Air Conditioner Sizing

There is no challenge to air conditioner sizing when using window, sleeve, or portable units. These air conditioners will have a label on the back which tells you the output of the unit as well as the square footage (or cubic footage) it can cool.

When dealing with central air conditioner sizing, the homeowner will probably leave it up to the HVAC contractor or an architect/engineer to determine the appropriate size of the units and ducts. But this usually results in the units being oversized, which needlessly spends your money in the future.

Ways To Determine The Size Of Your Air Conditioner

Compare your home to similar homes in your area. Does your neighbor have the same size home as you? If he has a properly sized air conditioner, then the same size unit may also work for you.

Ask a contractor. Most air conditioning professionals give free in-home estimates for installing new air conditioning systems, during which they will recommend a unit size.

While we don't encourage using contractors for quotes if you don't intend to hire them, a contractor familiar with the homes in your neighborhood will likely be able to give you an idea over the phone of what size you might need.

If you want to know how they go about it (and maybe try to check their calculations), here's an explanation of what they do.

Comfort, Psychrometric and Duct Sizing Charts' Used in Air Conditioner Sizing

This method of sizing is used for central air conditioning units - where you'll have multiple rooms to cool, with fresh air intake from outside.

Here's the checklist for AC sizing using this method:

1. Use a comfort chart, to set the desired indoor conditions. A comfort chart is formulated by ASHRAE, and shows a range of comfort levels for human beings. The comfort levels are provided in terms of humidity, and temperature.

2. Next, the "Manual J Method" is used to find out the latent heat gain and loss in each room within a home. Room slope will be assigned for all rooms, individually. It is carried out by dividing sensible heat into total heat (sensible, plus latent heat).

3. Now the psychrometric chart is used. This chart will eventually determine the minimum air conditioner size for your residence. The size is determined by finding out the following values:

Mass flow rate of air into each room,

Fresh air to recirculated air ratio,

And finally, the air conditioner size from cooling load value.

Data that you will need includes: the volumetric air requirement per person, of 5 litres/person/compartment; second, outside and inside average temperature and humidity, and room slope values.

4. Ducts will be the final item to be sized, as this is based on the air mass flow rate. Duct sizing charts need to be used for this, with prior information of maximum air draft at design condition, and duct resistance values.

Look at the name plate on the outdoor condensing unit and locate the model number (not serial number). You are looking for 2 digits in the model number that match the numbers below to indicate Tons or BTU.

18 = 1.5 Ton (18,000 BTU)
24 = 2 Ton (24,000 BTU)
30 = 2.5 Ton (30,000 BTU)
36 = 3 Ton (36,000 BTU)
42 = 3.5 Ton (42,000 BTU)
48 = 4 Ton (48,000 BTU)
60 = 5 Ton (60,000 BTU)

If you want more information about sizing your air conditioner, I recommend a visit to www.alpinehomeair.com. Their website was helpful to me when composing this page.

Feel free to contact me if you have questions about anything else by clicking here.

Return From Air Conditioner Sizing To Home Remodeling Ideas.

Return From Air Conditioner Sizing To The Home Page.

See what your project will cost - call 866-452-3059 for free quotes with no obligation to hire.