Question:
Our house has 2 Central Air Conditioning(CAC units, one for each floor. They work fine but I'm not
certain if I'm correctly sealing off rooms that are not being used. Do I
need to close both vents of the returns and keep the door closed? Also,
does turning off the lower floor CAC at night help lower our electricity
bill?
What kind of system performance problems would sealing off a room cause?
Answer:
Central air conditioning systems are designed with specific volume (airflow)
and pressure to accommodate the rooms to be conditioned. If you close off a
supply vent (and / or a return vent) you are likely to create system
performance problems elsewhere.
And yes, turning off the A/C (or raising the temperature with a setback
thermostat) will save money.
http://www.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12720
It depends on the specifics of your system design. In general closing off
supply registers will increase the static pressure in the system. Depending
on how the ducts were sized this can result in higher air velocity in the
remaining ducts thus more noise and lower air temperature drop across the
coil; this results in less effective humidity removal. Closing off a return
register can reduce air velocity and cause icing of the coil.
