QwikSEER White Paper

evaporator coil only increases the compressor lift by about a degree or less (i.e., lowers the

evaporator temperature by 1 degree or less), then we have found that the savings in power

consumption by the blower motor is greater than the increased power consumption of the

compressor, the difference being the energy saved. This is in part because the motor power

curve for the compressor is relatively flat, that is an unloaded compressor motor draws almost

the same power as a loaded motor. This is why we see the most benefit at very low lift, which

also happens to be the place where the typical A/C operates most.

The best way to illustrate this effect is with a specific case. For a 5 HP compressor motor, the

electric motor is about 90% efficient at full load, but only about 30% efficient at 10% of full

load. Therefore for a full load of 5 HP (3728 W), the motor power consumption is 4142 W

(3728/0.90), but at 10% load (0.5 HP or 372.8 W), the power draw is 1,242 W (372.8/0.3).

Increasing the load on the compressor by a factor of 10 (from 0.5 HP to 5 HP), only increased

the power draw by a factor of 3.3 and not by a factor of 10. Therefore, increasing the

compressor motor load a little (by reducing the fan speed and increasing the lift by only 1

degree) does not increase the compressor power draw very much (does not increase it linearly),

but it reduces the blower motor power draw linearly (since different winding have been activated

on the blower motor to keep the efficiency constant). The net result is a reduction in the total

energy consumed by the A/C unit.

A high latent load (lots of humidity) in the conditioned air makes the positive effects of reducing

the airflow even better, since a smaller fraction of the cooling is supplied by sensible air cooling.

The lower airflow allows more residence time for the moisture to be removed from the air and

the lower temperature coil, increases the moisture removal

Installation Effects

The installation-specific effects are related to the configuration of the supply ducts and the return

air ducts, and the type of blower motor. The pressure drop in the supply air ducting can differ

from installation to installation and can even change over time in a single installation because of

changes to the supply register settings. Similarly, the pressure drop in the return air ducting can

differ from installation to installation and can also change over time in a single installation

because of changes in the pressure drop across the air filter (due to changes in the type and

cleanliness of the air filter).

The type of blower motor affects how dramatically a change in pressure drop affects the

evaporator airflow. Constant blower motor speed is not the same as constant airflow rate.

PSC Motors

Different PSC motor speed taps correlate to different motor speeds. Once a technician selects a

speed tap, the PSC motor operates at a single speed. The resulting airflow that is achieved at a

selected speed is greatly affected by the pressure drop in the ductwork. Therefore, the likelihood

that the actual airflow achieved in a specific installation is the design airflow specified by the

manufacturer is unlikely. This configuration cannot alter the airflow as the environmental effects

change (outdoor air temperature, return air temperature, and humidity). To ensure adequate

airflow in the worst environmental conditions, excess airflow is typically supplied.