All air conditioners, except swamp coolers, work by removing both heat and humidity. Humidity condenses from the air when it passes over the air
conditioner's cooling coil. Therefore, a unit that is too large will cycle too rapidly to remove humidity properly. As a result, the room will feel damp
and less comfortable.
The properly sized unit will effectively remove humidity as it cools the space.
Then there is the cost issue. A larger unit will cost more to purchase, and will consume more energy to operate. Running a smaller properly sized unit
for a longer period of time will use less energy to cool a room than running a larger unit for a shorter time.
"Rule of Thumb" sizing for room air conditioners
Capacity is measured in BTUs, or British Thermal Units, per hour.
To figure the size unit you need, find the square footage of the space you intend to cool. Square footage is width of the space X length of the space.
- 100 to 150 square feet = 5,000 BTUs
- 150 to 250 square feet = 6,000 BTUs
- 250 to 300 square feet = 7,000 BTUs
- 300 to 350 square feet = 8,000 BTUs
- 350 to 400 square feet = 9,000 BTUs
- 400 to 450 square feet = 10,000 BTUs
- 450 to 550 square feet = 12,000 BTUs
- 550 to 700 square feet = 14,000 BTUs
- 700 to 1,000 square feet = 18,000 BTUs
- 1,000 to 1,400 square feet = 24,000 BTUs
Add 600 btu for each person regularly occupying the space, in excess of 2 occupants.
Add 10% capacity if the space is heavily in the sun.
Add 4,000 btu if the unit covers a kitchen.
All air conditioners have a filtration system to keep the coils free from blockage. THIS IS VERY IMPORTANT! Clean or replace the filters often. Make
sure, upon purchase, that the unit you select is easily serviceable.
Just the same as a conventional air conditioning system, a window AC unit's
EER and SEER ratings
should be considered before purchasing. These ratings are energy efficiency ratios. The higher the number, the less the unit will cost to operate.
Also check out Energy Star ratings. Some window AC units are built to meet strict
criteria, such as, high efficiency compressors,
high efficiency fan motors, cycling limiters, and timed on/off circuits.
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