on the 5 cubic feet dial. The time scales with the heating value, so use the figure from your gas supplier.
How close does the clocked input have to be to the rating plate?
The Carrier instructions say within plus or minus 2 percent of the rating plate input, or of the input as adjusted for altitude. For 80,000 BTU/h that is 78,400 to 81,600 BTU/h. Other manufacturers set their own tolerance in their instructions.
Which dial on the gas meter do I time?
A test dial whose size is printed beside it. HVAC School times the smallest one, often 1/2 cubic foot, and Lennox the 1 cubic foot dial. Carrier says the 2 or 5 cubic feet dial gives a more accurate measurement, because a longer time makes a stopwatch error a smaller share of it. Timing two or three revolutions does the same on a small dial.
Where do I get the heating value of the gas?
From your gas supplier. Lennox and Carrier both send you to the utility for it, and Carrier asks for the expected in-season figure. 1,000 BTU per cubic foot is a round number used in examples, and HVAC School puts the US average closer to 1,040. Supplier figures at high elevation can be lower.
What do I do if the clocked input is too high or too low?
First confirm the heating value, the dial size and the timing, and that nothing else on the meter was burning. Carrier then has you raise the manifold pressure to increase the input or lower it to decrease the input, and clock again. At a different heating value or altitude the orifice may need changing to the size in the manufacturer's tables. Carrier says never to redrill an orifice.
Does a 2 psi gas meter change the calculation?
Yes. The dial counts cubic feet at the meter pressure, which hold more gas than at standard low pressure. HVAC School multiplies the flow by (psig + 14.73) / 14.73, which is 1.14 at 2 psig. Enter the meter pressure on this page to apply it, and ask the supplier which base it bills on.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup.
Used on the same job
- Furnace size calculator A preliminary heating load from a ZIP code, to see whether the input you clocked fits the house.
- HVAC CFM calculator The airflow a furnace output needs at the nameplate temperature rise.
- Boiler size calculator Boiler sizing from the heating load. A gas boiler is clocked the same way.
When the input is off, check the gas side first
Before the manifold pressure is touched, the heating value, the dial size and the timing have to be right, and nothing else on the meter can be burning. At elevation the target is the derated input from the furnace altitude derate calculator, and the gas line has to carry the input, which the gas pipe sizing calculator checks. You can also ask the assistant to walk through a reading with you.
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