I was trying to gauge the cost(for natural gas) when my furnace has run(consumed gas) for an hour's time. I did it two ways.

First I just took the BTU input rating of the furnace and did the calculations from there. Then someone pointed out that the BTU input rating may not be an actual value.

So secondly I needed to determine the amount of gas my furnace is actually consuming per hour.

Here are the particlars:

1. I have Goodman model GMPN100-4 furnace. (natural gas, single stage, pilotless and an input rating of 100,000 BTU per Hour)

FIRST WAY USING BTU INPUT RATING:

1.One cubic foot of natural gas has about 1,030 BTU.

2.Divide the furnace input rating(in my case 100,000) by 1030 to get the number of cubic feet of gas the furnace will use in one hour. So 100,000(BTU) divided by 1030(BTU per Cubic Foot) is about 97 Cub Feet.

3.My supplier's bill is based on units of one hundred cubic feet(CCF) so I divide 97 cubic feet by 100 to determine how many CCF the furnace will burn per hour. This turns out to be 0.97 CCF

4.My supplier charges $.745 per CCF so it costs me 0.97 times $.745 = $.72 per hour for natural gas for my furnace to run.

SECOND WAY BY DETERMINING ACTUAL GAS CONSUMPTION:

As stated above someone pointed out that the BTU input rating may not be an actual value. So I wanted to determine how much gas the furnace was consuming per hour.

My gas meter has a 2 cubic foot dial. I set a video cam in front of the gas meter in the morning knowing the furnace would be on for at least 20 minutes to bring the house up to temperature from the night's setback. The furnace did run for about 20 minutes but I stopped my readings at 10 minutes. Here are some findings:

In 5 minutes the furnace consumed 7.6 cubic feet of gas.

In 10 minutes the furnace consumed 15.2 cubic feet of gas.

So 15.2 cubic feet of gas in 10 minutes extrapolated out to 60 minutes would be 91.2 cubic feet gas per hour or 0.912 CCF/hr.

At $.745 per CCF the cost is about $.68 per hour.

From the input rating data (100,000 BTU/hr) I calcualted .97 CCF/hr. So the actual gas consumption was a little bit less that the input rating.

NOTES:

1. No other gas consuming appliances(oven, stovetop or hot water heater) were firing at the time of testing.

2. I understand that outside temp, thermostat setting and how well my home is insulated are factors as to HOW OFTEN the furnace will run. That wasn't my concern. I only wanted to know how much gas was consumed when the furnace ran for an hour. How that hour was achieved was not important. It could of been 60 continuous minutes or six 10-minutes cycles. In my case I took a 10 minute run cycle and multiplied that amount of gas usage by 6.

I have since hooked up the following 24V hour meter to the gas valve:

http://www.grainger.com/Grainger/RED...PV9?Pid=search

The meter progresses in tenths of an hour increments anytime the gas valve is open. I can tell how long the furnace runs each day, month or season. This helps with filter changes also. I've found that about every 125 hours the filter is dirty enough to be replaced.