How to Estimate a Gas Appliance's Use From Its Input Rating

Why this matters

A customer whose gas bill tripled in July wants to know which appliance did it, and "probably the pool heater" is a guess, and a wrong guess sends them to the wrong repair. Every gas appliance carries the one number that settles it: its input rating in BTU per hour. Multiply that by the hours the burner actually fires and you have therms, which is what the bill is counted in. The estimate is only as good as the run hours you feed it, so most of this card is about getting those hours honestly and then checking the answer against the meter rather than trusting the arithmetic.

Before you start: gas odor and the burner compartment

• If you smell gas at the meter or in the house, stop. Everyone leaves the building, nobody operates a light switch, appliance or phone inside, and the call to the gas utility or 911 is made from outside and away from the building. Nothing on this card is done until the utility clears it.
• Reading a rating plate is a look, not a job. Where the plate sits behind a burner access panel, read it with the appliance off and the panel back on before it fires again. Do not reach into a firing burner compartment, and keep a personal CO monitor on you whenever you stand beside an appliance while it fires.
• This is estimating, not adjustment. If the numbers point to an appliance burning more than its rating, the fix (orifices, manifold pressure) is gas work your state or locality licenses. Record what you found and hand it to the qualified gas technician.

The formula

Therms = input (BTU/h) x burner hours / 100,000

Cost = therms x the price per therm on the bill

A therm is 100,000 BTU. The input figure is the gas the burner consumes, which is what the meter counts and the utility bills. Do not use the output or heating capacity figure that sits beside it on many plates; output is input times efficiency, and using it understates the therms by the efficiency loss.

Units check: BTU per hour x hours = BTU; BTU / (BTU per therm) = therms.

Step 1: Read the rating plate, and read it completely

Find the plate (inside the burner door on a furnace, on the jacket of a tank water heater, on the jacket or inside the cabinet on a pool heater, behind the kick panel or on the back of a range or dryer). Copy down:

• Input in BTU/h. For a two-stage appliance there are two figures; for a modulating one there is a range. Note both ends.
• The gas type. Natural gas and propane plates read differently, and an appliance converted in the field should carry a conversion label. A propane input on a natural gas house is a red flag for someone else to look at, not a number to use.
• Any altitude note. Plates state input at or near sea level. Above a stated elevation, the installation instructions call for a lower input, and the installed appliance may have been derated. Use the derated input if the manual or a label says one applies.

What breaks if you skip it: taking the input from the model number or a spec sheet for "the same size" unit. Water heaters with the same tank volume are built at noticeably different inputs, and pool heaters come in steps that differ by a factor of two or more.

Step 2: Estimate burner hours, not appliance-on hours

This is the step the estimate lives or dies on. The burner fires only when the appliance is calling for heat, and almost every appliance spends much of its "on" time with the burner off. See related: Duty Cycle: Why Run Hours, Not Clock Hours, Set the Cost.

Sources, best first:

1. A burner-hours counter or heating log in the appliance's control, where it has one. Read it now and again at a known later time.
2. A thermostat's runtime history, for a furnace on a single-stage call. It overstates slightly, because a call can continue while a limit holds the burner off.
3. Timing cycles. Watch the appliance through a few cycles, time how long the burner fires and how often, and scale to a day.
4. Working from the load. For a water heater, hot water used per day times the temperature rise gives the heat needed, which you can turn into burner time.
5. The owner's account. Useful for the schedule (which evenings, which season), weak for the hours, because people report when the appliance is switched on, not when it fires.

Write the source next to every hour figure. An estimate built on source 5 is a hypothesis.

Step 3: Build the estimate appliance by appliance, then add it up

One line per gas appliance, including the ones the customer forgot (a garage heater, a fireplace, a grill on a gas line, a generator that exercises weekly). See related: The Loads a Customer Forgets They Own. Total the lines and set them beside the bill's therms for the same number of days.

Step 4: Check against the bill, then against the meter

The estimate should land within shouting distance of the bill. If it falls well short, something is missing from the list or one of the hour figures is low. If it lands well over, an hour figure is high.

Then check the one input you have taken on faith: that the appliance actually burns what its plate says. Clocking the meter with only that appliance firing measures its real input. Reading Gas Meter + Usage Reference covers reading the dials and the test hand, and How to Clock a Gas Meter to Check Input walks the full procedure, including turning everything else off and putting it back.

Worked example: the July bill

The call. A family of four's gas bill for a 30-day July period shows 96 therms. The same 30-day period last July showed 22 therms. Since then they have had a gas pool heater installed. The owner says it runs "a couple of hours most evenings." Both periods are 30 days, so no per-day correction is needed (How to Compare Two Bills With Different Billing Days covers the case where they differ).

The rating plates. Water heater, 40,000 BTU/h input. Dryer, 22,000 BTU/h. Range, four cooktop burners at 9,000 BTU/h each and an oven at 16,000 BTU/h. Pool heater, 300,000 BTU/h, natural gas, no altitude derate.

The estimate sheet, before the pool heater:

GAS USE ESTIMATE - 30-DAY PERIOD
APPLIANCE    INPUT BTU/H  BURNER HRS  SOURCE      THERMS
Water heater  40,000      36.0        load+cycles  14.40
Dryer         22,000      10.3        loads x 0.6   2.27
Range top      9,000      15.0        owner         1.35
Oven          16,000       3.0        owner         0.48
                                      SUBTOTAL     18.50
Last July billed                                   22.00
Unexplained (pilots, use not listed)                3.50

How the water heater's 36.0 hours were reached: about 60 gallons of hot water a day at a 60 F rise is 60 x 8.33 x 60 = 29,988 BTU a day into the water. At a recovery efficiency of 0.78 (read off the unit's documentation; illustrative here) that takes 29,988 / 0.78 = 38,446 BTU of input, or 38,446 / 40,000 = 0.96 hours of burner time. Timing the burner across a quiet evening with no draws showed two standby reheats of about 7 minutes each, another 0.23 hours. That is 1.19, rounded to 1.2 hours a day, or 36.0 hours in 30 days. 40,000 x 36.0 / 100,000 = 14.40 therms.

The dryer line is 4 loads a week over 30 days (4.29 weeks, so 17.1 loads) at about 0.6 burner hours per load: 10.3 hours, and 22,000 x 10.3 / 100,000 = 2.27 therms.

The sheet accounts for 18.5 of last July's 22 therms. A remainder of 3.5 therms in a month is small and plausible (a standing pilot burns continuously, and usage estimates are loose); it is not worth chasing.

The gap the pool heater has to explain. This July's 96 therms, less last July's 22, is 74 therms. At 300,000 BTU/h, 74 therms is 74 x 100,000 / 300,000 = 24.7 burner hours across the month.

The owner's account against that. "A couple of hours most evenings" reads as something like 20 evenings at 2 hours, 40 hours, which would be 40 x 300,000 / 100,000 = 120 therms, more than the whole gap. The two numbers disagree, and the reason is the duty cycle: the owner was describing the pump's time-clock window. The heater fires only when the water is below its setpoint, so burner hours inside a 40-hour window can be well short of 40.

Checking it rather than arguing it. Two measurements settle it:

1. Clocking the meter with only the pool heater firing, on a 2 cubic foot test dial, gave 25.0 seconds a revolution. That is 3,600 / 25.0 = 144 revolutions an hour, x 2 = 288 cubic feet an hour, x the bill's 1,035 BTU per cubic foot = 298,080 BTU/h. The heater burns its rating (within 1 percent), so the plate figure can be trusted.
2. A one-week meter read. The tech read the meter at the visit and the owner read it again 7 days later, noting the evenings the heater ran (5). The register moved 20.3 CCF. At the bill's therm factor of 1.035, that is 20.3 x 1.035 = 21.0 therms. The house's non-pool use runs 22 / 30 = 0.73 therms a day, so 7 days of it is 5.1 therms, leaving 21.0 minus 5.1 = 15.9 therms for the pool heater. 15.9 x 100,000 / 300,000 = 5.3 burner hours over 5 evenings, about 1.06 hours an evening.

The answer closes. The monthly gap needs 24.7 burner hours; at 1.06 hours an evening that is 24.7 / 1.06 = 23 heating evenings in a 30-day July, which fits "most evenings." The pool heater accounts for the increase. At the bill's rate, it cost 74 times the price per therm for the month, and every hour it fires costs 3 times the price per therm (300,000 / 100,000).

What you tell the customer. The heater is working as rated; it is the hours that make the bill. Their levers are the setpoint, a cover (which cuts the heat lost overnight and so the hours to reheat), and the schedule. Give them the per-hour figure, not a promise of a monthly number. See related: Savings Claims Without Overpromising: The Honest Language.

What changes the method

• A clocked input well off the plate. Use the clocked figure for the estimate and hand the discrepancy to the gas technician; an appliance over its rating is a combustion question first and a bill question second.
• A modulating appliance. A modulating furnace or boiler spends most hours below its top input, so the plate's maximum times the hours overstates. Use the control's own consumption or modulation history if it keeps one, or clock the meter at the firing rates it actually holds.
• Winter instead of summer. Space heating swamps every other line and varies with the weather, so the appliance-by-appliance sheet is a summer tool. In winter, separate the heating from the base use first. See related: How to Weather-Normalize Two Winters of Bills.

How to know the estimate is good enough

• Every hour figure on the sheet names its source.
• The non-heating lines account for most of a mild month's bill, as they did here (18.5 of 22 therms).
• The suspect appliance's input has been confirmed by clocking, not assumed.
• The hours that explain the gap are consistent with an independent measurement, as the 23 evenings were with a 5-evening week.

References

• See related: Reading Gas Meter + Usage Reference
• See related: How to Clock a Gas Meter to Check Input
• See related: Anatomy of a Natural Gas Bill and the Therm Conversion
• See related: Duty Cycle: Why Run Hours, Not Clock Hours, Set the Cost
• Appliance rating plates and installation instructions, which state the input, the gas type and any altitude derate