How to Find the BTU Input of a Gas Furnace Without a Label?
If the label on your gas furnace is missing, you can determine the BTU input using a step-by-step process involving gas flow measurements and calculations. This article will guide you through the necessary steps to find the BTU input of your gas furnace, even without the manufacturer’s label.
What Tools are Required to Measure the BTU Input?
To measure the BTU input of a gas furnace without a label, you will need the following tools:
- Stopwatch
- Gas meter with a readable dial
- Calculator
Step-by-Step Process to Measure the BTU Input
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Ensure All Other Gas Appliances Are Off: Make sure all other gas-burning appliances in your home are turned off to avoid skewing the measurements.
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Measure Gas Flow: Use a stopwatch to measure the time it takes for one complete revolution of the smallest reading dial on the gas meter, usually the 1/2, 1, or 5 cubic feet indicator. Calculate the number of revolutions per minute (RPM) by dividing 60 seconds by the time for one revolution. Multiply the RPM by 30 to get the total cubic feet per hour (CFH) of gas consumed.
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Calculate BTU Input: Multiply the CFH by the caloric content of the gas, which varies by supplier and altitude. For example, in most low-altitude areas, natural gas has a heating value of approximately 1,000 BTUs per cubic foot.
Example calculation: If the gas meter shows 2.5 revolutions per minute, then:
– 2.5 RPM × 30 = 75 CFH
– 75 CFH × 1,000 BTUs/CF = 75,000 BTUs/Hr
How Does the Number of Burners Affect the BTU Input?
The BTU input of a gas furnace is directly related to the number and size of the burners. Each burner has an orifice that determines the gas flow rate, and the total BTU input is calculated by multiplying the BTU rating per burner by the number of burners.
For example, if each burner is rated at 25,000 BTUs and there are four burners, the total BTU input would be:
– 4 burners × 25,000 BTUs/burner = 100,000 BTUs
The total BTU input increases linearly with the number of burners, but the efficiency and performance can be affected by the burner configuration, ductwork, and overall system design.
How Do Altitude and Gas Supplier Affect the BTU Input?
Altitude:
At higher altitudes, the air pressure is lower, which can affect the gas flow rate and thus the BTU input. The caloric content of gas may also vary slightly with altitude, but the primary concern is the reduced air pressure affecting burner performance.
Example: In Denver, the caloric content of natural gas can vary between 1,050 BTUs per cubic foot at the wellhead and 600 BTUs per cubic foot at 10,000 feet above sea level.
Gas Supplier:
The caloric content of gas can vary by supplier. It is crucial to check with your fuel supplier to determine the exact caloric content of the gas being used.
Example: In most low-altitude areas, natural gas has a heating value of around 1,000 BTUs per cubic foot, but this can vary.
