What is the Nominal Rating and Output of a Two-Stage Gas Furnace?
A two-stage gas furnace has a nominal rating that indicates its maximum input capacity, typically measured in British Thermal Units per hour (BTU/Hr). This rating is crucial for understanding the furnace’s performance during both stages of operation.
Nominal Rating: Definition and Influence on Output
- Nominal Rating: The nominal rating is the maximum input capacity of the furnace, which is usually specified at normal gas pressure. For example, a Trane XV80i furnace might have a nominal rating of 100,000 BTU/Hr.
- First and Second Stage Output: During operation, the furnace can switch between two stages:
- First Stage: This stage typically operates at a lower capacity, often around 50-60% of the nominal rating. For a 100,000 BTU/Hr furnace, the first stage might output around 52,000 BTU/Hr.
- Second Stage: When more heat is required, the furnace switches to the second stage, which operates at or near the nominal rating. For the same furnace, the second stage would output around 79,000 BTU/Hr.
Calculations and Efficiency
The efficiency of a two-stage furnace is influenced by its ability to operate at lower capacities for longer periods, reducing short-cycling and increasing overall efficiency. The Annual Fuel Utilization Efficiency (AFUE) rating, which measures the furnace’s efficiency, is calculated based on the weighted average of the first and second stages. The AFUE calculation weights the first stage more heavily because it operates for a larger portion of the time.
Why is My Gas Furnace Only Lighting the First Burner?
If a gas furnace only lights the first burner and fails to engage the second stage, several issues could be responsible:
Potential Causes
- Ignition System Problems: Issues with the ignition system, such as a faulty igniter or spark electrode, can prevent the second stage from lighting.
- Gas Supply Issues: Insufficient gas pressure or a blockage in the gas line can prevent the second stage from receiving the necessary gas flow.
- Control Board Failures: The control board may not be sending the proper signal to engage the second stage. This could be due to a malfunctioning thermostat, a faulty control board, or incorrect wiring.
Troubleshooting Steps
- Check Gas Pressure: Ensure that the gas pressure is within the specified range (e.g., 5.0″ W.C. to 10.0″ W.C. for natural gas).
- Inspect Ignition System: Verify that the igniter and spark electrode are functioning correctly. Clean or replace these components if necessary.
- Verify Thermostat Settings: Ensure the thermostat is set correctly and that it is compatible with a two-stage furnace (it should have a “W2” terminal in addition to a “W1” terminal).
- Check Control Board: Inspect the control board for any signs of damage or malfunction. Check the wiring connections to ensure they are secure and correct.
- Test Gas Valve: The gas valve should be functioning correctly to supply gas to both stages. Check for any blockages or malfunctions in the gas valve.
What are the Design Elements of a Gas Furnace: Orifices and Gas Valve?
The orifices and gas valve are critical components in the design of a gas furnace, responsible for regulating gas flow and maintaining optimal combustion efficiency.
Orifice Sizes
The orifices in a gas furnace are critical for regulating gas flow to each burner. The size of the orifices determines the amount of gas that can flow to the burners during each stage.
- First Stage Orifices: These are typically smaller to restrict gas flow and reduce the heating capacity during the first stage.
- Second Stage Orifices: These are larger to allow more gas flow and increase the heating capacity during the second stage.
Gas Valve Types
The gas valve is responsible for controlling the gas flow to the burners. There are different types of gas valves, but for two-stage furnaces, a dual-stage gas valve is used.
- Dual-Stage Gas Valve: This valve has two positions, one for each stage of operation. It ensures that the correct amount of gas is supplied to the burners based on the stage of operation.
- Specifications: The gas valve must be compatible with the furnace’s design and the type of gas being used (natural gas or propane). The valve must also comply with local codes and the National Fuel Gas Code.
Performance and Efficiency
The orifices and gas valve work together to maintain optimal combustion efficiency. Properly sized orifices ensure that the correct amount of gas is supplied to the burners, while the gas valve ensures that this gas flow is regulated correctly between the two stages. This regulation is crucial for maintaining efficient combustion, reducing short-cycling, and increasing overall furnace efficiency.
Reference:
- HVAC-Talk Forum – Discussion on two-stage furnace ratings and operation.
- Amana Two-Stage Variable Speed Gas-Fired Furnace Installation Instructions – Detailed instructions on installation, combustion, and ventilation requirements.
- Bogleheads Forum – Discussion on the efficiency and operation of two-stage furnaces, including AFUE calculations and comfort benefits.
