Where 1.08 comes from
It is not a magic number and it is not fixed, which is why this page exposes the two figures behind it rather than burying them. A cubic foot of air at sea level and ordinary temperatures weighs about 0.075 pounds. One CFM for an hour is 60 cubic feet, so it is 4.5 pounds of air an hour. Air takes about 0.24 BTU to raise one pound by one degree. Multiply: 60 times 0.075 times 0.24 is 1.08 BTU per hour for each CFM and each degree of rise.
Both inputs move. Density falls with altitude, so at 5,000 feet the same arithmetic gives roughly 0.062 times 60 times 0.24, or about 0.89 — seventeen percent less heat carried by the same volume of air. Ignore that in Denver and every airflow figure comes out seventeen percent low. Density also falls as air warms, which is a smaller effect at furnace return temperatures and a real one in a hot attic.
The formula, and its two cousins
| What is being carried | Constant | Where it comes from |
|---|---|---|
| Sensible heat, per degree | 1.08 | 60 x 0.075 x 0.24 BTU per lb per °F |
| Latent heat, per grain of moisture | 0.68 | 60 x 0.075 x 1,054 / 7,000 grains per lb |
| Total heat, per BTU per lb of enthalpy | 4.5 | 60 x 0.075, the mass flow itself |
All three are the same mass flow with a different property multiplied onto it. The one that trips people is the water constant, 500, which is 8.33 pounds per gallon times 60 minutes times 1 BTU per pound per degree. That belongs to gpm and hydronic pipe, not to CFM, and it turns up in the hydronic flow calculator. Mixing them up produces an answer that is wrong by a factor of about 460.
The probe placement that quietly ruins this
The supply thermometer has to be out of the heat exchanger line of sight. A probe that can see glowing metal reads radiant heat as well as air temperature and reports a rise several degrees higher than the air is actually carrying — which, run through the arithmetic backwards, understates the airflow. The usual answer is to take the reading around a bend or far enough downstream that nothing hot is visible from the probe tip, and to check that moving the probe a few inches does not move the reading.
The return probe has the opposite failure. If any supply air is finding its way back into the return — a leaking plenum, a bypass damper, a return in the same room as a supply register — the return reads warm and the rise reads small, which overstates airflow. Both errors are placement, not instrument, and neither shows up as anything odd in the number itself.
What the defaults produce
An 80,000 BTU/h input at 80% gives 64,000 BTU/h into the air. A rise of 65 degrees at a constant of 1.08 gives 64,000 divided by 70.2, or about 912 CFM. That is 142 CFM for every 10,000 BTU/h of output. Add a 600 watt blower inside the airstream and the heat into the air goes to 66,047, and the airflow figure rises to about 941 CFM on the same rise.
The sensitivity line is worth watching. On those defaults, one degree of thermometer error moves the answer by about 14 CFM, or 1.5 percent. Shrink the rise to 25 degrees and the same one degree is worth 4 percent. This measurement is at its best on a big rise and at its worst on a small one, which is exactly backwards from where a small rise makes you want to check.
Checking the answer another way
Airflow measured two independent ways that agree is worth far more than either alone. The external static pressure calculator reads airflow off the blower table at your measured static, which uses none of the same instruments. On a cooling coil the coil enthalpy calculator does the same job with total heat instead of sensible heat. If the static and the rise disagree badly, one of the two measurements is wrong before either result is.
For what the fan is spending to move that air, the blower watt draw calculator; for the load the airflow is meant to serve, the heat loss calculator.
A fuel-burning appliance and its flue are licensed work. Combustion testing, draft, gas pressure and heat exchanger condition are not covered here and are not something to work out from a temperature reading. Carbon monoxide has no smell and the symptoms are ordinary until they are not.
Questions people ask
What temperature rise should a furnace have?
Whatever the range on the data plate for that appliance says, which is why this page asks you to type it in rather than offering one. Ranges differ between models and between firing stages on the same model. The page restates the range you entered beside your measurement and reports where the reading falls relative to it, and that is deliberately as far as it goes — a reading outside the range is a signal to investigate, not a diagnosis, and what to do about it on a fuel-burning appliance is licensed work.
Why does my airflow answer change so much for a small temperature change?
Because the rise is in the denominator. Airflow is heat divided by the constant times the rise, so a rise of 25 degrees with a one degree error is a 4 percent error in the answer, while a rise of 65 degrees with the same one degree error is 1.5 percent. The page reports that sensitivity directly. The practical consequence is that the two thermometers agreeing with each other matters more than either being accurate in absolute terms, because a shared offset cancels in the subtraction and a difference between them does not.
Do I use AFUE for the efficiency field?
No, and this is a common substitution that quietly biases the answer. A seasonal figure includes cycling and standby losses across a whole heating season; what this arithmetic needs is the steady-state output while the burner is firing, which is what the plate output divided by the plate input gives. On many appliances the two figures are close enough that it hardly shows, and on others they are not. If the plate lists an output rating, use that divided by the input rather than any seasonal number.
Does the blower motor heat really matter?
It depends entirely on what else is going on. On the defaults here a 600 watt blower adds about 2,047 BTU/h to 64,000, which is 3.1 percent — real but small. In fan-only operation, or on a heat pump running on the fan alone, the motor is the only heat source in the airstream and it is the whole of the measured rise. An air handler with the motor outside the airstream contributes nothing at all. Enter zero if you are not sure; the field exists so the effect is visible rather than assumed.
Can I use this on a heat pump or an electric furnace?
The arithmetic is the same but the input field is not. On electric resistance heat the heat into the air is the element wattage times 3.412, so enter that as the input with the efficiency at 100 and the answer is correct. On a heat pump in heating there is no fixed input figure to enter — the capacity varies with outdoor temperature — so the rise tells you very little about airflow unless you already know the capacity at that moment. On a heat pump in cooling, the coil enthalpy page is the right tool instead.