Anatomy of a Natural Gas Bill and the Therm Conversion

Why this matters

A gas bill measures one thing and charges for another. The meter counts volume, in cubic feet. The utility bills for heat, in therms. Between the two sits a conversion factor printed on the bill that changes month to month, and most people who read gas bills, techs included, skip it. That is how a customer ends up comparing CCF on one bill to therms on another, or concluding the furnace burned 24 percent more gas when it burned 16 percent more per day. Read the conversion and the bill becomes a measurement of heat delivered to the house, which is the number you can check against the equipment's input rating.

If the call comes with a smell of gas, it is not a bill call. Tell the customer to leave the house now, not to touch light switches or appliances, not to use a phone inside, and to call the gas utility's emergency line from outside. The bill can wait.

The units, stated once

• CCF is 100 cubic feet of gas. The C is the Roman numeral for 100. MCF is 1,000 cubic feet, so 1 MCF is 10 CCF.
• BTU (British thermal unit) is the heat needed to raise one pound of water one degree Fahrenheit (see related: What a BTU Actually Is).
• Therm is 100,000 BTU of heat.
• Dekatherm (Dth) is 10 therms, which is 1,000,000 BTU. Wholesale gas is traded in dekatherms, so it shows up on supply lines and in the fine print.

The conversion the bill performs:

therms = CCF x BTU factor

The BTU factor, also called the heat content, therm factor or conversion factor, is how many therms one CCF of the gas delivered that month actually contained. Pipeline natural gas commonly carries around 1,000 to 1,100 BTU per cubic foot, with the US average a little over 1,030, so the factor sits a little above 1.0 on most bills. It varies with the gas the utility received that month, so it is printed per bill, not once a year.

Some bills also carry a pressure or altitude correction, because a cubic foot of gas measured at a higher delivery pressure or a lower air pressure holds a different amount of gas. Where it appears, it is another multiplier and the bill shows it. Use the factors exactly as printed rather than estimating your own.

The bill, laid out

SERVICE 11/28 TO 12/29          BILLING DAYS: 31
---------------------------------------------------
METER    PREV    CURR    CCF  BTU FACTOR  THERMS
G-5521   4,812   4,931   119  1.037       123.4  [1]
---------------------------------------------------
DELIVERY
 Customer charge     fixed C per month         [2]
 Distribution        123.4 therms x D          [3]
SUPPLY
 Gas cost            123.4 therms x G          [4]
RIDERS
 Program surcharges  per therm or fixed        [5]
TAXES                percent of the above      [6]
---------------------------------------------------
AVG DAILY USE 3.98 THERMS

Run the arithmetic: 4,931 minus 4,812 is 119 CCF. 119 times 1.037 is 123.4 therms. 123.4 therms over 31 days is 3.98 therms a day. All three printed values close.

[1] The meter block. Volume read, conversion applied, heat billed. Like the electric bill, there is a read code next to the read, actual or estimated, and the same catch-up trap (see related: Estimated Versus Actual Meter Reads and the Catch-Up Bill). How to read the meter face itself, and check it against the bill, is in Reading Gas Meter + Usage Reference.

[2] Customer charge. Fixed per billing period, regardless of use. On a gas account this is often a larger share of a summer bill than of a winter one, because summer use is small. A customer with a gas range and water heater only may see the fixed charge as the biggest line in July.

[3] Distribution. The utility's charge for moving gas through its pipes, per therm. Some utilities charge it in blocks, with the first set of therms at one rate and the rest at another.

[4] Gas cost. The commodity itself, per therm, often called the purchased gas adjustment or gas supply charge. It is passed through at what the utility paid and it can move every month. In states that allow it, a third-party supplier's rate replaces this line (see related: Third-Party Energy Suppliers and the Variable-Rate Jump). A winter with high wholesale gas prices raises this line on every bill in the service area, regardless of what anyone's furnace did.

[5] Riders. Program and recovery charges, per therm or fixed, as on an electric bill.

[6] Taxes. Usually a percentage of the lines above.

Some utilities bill per CCF rather than per therm, folding the heat content into the rate. The arithmetic still holds; you just have to convert yourself if you want therms, and you need the heat content from the utility's published figures to do it.

Worked example: December against January

The customer's January bill looks alarming next to December's, and they want to know whether the furnace is failing.

Check January's therms. 148 times 1.029 is 152.3. The factor fell, so the January gas carried slightly less heat per cubic foot.

Raw volume. 148 divided by 119 is 1.24. That is the 24 percent the customer saw if they compared CCF.

Heat billed. 152.3 divided by 123.4 is 1.23. Slightly less than the volume rise, because the January gas was leaner.

Heat per day. 152.3 over 33 days is 4.62 therms a day. 4.62 divided by 3.98 is 1.16. So the house used 16 percent more heat per day, not 24 percent more gas. The other eight points or so are two extra billing days and the comparison of CCF to CCF across different heat contents.

Check the split. Days: 33 divided by 31 is 1.065. Heat per day: 1.16. 1.065 times 1.16 is 1.235, which matches the 1.23 therm ratio within rounding. The CCF figure runs one point higher because it also carries the factor change: 1.037 divided by 1.029 is about 1.008.

Is 16 percent a lot? In most heating climates January is colder than December, and a 16 percent rise in daily heat for a colder month is ordinary. The real question is whether it is more than the weather explains, which is the job of heating degree days (see related: How to Weather-Normalize Two Winters of Bills).

Turning therms into burner hours

The therm is what makes a gas bill useful to a tech: it is the same unit as the appliance input rating, once you multiply by 100,000.

Take the January figure of 4.62 therms a day. Say the summer bills, when only the water heater and range run, average 0.9 therms a day. Assume those loads stay about the same in winter (in practice a water heater burns somewhat more in winter, because the incoming water is colder, so this leaves the furnace share slightly high). Heating then accounts for 4.62 minus 0.9, which is 3.72 therms a day, or 372,000 BTU.

The furnace nameplate says 80,000 BTU per hour input. 372,000 divided by 80,000 is 4.65 hours of burner run a day. Check it back: 4.65 hours times 80,000 BTU per hour is 372,000 BTU, which is 3.72 therms. That is a plausible January run time, and it is now something you can compare with a runtime counter or thermostat history. The full method, including two-stage and modulating furnaces where nameplate input is a maximum, belongs to a sibling (see related: How to Estimate a Gas Appliance's Use From Its Input Rating).

Notice what the arithmetic used: input, not output. A 95 percent furnace and an 80 percent furnace both burn their rated input; the difference is how much of it reaches the house. Efficiency belongs in comparisons between fuels, not in reading a gas bill (see related: COP, AFUE and Delivered Heat: Putting Every Fuel on One Scale).

What changes the reading

A second gas meter or a shared meter. Duplexes and some older houses share a meter, so the bill carries two households' heat. Check which units the meter serves before you attribute use to one furnace.

A new gas appliance. A pool heater, a generator, a gas dryer or a fireplace insert can each add more than the furnace changed. A standby generator exercises on a schedule and burns gas every week of the year.

A pilot light. Standing pilots on older water heaters, fireplaces and some furnaces burn gas around the clock. Each one is small per hour and continuous, so it shows up most in the summer baseline.

A rate change rather than a use change. If therms per day did not move but the bill did, the cause is on lines [2] through [6]. Compare the per-therm values of the gas cost line on both bills before you look at the equipment.

Where readers go wrong

Comparing CCF to therms. A bill that shows both invites it. Pick therms and stay there, because therms carry the heat content and CCF does not.

Assuming the factor is 1. It is close to 1, which is why skipping it feels harmless. On a single bill it shifts the answer by a few percent. On a comparison between two bills, the two factors can push in opposite directions and a few points of change are pure conversion.

Reading the heat content as efficiency. The BTU factor describes the gas, not the appliance. A higher factor means more heat per cubic foot delivered to the meter. It says nothing about how well the furnace uses it.

Treating a dekatherm on the supply line as a therm. Supply contracts and wholesale notes quote prices per dekatherm, ten times the therm. A per-dekatherm figure read as per-therm makes gas look ten times dearer.

How to verify you read it right

• Current read minus previous read equals CCF used, and CCF times the printed BTU factor (and any pressure factor) equals therms billed.
• Therms divided by billing days matches the average daily use on the bill.
• A ratio between two bills is quoted in therms per day, and you can say how much of the raw change came from days and how much from the factor.
• Any burner-hour estimate multiplies back to the therms you started from.

References

• See related: Reading Gas Meter + Usage Reference
• See related: What a BTU Actually Is
• See related: How to Weather-Normalize Two Winters of Bills
• See related: How to Estimate a Gas Appliance's Use From Its Input Rating
• The customer's gas utility tariff, filed with the state utility commission, which defines the heat-content and pressure factors applied on that bill