To calculate gas load (the amount of gas an appliance or system requires), divide the appliance's input rating (in Btu/hr) by the heating value of the gas (in Btu/cubic foot).
Here's a more detailed breakdown:
1. Understanding the Components:
- Gas Load: This is the volume of gas (typically measured in cubic feet per hour, or CFH) needed to operate an appliance or system at its designed capacity.
- Appliance Input Rating (Btu/hr): This specifies the rate at which the appliance consumes energy, measured in British thermal units per hour (Btu/hr). It's usually found on the appliance's nameplate.
- Gas Heating Value (Btu/cubic foot): This indicates the amount of heat energy released when one cubic foot of gas is burned. Natural gas typically has a heating value around 1,000 to 1,100 Btu/cubic foot. Check with your gas supplier for the precise value in your area.
2. The Calculation Formula:
The fundamental formula is:
Gas Load (CFH) = Appliance Input Rating (Btu/hr) / Gas Heating Value (Btu/cubic foot)
3. Example:
Let's say you have a furnace with an input rating of 100,000 Btu/hr, and the natural gas has a heating value of 1,000 Btu/cubic foot.
Gas Load = 100,000 Btu/hr / 1,000 Btu/cubic foot = 100 CFH
This means the furnace requires 100 cubic feet of gas per hour to operate at its full capacity.
4. Considerations:
- Units: Ensure that all units are consistent before performing the calculation.
- Gas Type: The heating value of gas varies depending on its composition (e.g., natural gas vs. propane). Use the correct heating value for the type of gas you are using.
- Altitude: At higher altitudes, the air is thinner, which can affect combustion efficiency. This might require adjustments to the gas pressure or burner settings to maintain proper gas load. Consult a qualified technician for adjustments.
- Multiple Appliances: If you're calculating the total gas load for a system with multiple appliances, calculate the gas load for each appliance individually and then add them together.
- Safety Factors: When sizing gas lines, it's crucial to factor in safety margins and potential future expansion. Consult relevant building codes and regulations, and seek professional guidance from a qualified HVAC technician or gas fitter. The "Longest Length Method" is often referenced in code, and must be applied by qualified persons.
5. Summary
Calculating gas load is a straightforward process involving dividing the appliance input rating by the gas heating value. Accurate calculations are essential for proper appliance operation, safety, and gas line sizing. Remember to consult local codes, regulations, and qualified professionals for optimal results.
