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Calculator · Furnace airflow
Furnace temperature rise calculator
The temperature rise of a gas furnace is the supply air temperature minus the return air temperature, read once the furnace has settled. The manufacturer prints an allowed range on the rating plate, and a furnace outside it is moving the wrong amount of air for its heat. With 118°F supply and 68°F return the rise is 50°F, inside a 35 to 65°F range. With the furnace output, 76,800 BTU/h here, the same rise means about 1,422 CFM. This page checks a measured rise against your range, estimates the airflow, or takes a known CFM and gives the rise to expect. For the design airflow from an output and a nameplate rise, use the HVAC CFM calculator. For the cooling version of this reading, use the AC temperature split calculator.
- Fire the furnace for about ten minutes with the filter in, the registers open and the blower door on. Put a thermometer in the return duct and one in the supply duct, close to the furnace and out of sight of the heat exchanger.
- Pick the measured mode, enter the return and supply temperatures and the low and high limits of the temperature rise range from the rating plate. Add the furnace output, or its input and AFUE, to get an airflow estimate, and the elevation if it is above sea level.
- The result shows the rise and whether it is below, within or above the range. A rise above the range means too little air, and a rise below it means too much. Change the blower speed, then read again after the furnace settles. The expected mode gives the rise a known CFM should produce.
Example result for the default inputs
Return 68°F, supply 118°F, sea level.
Measured temperature rise
50.0 °F
Against the rating plate range
Within the range
Airflow at this rise, about
1,422 CFM
| Return air | 68.0°F |
|---|---|
| Supply air | 118.0°F |
| Rise, supply minus return | 50.0°F |
| Rating plate range | 35 to 65°F |
| Middle of the range | 50.0°F |
| Furnace output | 76,800 BTU/h |
| Airflow at the measured rise | 1,422 CFM |
| Airflow at the top of the range | 1,094 CFM |
| Airflow at the middle of the range | 1,422 CFM |
| Airflow at the bottom of the range | 2,032 CFM |
| Sensible heat constant at this elevation | 1.080 |
- Read both temperatures after the furnace has run long enough to settle, with the registers open and the blower door on, and keep the probes out of sight of the heat exchanger. Radiant heat from it raises the supply reading.
- The airflow is only as good as the output. It assumes the furnace fires at its rated input and that the readings are the whole rise across the heat exchanger. Blower motor heat, duct leakage and a gas input off the plate rate all move it. Use it to see if the blower is near its table, and confirm with a flow hood or the blower table at the measured static pressure.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the temperature rise and airflow are calculated
Temperature rise is how much the furnace warms the air that passes through it. Subtract the return air temperature from the supply air temperature, once the readings have settled. The manufacturer prints an allowed range on the rating plate. The same heat balance that sets the rise also links the furnace output, the airflow and the rise, so any two of them give the third. This page does both: it judges a measured rise against your range and, with the output, estimates the airflow, or it takes the airflow and gives the rise to expect.
- Temperature rise, measured
rise = supply air temperature - return air temperatureThe Amana and Carrier installation instructions give the same step: place thermometers in the return and supply ducts as close to the furnace as possible, out of sight of the heat exchanger, let the readings stabilize, and subtract the return temperature from the supply temperature. With 118°F supply and 68°F return the rise is 50°F.- Verdict
below: rise < low limit; within: low limit <= rise <= high limit; above: rise > high limitThe limits are the range on the rating plate, entered by you. This page compares the rise at the one decimal it prints, so a reading shown as 35.0°F is not called below a 35°F limit. The default range, 35 to 65°F, is the one printed for the Goodman GMVC960803BNB.- Airflow from the rise
CFM = output / (1.08 x k x rise)1.08 is 60 minutes per hour x 0.075 lb/ft3 x 0.24 BTU/lb F, the standard air value from ASHRAE Fundamentals Chapter 1. k is the air density ratio at your elevation, 1 at sea level. For 76,800 BTU/h at a 50°F rise the airflow is about 1,422 CFM. The Goodman sheet prints 1,400 CFM at 50°F for this model, a little lower.- Rise from the airflow
rise = output / (1.08 x k x CFM)The same relation solved for the rise. 76,800 BTU/h at 1,400 CFM gives 50.8°F, against the 50°F printed beside that CFM.- Output from input
output = input x AFUEUsed when only the input is known. 80,000 BTU/h at 96 AFUE gives 76,800 BTU/h, the output the Goodman sheet prints. AFUE is a seasonal figure, so for older or single-stage furnaces this is an estimate and the printed output governs.- Airflow window
CFM at the high limit, the middle and the low limit of the rangeThe airflow falls as the rise goes up. For 76,800 BTU/h and 35 to 65°F the window is 1,094 CFM at the top of the range, 1,422 CFM at the middle and 2,032 CFM at the bottom. Carrier says to adjust the rise to the midpoint of the range or slightly above.- Elevation
k = standard atmosphere pressure at elevation / 14.696 psiaThinner air carries less heat per cubic foot. At 5,280 ft the constant falls from 1.08 to 0.889, which is 82 percent of the sea level value. This is the same ASHRAE correction the AC temperature split calculator uses. The page does not derate the furnace output. Enter the output at the derated input.
Worked examples from published furnace data
The default furnace is the Goodman GMVC960803BNB, 80,000 BTU/h input at 96 AFUE. Its spec sheet prints an output of 76,800 BTU/h, a rise range of 35 to 65°F and 1,400 CFM at 50°F. From the heat balance that is 1,422 CFM at 50°F. The table below runs the printed recommended CFM and rise of all seven GMVC96 models through the same relation. Each calculated airflow is within 2 percent of the printed one, which is the sheet's rounding.
| Model | Rise range, °F | Output, BTU/h | Printed CFM at rise | Calculated CFM | Difference |
|---|---|---|---|---|---|
| 0403BNB | 20 to 50 | 38,400 | 1,010 at 35°F | 1,016 | 0.6% |
| 0603BNB | 35 to 65 | 57,600 | 1,072 at 50°F | 1,067 | -0.5% |
| 0803BNB | 35 to 65 | 76,800 | 1,400 at 50°F | 1,422 | 1.6% |
| 0804CNB | 25 to 55 | 76,800 | 1,760 at 40°F | 1,778 | 1.0% |
| 1005CNB | 35 to 65 | 96,000 | 1,770 at 50°F | 1,778 | 0.4% |
| 1005DNB | 30 to 60 | 96,000 | 1,980 at 45°F | 1,975 | -0.2% |
| 1205DNB | 35 to 65 | 115,200 | 2,150 at 50°F | 2,133 | -0.8% |
Eight cases below run through the same functions the form uses. The first three show what a reading means on the default furnace: in range, too cool at the supply and too hot at the supply. Most expected rows use the sheet's own CFM. The last row is an example chosen for the page, with no product data behind it.
| Case | Setup | Result |
|---|---|---|
| Goodman GMVC960803BNB, readings in the middle of its range | Return 68°F, supply 118°F, plate 35 to 65°F, output 76,800 BTU/h | Within the range: rise 50.0°F, about 1,422 CFM |
| Same furnace, supply air too cool (too much air) | Return 70°F, supply 100°F, plate 35 to 65°F, output 76,800 BTU/h | Below the range: rise 30.0°F, about 2,370 CFM |
| Same furnace, supply air too hot (too little air) | Return 65°F, supply 135°F, plate 35 to 65°F, output 76,800 BTU/h | Above the range: rise 70.0°F, about 1,016 CFM |
| Goodman GMVC961005DNB, readings only | Return 66°F, supply 121°F, plate 30 to 60°F | Within the range: rise 55.0°F |
| Goodman GMVC960403BNB at its recommended CFM | Output 38,400 BTU/h, 1,010 CFM, plate 20 to 50°F | Within the range: expected rise 35.2°F |
| Goodman GMVC961005DNB at its recommended CFM | Output 96,000 BTU/h, 1,980 CFM, plate 30 to 60°F | Within the range: expected rise 44.9°F |
| GMVC960803BNB output and CFM at 5,280 ft, output not derated | Output 76,800 BTU/h, 1,400 CFM, plate 35 to 65°F, 5,280 ft | Within the range: expected rise 61.7°F |
| An 80,000 BTU/h output on a restricted duct (example chosen for the page, no product data) | Output 80,000 BTU/h, 1,000 CFM, plate 40 to 70°F | Above the range: expected rise 74.1°F |
The 5,280 ft row keeps the output and the CFM of the default furnace. Thinner air raises the rise to 61.7°F, close to the top of the range. At that elevation the manufacturer also reduces the input, so enter the derated output for a real check.
Standards and data sources
- Goodman GMVC96 gas furnace, Product Specifications SS-GMVC96 (12/20) - the temperature rise range printed for each model, the output and AFUE, and the airflow table: recommended, lowest and maximum CFM with the rise printed beside each. For the GMVC960803BNB the sheet gives 35 to 65°F, an output of 76,800 BTU/h and 1,400 CFM at 50°F. The tests check all 21 CFM and rise pairs for the seven models against the formula.
- Amana AKSS9 / GKS9 gas-fired warm air furnace, Installation Instructions (B10259--216) - the measurement procedure: run the furnace about ten minutes with the registers open, put thermometers in the return and supply ducts as close to the furnace as possible without a view of the heat exchanger, let them stabilize and subtract. It also says to adjust the rise with the blower speed. Its blower table prints CFM and rise pairs, which the tests check against one output.
- Carrier 820SA/821SA gas furnace, Installation, Start-Up, Operating and Service and Maintenance Instructions (II820SA-02) - the same procedure with the blower access door on, the first things to check when the rise is outside the range (gas input, altitude derate, duct static pressure above 0.50 in. w.c.), the instruction to set the rise to the midpoint of the range or slightly above, the 60°F minimum return air for that non-condensing furnace, and the input reduction of 4 percent for each 1,000 ft above sea level, once the site is above 2,000 ft.
- ASHRAE Handbook, Fundamentals, Chapter 1 (psychrometrics, standard atmosphere) - the sensible heat constant 1.08 (60 min/h x 0.075 lb/ft3 x 0.24 BTU/lb F) and the standard atmosphere pressure at elevation that scales it. The same constant and correction are used on the HVAC CFM and AC temperature split calculators.
Limitations of this calculator
- The rise range is entered, not looked up. The page does not know your furnace, and the range on its rating plate, which can differ from a sister model, is the only one that counts.
- The airflow is a heat balance estimate. It assumes the furnace fires at its rated input and that the whole output goes into the air that passes the probes. A gas input off the plate rate, blower motor heat, a leaky duct before the probes and a different gas heating value each move it. Use a flow hood, a duct traverse or the blower table at the measured static pressure when the number matters.
- The probes are assumed to be placed as the manufacturers describe, close to the furnace, with no view of the heat exchanger, after the readings settle. Radiant heat on the supply probe reads high, and a return probe near a cold wall or an open register reads low.
- The furnace output is not derated here. The Carrier manual reduces the input 4 percent for each 1,000 ft above sea level, once the site is above 2,000 ft. At elevation, enter the output at the derated input. The Amana table notes that a properly derated unit has about the same rise at a given CFM.
- Two-stage and modulating furnaces fire at more than one rate. Run the check at the firing rate you read, with the output for that rate. The Goodman sheet gives low heat CFM as 0.7 times the high heat CFM and expects the low heat rise to equal the high heat rise within 5 percent.
- The rise range is one of several limits. The rating plate also gives a maximum external static pressure, and the gas input has to match the plate rate. The page checks the rise only.
- Heating with a gas furnace only. Heat pump and electric heat rise follow different instructions and a different output basis.
- This is a field check, not a commissioning report. A licensed contractor, the furnace manufacturer's instructions and the authority having jurisdiction decide the installation. The page makes no claim of code compliance.
Common temperature rise mistakes
- Taking the readings before the furnace settles. The Amana instructions run the furnace about ten minutes with the burners firing and all registers and dampers in their final position, then say to allow adequate time for the readings to stabilize. A reading taken during warm-up is low and climbing, so the furnace looks like it moves too much air.
- Putting the supply probe where it sees the heat exchanger. Both Amana and Carrier warn that the thermometers must not see radiant heat from the heat exchanger. A probe in line of sight reads high. A supply reading that is too high makes the rise look high, which sends you to cut airflow that was fine.
- Reading with the blower door off. The Carrier instructions say the blower access door must be installed when the rise is read, and that leaving it off gives incorrect temperature measurements. Cooler room air gets drawn in at the return side and the rise reads wrong.
- Using the input where the output belongs. The airflow depends on the heat that goes into the air, the output. Typing the 80,000 BTU/h input where the 76,800 BTU/h output belongs gives 1,481 CFM at a 50°F rise instead of 1,422 CFM, 4.2 percent too high. The error is the AFUE turned upside down, so it is small on a 96 AFUE furnace and 25 percent on an 80 AFUE one.
- Ignoring elevation. At 5,280 ft the air carries 82 percent of the heat per cubic foot that it does at sea level. The same 76,800 BTU/h at 1,400 CFM gives 61.7°F there, not 50.8°F. A calculation with the sea level constant would show a rise 11°F too low. The input is also reduced at altitude, so the actual output is lower than the plate figure.
- Letting a small probe error set the airflow. The airflow goes as one over the rise, so a few degrees of error moves it a lot. A supply probe that reads 8°F low turns a 50°F rise into 42°F and the estimate from 1,422 to 1,693 CFM. A probe that reads 8°F high gives 58°F and 1,226 CFM. Treat the airflow as a check on the blower, not a measurement.
Common questions about furnace temperature rise
What is the temperature rise of a furnace?
It is the supply air temperature minus the return air temperature, read at the furnace once it has settled. A furnace that is moving the right amount of air for its output warms the air by an amount inside the range on its rating plate. With 118°F supply and 68°F return, the rise is 50°F.
What should the temperature rise be?
Whatever the furnace's rating plate says. It is different for each model. The Goodman GMVC96 sheet prints 20 to 50°F for one model, 25 to 55°F for another, 30 to 60°F for a third and 35 to 65°F for the rest. The Carrier instructions say to adjust the rise to the midpoint of the range or slightly above. For a 35 to 65°F range that is 50°F.
How do you measure the temperature rise?
Run the furnace with the burners firing for about ten minutes, with the registers open, the filter in and the blower door on. Put a thermometer in the return duct and one in the supply duct, as close to the furnace as possible and out of sight of the heat exchanger. Wait for both readings to stabilize, then subtract the return temperature from the supply temperature. This is the procedure in the Amana and Carrier installation instructions.
How do I calculate the CFM from the temperature rise?
Divide the furnace output in BTU/h by 1.08 times the rise. For 76,800 BTU/h at a 50°F rise that is about 1,422 CFM. Use the output, not the input, and read the rise after the furnace settles. The result is an estimate. The Goodman sheet prints 1,400 CFM at 50°F for this model.
What does a high temperature rise mean?
Too little air for the heat, or too much heat for the air. Check for a dirty filter, a closed register or damper, a restricted return or supply, a blower speed set too low and a gas input above the plate rate. The Carrier instructions also name duct static pressure above 0.50 in. w.c. The Amana instructions warn that an incorrect rise can overheat the heat exchanger. Raise the blower speed to bring the rise down.
What does a low temperature rise mean?
Too much air for the heat, or too little heat. Check the gas input rate and the altitude derate, then lower the blower speed. A low rise is not harmless. The Amana instructions say an incorrect rise can cause condensing in the heat exchanger, and the Carrier instructions say a low rise can cause low vent gas temperature and condensation problems. The Carrier instructions also ask for return air of at least 60°F on that non-condensing furnace.
Does altitude change the temperature rise?
Yes, in two ways. The air is thinner, so each CFM carries less heat. At 5,280 ft the sensible heat constant is 0.889 instead of 1.08. And the manufacturer reduces the input, 4 percent for each 1,000 ft above sea level, once the site is above 2,000 ft in the Carrier instructions, so the output is lower. Enter the elevation and the derated output. The Amana table notes that a properly derated unit has about the same rise at a given CFM.
Can I use the input and AFUE if I do not know the output?
Yes. Output is input times AFUE. 80,000 BTU/h at 96 AFUE is 76,800 BTU/h, the output the Goodman sheet prints. AFUE is a seasonal figure, so for an older or single-stage furnace the result is an estimate. When the spec sheet prints the output, enter that.
Is the temperature rise method an accurate way to measure airflow?
It is a good check and a rough measurement. The airflow goes as one over the rise, so a 8°F low reading on a 50°F rise moves the estimate from 1,422 to 1,693 CFM. It also assumes the furnace fires at its rated input. Use it to see whether the blower is near its table, and use a flow hood or the blower table at the measured static pressure when you need the number.
How is this different from the HVAC CFM calculator?
The HVAC CFM calculator gives a design CFM from a furnace output and the nameplate rise, which is the airflow to aim for. This page checks a furnace that is already running. It takes the rise you measured, says whether it is inside the range, and estimates the airflow the blower is moving, or it takes the blower CFM and gives the rise to expect.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup.
Used on the same job
- Combustion air calculator Combustion air openings for the furnace room, from the input rating.
- Gas pipe sizing calculator Size the gas line for the furnace input, the rate the airflow check assumes.
- Duct sizing calculator Duct sizes for the airflow the furnace should move when the rise points to a restriction.
When the rise is out of range, look at the duct
A high rise usually traces back to the air side: a dirty filter, a restricted return or supply, or a blower speed set too low. The static pressure calculator shows how the duct static pressure changes the airflow, and the duct sizing calculator checks whether the ducts can carry the airflow. You can also ask the assistant to walk through the readings with you. The furnace on this page, with 76,800 BTU/h of output, is an example, not a typical house.
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