QwikSEER White Paper

the compressor contactor, 24 VAC across the W (white) and C activates the heat contactor, and

24 volts across either O (orange) or B (blue) to C actives the heat pump reversing valve. In

addition, 24 VAC is continuously supplied to the thermostat, if needed, via R (red) and C wires.

Digital Thermostats

On a digital thermostat, 24 VAC is still supplied to the thermostat via the R (red) and C

(common) wires; however, all commands from the thermostat to the system are via two or more

digital lines. Such digital thermostats typically also control compressor speed and/or blower

motor speed among other things. Examples of digital thermostats are the Carrier Infinity

Control, Lennox icomfort Touch

, and Goodman ComfortNet

. The ability to adjust the

blower speed and compressor speed, provides a higher EER rating, however these systems are

also more expensive.

Effects of Conditioned Air Flow Rate on Efficiency

To meet more stringent energy efficiency requirements for air conditioning systems, some

manufacturers are trying to optimize the operation of air handler blower motors. An inefficient or

oversized blower motor decreases system efficiency because this motor draws additional power

and the power consumed by the blower motor heats the air that is being cooled. Unfortunately,

optimal air handler airflow is affected by several factors, which can be divided into two

categories, environmental effects (which can continuously vary throughout any given day) and

installation-specific effects (which can vary from installation to installation or from technician to

technician). Equipment manufacturers have focused most of their efforts on minimizing the

negative effects of installation-specific variations so that their units operate properly, in spite of

less-than-perfect installation conditions.

For PSC blower motors, manufacturers typically provide excess fan capacity to accommodate

less than ideal ductwork systems that may have higher than normal pressure drops. Likewise,

many technicians simply use the highest blower speed to assure that sufficient air is moving

across the coil to avoid coil freeze-ups, and minimize callbacks. In addition to wasting energy,

the higher flow rate will reduce humidity removal, due to the higher coil surface temperature and

reduced residence time (the time the air is exposed to the coil surfaces).

Environmental Effects

The environmental effects are related to outdoor temperature, return air temperature, and return

air humidity. This section describes the environmental conditions that would benefit from

reduced airflow (and reduced energy consumption) if the air handler blower motor had the

capability to adjust airflow. Although some high-end systems (high EER systems) with digital

thermostats and continuously variable speed ECM motors already control evaporator blower

airflow and/or compressor speed to improve the EER, typical systems using analog thermostats

do not have this sophistication.

The heat removed by the evaporator will always match the heat carried from the air by the

blower, the question is how to best adjust the mass flow rate of conditioned air, that is how to

best adjust the blower motor speed. We have found that under certain situations, a lower blower

shaft speed, which reduces air flow rate, and reduces heat transfer, also reduces overall power

draw. For example, when the lift of the A/C unit is small, if reducing the air flow across the