How to Clock a Gas Meter to Check Input

Why this matters

An appliance's rating plate says what it should burn. Whether it actually burns that depends on the orifices, the manifold pressure, the gas supply and the gas itself, and none of those are visible from the front of the unit. A furnace firing a quarter over its rating can overheat a heat exchanger and make carbon monoxide. One firing well under can condense in the heat exchanger and vent, and short the customer on heat. Clocking the meter measures the real input with nothing more than a stopwatch and the numbers already on the meter and the gas bill. It is the cross-check that tells you whether the combustion readings and the temperature rise are being taken on a correctly fired appliance in the first place.

Safety before anything else

• If you smell gas on arrival, do not clock anything. Everyone leaves the building, no light switches or appliances are operated, no phone is used inside, and the call to the gas utility or 911 is made from outside and away from the building.
• Check for a leak first. With every gas appliance off (step 1 below), watch the smallest test dial for a full two minutes. If it moves at all with no pilot burning, or faster than the slow creep of a standing pilot you left lit, gas is going somewhere it should not. Stop the clocking, treat it as a leak, and follow your shop's leak procedure or call the utility.
• Clocking is a measurement, not an adjustment. Changing manifold pressure, swapping orifices or converting an appliance between fuels is gas work that your state or locality licenses. If the clocked input is out of range and you are not the qualified gas technician on the job, record the number and hand it to the person who is.
• Run a personal CO monitor while the appliance fires. If it shows CO, stop, shut the appliance off and ventilate before going further; if it alarms or the reading keeps rising, get everyone out of the building too.

What you need before you start

From the meter: the test dial size, printed on the dial face of most residential diaphragm meters. Common sizes are 1/2, 1 and 2 cubic feet per revolution. Read it off the dial; do not assume.

From the gas bill: the heating value of the gas, which the utility prints as a BTU factor, a therm factor, or "therms per CCF". One therm is 100,000 BTU and one CCF is 100 cubic feet, so a factor of 1.035 therms per CCF is 103,500 BTU per 100 cu ft, or 1,035 BTU per cubic foot. Natural gas is commonly near 1,000 to 1,050 BTU per cubic foot at sea level, but the bill's number is the one for that address and that month. See related: Anatomy of a Natural Gas Bill and the Therm Conversion.

From the appliance: the rated input on the rating plate, in BTU per hour (BTU/h). For a two-stage or modulating appliance, the plate gives a high and low input; clock each stage you can lock the appliance into.

The steps

1. Turn off every other gas appliance in the building. Water heater to its vacation or pilot setting, range and oven off, dryer off, fireplace off, any pool heater or generator off. The meter counts all gas flowing through it, so anything else burning is added to your reading. A standing pilot left burning adds a small draw of its own; note it on the sheet rather than chase it. Write down each appliance you turned off and its setting, because step 6 puts each one back.

2. Fire the appliance being tested and let it reach steady state. Give it several minutes on high fire. Input during the first moments of ignition is not representative, and on a two-stage unit the controls may not hold high fire until the call has lasted a while. Lock the stage with the thermostat or the control's test mode if it has one. Hazard: stay clear of the burner compartment while it fires; keep the CO monitor on you.

3. Time one full revolution of the test dial, then a second. Start the stopwatch as the hand passes a mark and stop it when it passes the same mark again. Time a second revolution and compare. Two revolutions within a few tenths of a second of each other mean the input is steady. If they differ by more, the appliance is still settling or something else is drawing gas.

4. Compute the input.

BTU/h = (3,600 / seconds per revolution) x dial size (cu ft) x heating value (BTU/cu ft)

The 3,600 is the seconds in an hour, so 3,600 divided by seconds per revolution is revolutions per hour. Times cubic feet per revolution gives cubic feet per hour (CFH). Times BTU per cubic foot gives BTU per hour. The units cancel down the line, which is how you know the formula is the right way up. See related: Units Carried Through Every Line of a Calculation.

5. Compare with the rating plate, as a ratio, and with the manufacturer's allowed range from the installation instructions. Fuel gas codes as adopted in your jurisdiction (NFPA 54 or the International Fuel Gas Code) require appliances to be installed in accordance with those instructions, so the instructions set the range, not a rule of thumb.

6. Put everything back. Return every appliance from step 1 to the setting you wrote down. Confirm the water heater's burner relights and holds, pilot appliances have a pilot, and nothing you turned off is left off. A water heater left on vacation is the call you get the next morning.

The worksheet, worked

A single-stage furnace, plate input 100,000 BTU/h, on a meter with a 1/2 cu ft test dial.

GAS METER CLOCKING WORKSHEET
Appliance: furnace, single stage
Plate input: 100,000 BTU/h
Others off: WH (vacation), range, dryer
Leak check: dial still for 2 min - pass
Test dial: 1/2 cu ft per revolution

Rev 1: 18.0 s     Rev 2: 18.0 s     Use: 18.0 s

Revs per hour  = 3,600 / 18.0      = 200 rev/h
Gas flow       = 200 x 0.5 cu ft   = 100 CFH
Input at 1,000 = 100 x 1,000       = 100,000 BTU/h
Bill factor    = 1.035 therms/CCF  = 1,035 BTU/cu ft
Input at bill  = 100 x 1,035       = 103,500 BTU/h
Ratio to plate = 103,500 / 100,000 = 1.035 (3.5% over)
Restored: WH relit, range and dryer back on

Using the round 1,000 BTU per cubic foot, the furnace reads exactly its plate, 100,000 BTU/h. Using the bill's own 1,035, it reads 103,500 BTU/h, 3.5 percent over. Which is right? The bill's figure, because it is the gas actually delivered. Whether 3.5 percent over is acceptable is a question for the manufacturer's instructions, which often allow a small band around the rating; it is not something to settle by rounding the heating value until the number looks good.

Check the answer with a rough estimate. Half a cubic foot every 18 seconds is about a cubic foot every 36 seconds, which is 100 cubic feet in 3,600 seconds, an hour. At roughly 1,000 BTU each, that is about 100,000 BTU/h. The sheet agrees.

Two readings that are not fine

Fast dial: 15.0 seconds per revolution. 3,600 / 15.0 = 240 rev/h. 240 x 0.5 = 120 CFH. 120 x 1,035 = 124,200 BTU/h, which is 24.2 percent over the 100,000 BTU/h plate. That is overfiring. The usual causes are manifold pressure set high, orifices drilled for a different gas or the wrong size, or a natural gas appliance with a missing or wrong regulator setting. It goes with a combustion analysis for CO and a check of the temperature rise against the plate range, and it gets corrected by the qualified gas technician, not by turning the thermostat down.

Slow dial: 22.0 seconds per revolution. 3,600 / 22.0 = 163.6 rev/h. 163.6 x 0.5 = 81.8 CFH. 81.8 x 1,035 = 84,700 BTU/h, 15.3 percent under the plate. Underfiring has its own list: low supply pressure (often a gas line undersized for everything it serves, which shows up as a pressure drop when other appliances come on), low manifold pressure, restricted orifices, or a partially closed valve. The customer's complaint is usually not enough heat, and the hidden risk is flue gas condensing where the appliance was not built to handle it.

In both cases the clocked number is the start of a diagnosis, not the end of one. It tells you the input is wrong; the pressure readings tell you why.

What changes the answer

Altitude. Rating plates are usually stated at sea level, and appliances installed at elevation are derated per the manufacturer's high-altitude instructions. Compare the clocked input with the derated figure for that altitude, not with the plate. The utility's heating value per cubic foot is also typically lower at elevation, because the gas is less dense, which is another reason to use the bill's figure rather than a sea-level number.

Elevated delivery pressure. Most homes take gas at a low pressure measured in inches of water column, and the meter's cubic feet are close enough to standard that no correction is needed. Some services deliver at an elevated pressure (a 2 psi system, for example) and the utility applies a pressure factor to the meter reading on the bill. If the bill shows a pressure factor, multiply the CFH by it before applying the heating value.

A meter with no usable test dial. Some meters show only a digital index. Read the index at the start and end of a timed interval long enough that the change is many units of its smallest digit, so the reading resolution is small next to the change. A short interval on a coarse index gives a number that looks precise and is not; see related: Measurement Resolution and Honest Rounding.

Propane. Propane systems usually have no meter between the tank and the appliance, so this method does not apply. Input on propane is checked from manifold pressure and orifice size against the manufacturer's tables.

Input is not output. The clocked figure is the gas being burned. The heat delivered to the house is that times the appliance's efficiency. Do not compare a clocked input with an output figure from a sales sheet or a heat-load calculation.

References

• NFPA 54, National Fuel Gas Code, and the International Fuel Gas Code, in the edition adopted by your jurisdiction
• Appliance manufacturer's installation instructions (allowed input range, high-altitude derate)
• The utility's bill and tariff for the heating value and any pressure factor
• See related: Anatomy of a Natural Gas Bill and the Therm Conversion
• See related: Energy and Power Units: BTU, Kilowatts, Tons and Horsepower