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# Why does PSEG not use a constant therm calculation?

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Member Posts: 165
Therms should be a constant value equal to the CCF times a constant variable, right? An example is:
Usage of 9 on the meter, then CCF is 9 * 1.012 or 9.18, times 1.033 is 9.409.
PSEG does use 1.012 for the CCF as a constant month to month, but PSEG changes the 1.033 for the therms every month. Why? I use a spread sheet to anticipate my gas bill and it is always off because of the value to multiply by the CCF. I notice that it goes down each month thus far:
May- 1.036
June- 1.035
July- 1.033
Aug- 1.031
I'm not a plumber or hvac man and my thoughts in comments are purely for conversation.

• Member Posts: 23,544
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The therm value will change with the chemical makeup of the fuel. A therm is 100,000 BTU, and actually has only a convenient relationship to volume. A CCF is 100 cubic feet of gas -- in fact, any gas. Now methane, the primary constituent of natural gas, has a gross heating value of 1,011 BTU per cubic foot of gas at standard temperature and pressure. Therefore, a CCF actually has 101,100 BTU gross heating value. Propane and ethane, which are commonly also present in "natural" gas -- propane often being actually added -- have much higher gross heating values, so if they are present even in trace amounts, as they usually are, the therms per CCF will change.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 22,353
edited August 2022
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In winter months when NG supplies get low the do blend the gas and the BTU or calorific content drops. Then, burners are out of adjustment. The gas company had us put colored stickers on equipment when we readjusted. I recall it dipping in to the 900’s at one point
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 165
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Thanks guys, that makes sense. And it is very interesting.
I'm not a plumber or hvac man and my thoughts in comments are purely for conversation.
• Member Posts: 6,505
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Which is why you're probably better off with the analyzer than clocking the meter, if you can't get the exact btu content of what is getting delivered from the supplier.

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• Member Posts: 15,752
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You will never get things exact.

Look at oil. It's about 140,000 but/gallon.

But what is a gallon?

Temperature affects the volume of a gallon. Measure a gallon at 60 deg then heat it to 100 deg and the volume goes up because of expansion.

Interestingly cold oil will flow more gallons through a nozzle at a constant pressure than warmer oil with the same nozzle and pressure.

Gas is no different