Air Conditioning / Heating System Cost and Comfort Level

The basic air conditioning system for a residence is a single zone, split system air conditioner, heat pump or gas furnace.  The air distribution system is typically in the attic, basement or crawl space underfloor.

Comfort is closely related to the ability to closely control the mechanical operation of the system.  Air conditioning is controlled by a switch called a thermostat that turns the system on or off to supply the air needed to maintain the desired temperature set point.

One thermostat controls one zone.  Multiple cooling zones are required where the building heats or cools at different rates or at different times.  The most common example is a two story home.  The upstairs area is under the unconditioned attic and the downstairs area has conditioned space above it.  The upstairs space will require cooling when the downstairs space does not.  Additionally since heat rises, the heat from below will rise to the second floor.  

Common factors that create a zone:

  • Multiple floors.
  • Rooms with exterior walls.  Rooms with no exterior walls.
  • Rooms with high internal heat loads such as kitchens.
  • Rooms with large amounts of glass.  South glass gets sun all day and west glass gets sun in the afternoon.  North glass never gets direct sun unless it is reflected by another structure.
  • Unoccupied rooms can be separately zoned to save energy.
  • Bedrooms need control at night and gamerooms need control when occupied with several people.
  • Dining rooms are unoccupied most of the time and heavily occupied for short periods of time.

Levels of control

  • Single story home:  Single zone.  One system, one thermostat.
  • Two story home: 2 zones.  Upstairs zone with a thermostat.  Downstairs unit with a thermostat.
  • Large home: Multiple zones
  • Ultimate control:  Thermostat in every room.

Multiple zones require additional thermostats, hardware and units to provide the additional control.  More zones will increase system cost.    Many large homes have chill water systems because they are easily adapted to unlimited zoning and control.      

The level of humidity in a building can also have a significant effect on comfort.  The typical single zone residential system does not have humidity sensors for control.  Air conditioners remove moisture from the air (latent heat) as a function of their normal operation.  In areas where humidity is higher than normal additional hardware and control can be added to units to remove additional moisture.

Dehumidification can be accomplished with the following:

  • Multispeed or variable speed compressors in the condensing unit.
  • Stand alone refrigerated dehumidifiers.
  • Smaller AC unit that operates as the first stage of cooling controlled by a humidistat.
  • Dhinn coils.
  • Overcooling the air to dehumidify and reheating for comfort.