These eight easy steps will determine the BTUH output of a gas furnace.
Step 1: Determine the BTUH content of a cubic foot of natural gas. This information is available from your local gas supplier. If the actual heat content of the gas cannot be obtained, use a default value of 1,050 BTU per cubic foot of natural gas for your calculations. Beware, there will be some inaccuracy in your calculations performed in this procedure.
Step 2: Shut off all gas appliances in the home. The only gas appliance that should be operating in this test is the gas furnace. Set the blower to operate at cooling speed during heat mode operation.
Step 3: Install a gas pressure manometer onto the furnace manifold pressure tap (Fig. 1)
Step 4: Call for heat. Make sure all furnace stages are calling for heat and the burners are operating at high fire rate. Check manifold pressure versus the nameplate requirement. Make adjustment as needed.
Step 5: With the furnace operating at high fire rate and manifold pressure within manufacturer specifications, go to the home’s gas meter (Fig. 2). Time the number of seconds it takes for the gas meter’s smallest scale to complete one revolution on the dial.
Step 6: Plot the time measured on the chart (Fig. 3) and note the listed flow rate for that time. Determine the scale of the dial on the gas meter to calculate the cubic feet of gas being burned by the furnace per hour:
For 1 Cu. Ft. Dial:
Gas Flow CFH = Chart Flow Reading divided by 2
For ½ Cu. Ft. Dial:
Gas flow CFH = Chart Flow Reading divided by 4
For 5 Cu. Ft. Dial:
Gas Flow CFH = 10X Chart Flow Reading divided by 4
Step 7: Multiply the cubic feet of gas burned per hour by the heat content value obtained from the gas supplier. (1,050 BTU default average) The answer is BTUH input to the furnace. For example, lets say we timed 36 seconds per revolution on a 1 Cu. Ft. dial, the furnace is an 80% furnace.
- A. 36 seconds = 200 cubic feet per hour/2
- B. 36 seconds = 100 cubic feet per hour
- C. 100 x 1050 = 105,000 BTUH Input
Step 8: Multiply BTUH input obtained in step 7 by furnace efficiency (80%) to determine BTUH output.
105,000 BTUH Input x .8 = 84,000 BTUH Output