Why watts per CFM says more than watts
Watts on their own describe a motor. Watts per CFM describes a system, because the same motor spends more watts for less air whenever the static pressure it is fighting goes up. A blower drawing 691 watts and moving 1,150 CFM is at 0.601 W per CFM. Put a restrictive filter in front of it and it might draw 660 watts and move 980, which is 0.673 — twelve percent worse on the one number that counts, while the watt reading alone went down.
That is the number to record on a service sheet, because it is the one that stays comparable. Speed taps change, filters change, seasons change; watts per CFM tracks all of them in one figure and tells you which direction the system moved.
The two motor types behave completely differently
A permanent split capacitor motor runs at roughly a fixed speed. As static pressure rises it moves less air and, because it is doing less work, its watt draw often falls slightly. So a badly ducted PSC system announces itself as low airflow at ordinary-looking watts — which is exactly the case where a static pressure reading, not a power reading, is the instrument.
An electronically commutated motor does the opposite. It measures its own output and increases speed to hold the airflow it was told to deliver, so the airflow holds steady across a range of static and the power draw climbs to pay for it. A restricted ECM system does not present as low airflow until it runs out of authority; it presents as noise and watts. This is the reason watts per CFM is the more useful field measurement on modern equipment, and the reason a blower table is nearly useless there.
| Situation | Airflow | Watts | W per CFM |
|---|---|---|---|
| PSC, clean system | 1,150 | 691 | 0.601 |
| PSC, restrictive filter | 980 | 660 | 0.673 |
| ECM, clean system | 1,150 | 390 | 0.339 |
| ECM, restrictive filter | 1,140 | 620 | 0.544 |
Those rows are illustrations of the two behaviours rather than measurements of any product. The point is the pattern: the PSC row loses air and barely moves on watts, the ECM row holds air and pays in watts. Both are the same duct problem.
The heat the motor puts into the air
Every watt the blower draws ends up as heat, and if the motor sits in the airstream that heat goes into the supply air. Watts times 3.412 gives BTU per hour: 691 watts is 2,358 BTU/h. On 1,150 CFM, at 1.08 BTU per hour per CFM per degree, that is about 1.9 degrees of supply air temperature.
It is a small correction on a firing furnace and it is the whole story in fan-only operation, where the motor is the only heat source in the box. It also quietly inflates a measured temperature rise, which is why the temperature rise airflow calculator has a field for it. On the cooling side it works against you: the same heat has to be removed by the coil before any of the capacity reaches the house.
Volts times amps is not watts
A clamp meter gives amps and a voltmeter gives volts, and their product is apparent power in volt-amps. Real power is that multiplied by the power factor, which on an induction motor is not a constant and moves with load. Guessing it puts a systematic error into everything downstream — the annual cost, the heat into the air, the watts per CFM.
The page offers both modes for that reason. If you have a wattmeter on the blower circuit, use it and the power factor question disappears. If you only have a clamp meter, enter what you assumed for power factor so that the assumption is visible in the record rather than hidden inside a number.
Where the rest of the service sheet lives
The static pressure the watts are being spent against is the external static pressure calculator, and the design-side version of the same question is the duct static pressure calculator. For the airflow figure this page needs, the temperature rise airflow calculator on the heating side and the coil enthalpy calculator on the cooling side. If leakage is where the air is going, the duct leakage test calculator. For a fan running continuously as ventilation rather than as distribution, the whole house ventilation calculator.
The air handler is on a live circuit and the run capacitor holds a charge after the disconnect is pulled. Attics and crawl spaces where this equipment usually sits reach temperatures that put people down before they notice.
Questions people ask
What is a good watts per CFM figure?
This page does not state one, and the reason is that the honest comparison is against the same system on a different day rather than against a published number. Motor type alone shifts the figure substantially, and equipment ratings are measured under test conditions that no installed system reproduces. What the number is genuinely good for is trending: record it at commissioning, record it again after a filter change or a duct modification, and the direction of travel tells you something no single reading can.
Why did my watts go down when the airflow got worse?
That is the signature of a fixed-speed motor. A permanent split capacitor blower runs at roughly constant speed, so as static pressure rises it moves less air, does less work, and draws slightly less power. Watts alone will therefore reassure you while the system starves. Watts per CFM catches it because the airflow is in the denominator, and a static pressure reading catches it directly. An electronically commutated motor behaves the opposite way and climbs in watts to hold the airflow.
Do I need a wattmeter, or will a clamp meter do?
A clamp meter gives amps, and amps times volts is apparent power rather than real power. The gap between them is the power factor, which on a motor is not fixed and moves with load, so a clamp meter reading needs a power factor assumption before it means anything in watts. The page accepts either input for that reason. If the number is going into a record that someone will act on later, the wattmeter reading is the one worth having, because it removes the guess entirely.
Does the blower heat really matter to a measurement?
It depends what you are measuring. On the defaults here the motor adds 2,358 BTU/h to the air, which on 1,150 CFM is about 1.9 degrees of supply temperature. Against a furnace putting 64,000 BTU/h into the same air it is a small correction. In fan-only operation, or on a heat pump running the fan alone, it is the entire measured rise — which is why a temperature rise measured with the burner off tells you about the motor and nothing about anything else.
How do I get the airflow figure this page asks for?
Not from the electrical side, which is the honest limitation of this page. On a furnace, temperature rise against the fuel input gives it. On a cooling coil, the enthalpy change gives it. A blower table read at the measured static gives it on a fixed-speed unit but not on a variable-speed one. Each of those has its own page here, and the best practice is to get the figure two independent ways and be suspicious when they disagree by more than a few percent, because at that point one of the measurements is wrong before any result is.