Cooling loads should be determined by a qualified air-conditioning contractor, using a recognized sizing method such as that specified in CSA-F280-M90: "Determining the Required Capacity of Residential Space Heating and Cooling Appliances." Do not rely on simple rules of thumb for sizing, but insist on a thorough analysis from the sales representative.
Select a central air conditioner size or capacity to just meet the design cooling-load calculated. Oversizing the unit will result in short operating cycles, which will not adequately remove humidity, resulting in an unpleasantly cold and damp home. Undersizing the unit will keep it from attaining a comfortable temperature on the hottest days.
The equipment cost for a central air conditioning system is much more proportional to size than for heating equipment. Unnecessary oversizing will increase the purchase price and increase on-and-off cycling, which will decrease the unit's overall efficiency.
Installation – Central Air Conditioners
The cost of installing a central air conditioner will vary depending on the nature of the existing furnace, the need to modify existing ductwork, and the need to upgrade the electrical service to deal with the increased electrical load of the central air conditioner. The contractor who installs the air conditioner must follow the manufacturer's instructions carefully as well as these general guidelines:
- Place the outdoor unit or condenser in a cool, shaded place, where the waste heat can be readily rejected.
- Locate the outdoor unit where its noise will not be a problem for you or your neighbours. This generally means away from bedroom windows or patios and not between houses.
- In new construction, consider installing the central air conditioner outdoor unit on a frame mounted to the house. This avoids problems that can occur when backfill settles around the foundation, causing the outdoor unit to lose its level.
- A central air conditioner generally requires more airflow than the furnace needs for heating. Consider a two-speed fan motor, with the correct speed automatically selected depending on whether cooling or heating is called for.
- Keep refrigerant lines as short as possible. Where the lines pass through the outside wall, pack the surrounding space between the lines and the wall with a resilient material, such as plumber's putty. This will prevent noise or vibration problems and air leaks.
- Where an existing central air conditioner is being replaced, replace the existing indoor coil with one matched to the new outdoor unit. If the existing indoor coil is not matched, the new unit will not deliver its rated efficiency.