An undersized unit will work hard and not reach the set temperature on those very hot days or very cool days – Isn’t that the time when you want the air conditioner to work? Christmas day in Brisbane when all the family is over and it is 360 degrees!!! In addition, the system will be stressed and shorten the life of the components.

Ducted air conditioner systems have a 10-20 year life. It is very difficult and expensive to change a ducted system after the initial installation. If the system is the wrong size then you have to live with it. Getting the right size system is very important.

What sized air conditioning do I need?

Air conditioners these days are rated in Kilowatts Kw. Previous ratings were British Thermal Units – BTU and Horse Power –  HP.

ducted air con

Today’s KW ratings measure the output of the air conditioner. Ratings are given for Cooling and Heating. You will note that the KW output of an air conditioner is higher than the electrical KW input. The ratio of the KW output to the electrical input is called the COP (coefficient of performance).It is also a measure of the efficiency. The higher the COP the more efficient the system. COP is the same value as EER(energy efficiency ratio).

E.g. a Daikin 12.5kw Standard Inverter has a cooling capacity of 12.5kw and a heating capacity of 15kw and COP’s of 2.99 and 3.35 respectively. You will note that the heating cycle is more efficient than the cooling cycle.

Air conditioning calculator

The first step in sizing the ducted system is to calculate the total floor area of the daytime living areas. The reason for using this area is based on the assumption that the bedrooms are not being used during the day. At night time the heat load comes off the house (ie direct radiation from the sun) and also some/all of the living areas are not used at night.

Daytime living areas include kitchens, family rooms, lounges, media, study entrances, hallways, stairs etc. –  any area that will be used during the day except for utility rooms.

The rule of thumb calculation is to multiply the total daytime living area by these factors:

Ceiling Height 2400mm – Total daytime living area x 150watts/sqm

Ceiling Height 2700mm – Total daytime living area x 160watts/sqm

Ceiling Height 3000mm –   Total daytime living area x175 watts/sqm

You require more capacity for higher ceilings because air conditioning is all about the volume of air to be cooled or heated.

The above factors are based on the roof space being insulated with average window areas.

E.g. an average size Brisbane house has a daytime living area of approximately 85sqm and 2700mm ceilings needs a 85 x 160= 13.6kw

Under what circumstances will you need a larger capacity air conditioning system?

  1. Large areas of single glass windows particularly facing west.
  2. Most bedrooms will be used during the day as well as the living areas.
  3. Uninsulated roof space.

Under what circumstances will you need a smaller capacity system?

  1. High rated insulation in the roof space and walls.
  2. Ability to close off and not use daytime living areas on the very hot/cold days
  3. High rated roof space ventilation eg eave vents and whirly birds.
  4. High rated duct insulation eg ultra duct.

The above is a general guide. Always use an experienced specialist residential ducted air conditioning company to advise you on the KW capacity for your home. Get in touch with Crown Power today.