The Furnace Temperature Rise Calculation Guide explains how to determine the rise in air temperature produced by a residential or light-commercial furnace. This article covers the key formula, required measurements, examples, and troubleshooting steps to ensure proper furnace performance and efficient heating. It is useful for HVAC technicians, homeowners checking system health, and anyone needing accurate temperature rise calculations.
| Parameter | Symbol | Typical Unit |
|---|---|---|
| Airflow | CFM | Cubic Feet Per Minute |
| Heat Input | BTU/hr | British Thermal Units Per Hour |
| Combustion Efficiency | η | Decimal (0–1) |
| Heat Delivered To Air | BTU/hr | BTU/hr = Input × η |
| Specific Heat Of Air | Cp | 0.24 BTU/(lb·°F) |
| Density Of Air | ρ | 0.075 lb/ft³ |
What Is Furnace Temperature Rise?
Furnace temperature rise is the difference between the supply air temperature leaving the furnace and the return air temperature entering the furnace. It is a key performance metric used to verify furnace sizing and safe operation. Manufacturers specify a recommended temperature rise range for each furnace model. Staying within that range helps prevent short-cycling, heat exchanger stress, and inefficient operation.
Why Temperature Rise Matters
Temperature rise affects comfort, safety, and equipment longevity. A rise lower than the specified range may indicate excessive airflow, undersized furnace, or airflow restrictions downstream of the heat exchanger. A rise higher than specified may indicate restricted airflow through filters or ducts, blower failure, or an overfire condition. Proper temperature rise ensures the furnace transfers designed heat to the home without overheating the heat exchanger.
Key Variables In The Calculation
Calculating temperature rise requires knowledge of the furnace heat output and the airflow through the heat exchanger. Heat output is the usable BTU/hr delivered to the air, which equals the fuel input BTU/hr multiplied by the combustion or system efficiency. Airflow is measured in cubic feet per minute (CFM). Additional constants include the specific heat and density of air.
Core Formula For Temperature Rise
The fundamental calculation links heat delivered to air and mass flow of air. Use this formula to calculate theoretical rise: DeltaT (°F) = (BTU/hr Delivered) / (1.08 × CFM). The factor 1.08 combines air density and specific heat expressed in customary units.
Derivation And Units Explained
Derivation: Heat Transfer (BTU/hr) = Mass Flow (lb/hr) × Cp (BTU/lb·°F) × DeltaT (°F). Convert CFM to lb/hr using density: lb/hr = CFM × 60 × ρ. Combine constants: Cp × 60 × ρ ≈ 1.08. Therefore DeltaT = BTU/hr / (1.08 × CFM). Units must be consistent to yield degrees Fahrenheit.
Step-By-Step Calculation Procedure
Step 1: Determine Furnace Input Rating. This is typically on the furnace rating plate in BTU/hr. Step 2: Obtain Combustion/System Efficiency. Use manufacturer specs or measured efficiency. Step 3: Compute Heat Delivered: Input × Efficiency. Step 4: Measure or estimate CFM through the heat exchanger. Step 5: Apply DeltaT = Delivered BTU/hr ÷ (1.08 × CFM). Step 6: Compare result to manufacturer specified rise range.
How To Measure Airflow (CFM)
Airflow measurement methods vary. The most accurate methods include using an airflow capture hood at registers and measuring total supply air. Alternatively, one can measure static pressures and use blower performance curves to estimate CFM. A simpler field estimate uses return duct velocity or register airflow measurement tools. For best accuracy, sum supply register flows or use anemometers designed for HVAC measurements.
Example Calculations
Example 1: Typical 80,000 BTU Furnace
A furnace has an input of 80,000 BTU/hr and efficiency of 80% (0.80). Delivered heat = 80,000 × 0.80 = 64,000 BTU/hr. If measured CFM = 1,800, DeltaT = 64,000 ÷ (1.08 × 1,800) = 64,000 ÷ 1,944 ≈ 32.9°F. If the manufacturer specified range is 30–50°F, the result is acceptable.
Example 2: Low Airflow Scenario
Same furnace, delivered heat 64,000 BTU/hr, but CFM = 1,200 due to a clogged filter. DeltaT = 64,000 ÷ (1.08 × 1,200) = 64,000 ÷ 1,296 ≈ 49.4°F. This elevated rise signals restricted airflow and potential overheating risks.
Field Measurement Of Temperature Rise
A practical check uses thermometers to measure supply and return temperatures near the furnace. Measure return air temperature at the furnace cabinet inlet and supply air temperature downstream of the heat exchanger. Use stable readings while the furnace runs continuously for several minutes. DeltaT measured in the field should match calculated DeltaT within reasonable tolerance.
Manufacturer Specified Rise Range
Most residential gas furnaces list a recommended temperature rise range on the data plate. Typical ranges are 30–50°F for lower-output models and 35–60°F for high-efficiency condensing units. Always follow the furnace manufacturer’s specified range for that model to avoid voiding warranties and ensure safe operation.
Common Causes Of Abnormal Temperature Rise
- High Rise Causes: Restricted airflow (dirty filter, closed registers, blocked return), failing blower motor, undersized blower wheel, or duct restrictions.
- Low Rise Causes: Excessive airflow (incorrect fan speed, oversized blower), heat exchanger bypass (rare), or incorrect efficiency inputs in calculation.
- Inconsistent Rise: Intermittent control issues, cycling, or duct leakage causing variable airflow.
Troubleshooting Steps For High Temperature Rise
Step 1: Inspect and replace dirty filters. Step 2: Check and open closed supply registers and returns. Step 3: Measure static pressure across the furnace; high static indicates restriction. Step 4: Verify blower motor speed and belt condition on older units. Step 5: Clean evaporator coils or other obstructions that reduce airflow.
Troubleshooting Steps For Low Temperature Rise
Step 1: Verify fan speed setting and control board signals. Step 2: Inspect for leaks in ductwork that reduce air through the heat exchanger. Step 3: Confirm CFM calculation method and measurements. Step 4: Check for short-cycling or combustion inefficiency that reduce heat delivered to air.
Special Considerations For High-Efficiency Condensing Furnaces
Condensing furnaces have different heat transfer characteristics and often have lower permissible temperature rises. They can operate at higher airflow for condensation and efficiency reasons. Also, condensate management and secondary heat exchangers make pay attention to manufacturer DeltaT specifications critical for warranty and safe operation.
Impact Of Altitude And Air Density
Standard DeltaT formula assumes sea-level air density. At higher altitudes, air density is lower, reducing heat capacity per CFM slightly. For altitudes above 3,000 feet, adjust the 1.08 factor downward using local air density. For most residential calculations in the U.S., the standard factor is sufficient, but adjustments increase accuracy for high-altitude installations.
When To Call A Professional
If measured temperature rise remains outside the manufacturer range after simple troubleshooting, a trained HVAC technician should inspect the system. Professionals can perform accurate airflow testing, combustion analysis, heat exchanger inspection, and blower curve measurements. They can also test for heat exchanger cracks, which are a safety concern if overheating is present.
Checklist For Accurate Temperature Rise Measurement
- Ensure furnace has run long enough to reach steady-state operation.
- Measure temperatures close to the furnace cabinet at return and supply sides.
- Use accurate thermometers or digital thermocouples for ±0.5°F accuracy.
- Measure total supply or return CFM when possible, not estimates from single registers.
- Use manufacturer data for input rating and efficiency, or measured combustion efficiency if available.
Frequently Asked Questions
How Often Should Temperature Rise Be Checked?
Temperature rise should be checked during installation, after major service, and annually during preventive maintenance. Checking rise when comfort issues arise or when replacing filters or ductwork is also prudent.
Can Temperature Rise Diagnose Heat Exchanger Cracks?
Temperature rise cannot reliably detect cracks. However, unusual rise values combined with combustion anomalies, odd odors, or carbon monoxide alarms should prompt immediate heat exchanger inspection and combustion testing.
What Is Acceptable Measurement Tolerance?
Field-measured DeltaT generally has a tolerance of ±2–4°F compared to calculated values due to measurement variability. Larger deviations warrant investigation.
Quick Reference Formulas And Constants
| Formula | Use |
|---|---|
| DeltaT = Delivered BTU/hr ÷ (1.08 × CFM) | Main Temperature Rise Calculation |
| Delivered BTU/hr = Input BTU/hr × Efficiency (η) | Heat To Air |
| CFM ≈ Delivered BTU/hr ÷ (1.08 × DeltaT) | Estimate Required Airflow |
| 1.08 | Combined Cp×60×ρ at sea level (BTU/hr per °F per CFM) |
Useful Measurement Tools
- Digital thermometers or thermocouples for return and supply temps.
- Airflow capture hood or balometer for register and total CFM.
- Manometer for static pressure measurement across the furnace.
- Combustion analyzer for efficiency and safety testing.
Key Takeaways
Temperature rise is essential for verifying furnace performance. The calculation is straightforward when input BTU/hr, efficiency, and CFM are known. Accurate measurements and adherence to manufacturer-specified rise ranges protect equipment and occupants. Regular checks and prompt troubleshooting maintain system efficiency and safety.
For complex diagnostics or safety concerns such as suspected cracks or carbon monoxide, contact a licensed HVAC professional to perform comprehensive testing and repairs.