How to Clock a Gas Meter to Check Furnace Input

Why this matters

A manometer tells you the pressure at the manifold. It does not tell you how much gas went through the orifices, and the furnace only cares about the second number. Wrong orifices, a mislabeled conversion, a heating value the plate was not written for, or a manifold set to the wrong plate all produce a furnace whose pressure looks right and whose firing rate is not. The gas meter is a calibrated flow instrument already installed on every natural gas house, and timing it costs five minutes. It is the only field check that measures input directly, which is why combustion tuning starts from a clocked furnace rather than from a pressure reading (See related: Gas Furnace Combustion Analysis and Tuning).

Safety and start state

Safety first. If you smell gas at the meter or in the house, stop. Everyone leaves, nobody operates switches or phones inside, and the utility is called from outside. Do not touch the meter's valve, regulator or vent; they belong to the utility, and the clock needs nothing from them except a view of the dial. Wear a personal CO monitor when you are at the furnace, since you will run it at high fire for several minutes.

What you need:

• The rating plate input at each firing stage, in BTU/h (See related: Furnace Rating Plate Read Before Start-Up).
• The utility's heating value for the gas, in BTU per cubic foot. Utilities publish it, often on the bill or on request, and it varies by supplier and season. A figure around 1,000 to 1,050 BTU per cubic foot is common for pipeline natural gas at low elevation; use the utility's number, not that range.
• A stopwatch that reads tenths of a second, and the furnace's manifold already confirmed on plate (See related: How to Set Gas Manifold Pressure on a Furnace).
• Access to the meter face, and a second person or a radio if the meter is far from the furnace.

Check before you start: this meter feeds only the house you are working on, and there is no second furnace, pool heater or generator on it that might start mid-clock.

Read the meter face first

A residential diaphragm meter has a row of register dials that bill the customer and one larger test dial, usually marked with the volume one revolution represents: commonly 1/2, 1 or 2 cubic feet on residential meters, larger on commercial ones. The test dial is the one you time. Find its label before you start the watch; timing a 2 cubic foot dial as if it were a 1 cubic foot dial halves the answer, and the arithmetic will look perfectly clean.

Some newer meters are electronic with no test dial; they display flow or a pulse indicator, and the utility can say how to read it. If you cannot find a test dial and cannot get the utility to explain the display, you cannot clock this meter, and the input check falls back to orifice size and manifold pressure against the maker's tables.

The steps

1. Quiet every other gas load. Turn the water heater to its pilot or vacation setting, shut off the range and any gas fireplace, and confirm the dryer is not running. Turn appliances down rather than off where you can, so there is nothing to relight afterward. Why: the meter counts every cubic foot the house uses, and a water heater firing mid-clock adds its whole input to the furnace's. Skip it and the clock reads a furnace with a water heater bolted on.

2. Account for standing pilots. Any appliance with a standing pilot still burns a little gas. On a careful clock, time the dial with the furnace off and only the pilots burning, convert that to cubic feet per hour the same way as step 4, and subtract it from the furnace reading. On most houses it is a small correction; on a house with three standing pilots it is not.

3. Fire the furnace at the stage you are checking and wait. Call for high fire by the maker's method and let it run a couple of minutes so the valve and the meter are both at steady flow. On a two-stage furnace you clock high and low separately, each against its own plate input.

4. Time the test dial. Start the watch as the needle crosses a mark and stop it when it crosses the same mark again. Aim for at least 30 seconds of timing: on a fast-turning small dial, time two or more revolutions and divide. Why: a reaction error of half a second on a 34 second revolution is 0.5 / 34, about 1.5 percent, which is most of the tolerance you are checking against. On a 15 second revolution the same half second is over 3 percent.

5. Calculate cubic feet per hour. Divide 3,600 seconds in an hour by the seconds per revolution, then multiply by the dial size in cubic feet. That is metered flow.

6. Convert to BTU/h. Multiply cubic feet per hour by the utility's heating value. That is the furnace's actual input.

7. Compare to the plate. Divide actual by rated. Makers state the allowed tolerance in the installation instructions, often within 2 percent, and many say the furnace must never exceed rated input. Outside the tolerance, find out why before you adjust anything.

8. Restore every appliance. Return the water heater to its original setting and confirm its burner lights on the next call, return the range and fireplace to how you found them, and tell the customer you have done it. Skip it and the call that comes in tomorrow is about cold showers.

Worked example: a furnace that clocked 9 percent hot

A single-stage natural gas furnace, plate input 100,000 BTU/h, replacing an older unit. The installer set the manifold "by the book" without a manometer. The utility's heating value is 1,030 BTU per cubic foot, and the meter's test dial is marked 1 cubic foot. Water heater set to pilot, range off, no other pilots in the house.

METER CLOCK  single-stage furnace, NG
Plate input ................ 100,000 BTU/h
Heating value (utility) .... 1,030 BTU/cu ft
Test dial .................. 1 cu ft per rev

TARGET
  100,000 / 1,030 .......... 97.1 cu ft/h
  3,600 / 97.1 ............. 37.1 s per rev

AS FOUND
  Time, one rev ............ 34.0 s
  3,600 / 34.0 x 1 ......... 105.9 cu ft/h
  105.9 x 1,030 ............ 109,100 BTU/h
  109,100 / 100,000 ........ 1.091, 9.1 pct over

AFTER MANIFOLD RESET
  Time, one rev ............ 37.0 s
  3,600 / 37.0 x 1 ......... 97.3 cu ft/h
  97.3 x 1,030 ............. 100,200 BTU/h
  100,200 / 100,000 ........ 1.002, 0.2 pct over

Reading the as-found. The target says the dial should take 37.1 seconds per revolution; it took 34.0, so the furnace is moving gas faster than its rating. Checked from the other direction: 109,100 BTU/h divided by 1,030 is 105.9 cubic feet per hour, and 3,600 divided by 105.9 is 34.0 seconds. The two directions agree.

Finding the cause. A manometer on the outlet tap read 4.1 in w.c. against a plate value of 3.5. Flow through a fixed orifice goes roughly with the square root of the pressure across it, so setting the manifold to plate should cut flow by the square root of 3.5 / 4.1, which is the square root of 0.854, about 0.924. That predicts 109,100 x 0.924, about 100,800 BTU/h, and a revolution of 34.0 / 0.924, about 36.8 seconds.

After the reset. The tech set the manifold to 3.5, re-fired at high, waited two minutes, and timed 37.0 seconds. The prediction said 36.8; the measurement is within a fifth of a second, which is inside stopwatch error. Input now sits at 100,200 BTU/h, 0.2 percent over plate, inside a 2 percent tolerance.

Why the clock mattered here. Had the manifold read 3.5 and the clock still said 9 percent over, the pressure would not have been the cause, and turning the screw down to force the clock into line would have been the wrong fix. That pattern points at the orifices (wrong size for the gas, or an altitude kit that was never fitted) or at a heating value higher than the plate assumed. The orifice stamp gets checked against the maker's orifice table before anything else is changed.

What went on the ticket: dial size, seconds per revolution as found and as left, the heating value and where it came from, the manifold as found and as left, and the water heater restored to its setting with its burner seen lighting.

Where the clock lies

Elevated-pressure meter sets. The calculation above assumes the meter delivers near-atmospheric pressure, which is true of a standard residential set at a few inches of water column. Where the utility delivers 2 psi or more and the house carries line regulators, the meter measures gas at that higher pressure, and each metered cubic foot holds more gas than a cubic foot at the furnace. At 2 psi the factor is about (14.7 + 2) / 14.7, roughly 1.14 at sea level, which is a 14 percent error if ignored. The utility knows the pressure factor for its meter; ask for it rather than estimating.

Altitude. Utilities at elevation commonly deliver gas with a lower heating value per cubic foot, and furnaces there are derated. Use the utility's figure for that service and compare against the derated input from the maker's high-altitude table, not the sea-level plate.

A meter that is the problem. A meter that has been struck, frozen, or stops and starts on the test dial cannot be trusted, and meter accuracy is the utility's question, not yours. Report it and fall back to the orifice and manifold check.

Propane. There is no meter to clock on a typical propane house. Input on propane is confirmed by the orifice size from the conversion kit plus manifold pressure, then by combustion analysis.

How to verify the result

The clock is done right when the ticket shows a test dial size read off the meter face, a timing of at least 30 seconds, the heating value with its source, and an input within the maker's stated tolerance at each firing stage clocked. Only then does combustion tuning start, because an analyzer reading on a furnace firing 9 percent over plate tunes the wrong fire.

References

• Furnace rating plate and installation instructions: rated input by stage, input tolerance, orifice and high-altitude tables
• The serving gas utility: heating value for the service, meter pressure factor on elevated-pressure sets
• See related: How to Set Gas Manifold Pressure on a Furnace; Gas Furnace Combustion Analysis and Tuning
• See related: Furnace Rating Plate Read Before Start-Up