Air Conditioning Sizing

Air Conditioning Sizing

To buy an energy-efficient room air conditioner, you must first determine what capacity, or size, you need. This is important for two reasons:

·        An undersized unit won't cool adequately in extremely hot weather.

·        An oversized air conditioner can also adversely affect your comfort. The unit may switch on and off too often, without running long enough to dehumidify the room properly or cool the space uniformly. And it will consume more energy than necessary.

Use the simple calculation method described below to estimate the cooling capacity you'll need to maintain a temperature of 24°C with a humidity level of between 30 and 50 percent.

Step 1

Calculate the floor area of the space to be cooled.

To calculate floor area, multiply the length of each room or open space by its width.

e.g. 20 ft x 12 ft = 240 sq.ft

Step 2

Calculate the basic cooling capacity you'll need to meet the  cooling load.

Use this table to find the basic cooling capacity in Btu/h you'll need based on total floor area to be cooled.

BASIC COOLING CAPACITY
sq. ft.	Btu/h*
100-150	5 000
150-250	6 000
250-300	6 500
300-350	7 250
350-400	8 000
400-450	8 750
450-500	9 650
500-550	10 500
550-700	12 500
700-1 000	15 000
1 000-1 200	17 700
1 200-1 600	19 000-24 000
1 600-1 800	24 000-27 000
1 800-2 800	27 000-33 000

Thus 240 sq.ft room require 6,000 Btu/h (The average capacity of 1 hp air-conditioner is 9,000 Btu/h and 1½ hp is 12,000 Btu/h)

Note: If cooling capacity is more than 12 000 Btu/h, consider installing two smaller units. Otherwise, a single unit might require a larger amperage circuit (20 to 30 amperes) or a dedicated 240-volt circuit

Below for a more detailed sizing

Additional Sizing Considerations	Add To Or Subtract From The Basic Cooling Capacity +6000 Btu/h +
If the space faces due north or northeast or is heavily shaded, subtract 10 percent of the basic cooling capacity.	X 0.10 =  -
If the space is extremely sunny (e.g. windows face west and southwest), add 10 percent of the basic cooling capacity.	X 0.10 = +600 +
If the space is poorly insulated, add 15 percent of the basic cooling capacity.	X 0.15 =  +
If the space includes a working kitchen, increase cooling capacity by 4000 Btu/h.	+ 4000 = +4000 +
If more than two people regularly occupy the space, add 600 Btu/h for each additional person.	__ (number of additional people) x 600 = +
If the air conditioner will run only at night, subtract 30 percent of the basic cooling capacity.	X 0.30 =  -
Approximate size of room air conditioner to buy.	10 600Btu/h

This article serve as a guideline for a more comprehensive calculation do consult an air-conditioner manufacturer. These calculations are suitable for residential home applications.