How to Choose an Air Conditioner - BTU Chart
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How to choose an air conditioner that fits the building and your cooling needs?
How big an air conditioner do I need?
How much air conditioning do I need? How many BTUs or Tons of Air conditioning?
Can an air conditioner be too powerful for the building?
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How to choose an air conditioner for window or through-wall mounting, how much BTU capacity do you need?
Also see COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs and see RATED COOLING CAPACITY to determine the cooling capacity of existing air conditioning equipment.
Typical BTU Cooling Capacity Range for Air Conditioners
Portable, window, or through-wall air conditioners are typically described by their manufacturer as suited for:
Single Room Air Conditioner Capacity - typically for rooms up to 20' x 20' or 400 sq.ft. in area. BTUs in this product range are typically from 6,000 BTUh to
10,000 BTUh.
Multiple Room Air Conditioner Capacity - typically for a total area of up to 800 sq.ft. BTUs in this product range are typically from 10,000 BTUh to 16,000 BTUh.
Large Capacity Air Conditioner Capacity - typically for multiple rooms or very large rooms up to a total area from 900 sq. ft. to 2,000 sq.ft. BTUs in
this product range are typically from 16,000 to 28,000 BTUh.
Central Air Conditioning - typically to cool an entire floor or multiple floors in a home. Also see
A/C TYPES, ENERGY SOURCES
To determine the BTUs needed to cool a given space follow this procedure:
Calculate the total square feet to be cooled: Measure the size of the room (or rooms) to be cooled, to obtain total square feet. Multiply room length by width for
each room and if there are multiple rooms, add the room areas together to get a single number.
Read the Base BTUs needed from Table 1 below
Add additional BTUs for these factors:
+ 4,000 BTUs for each room below a ceiling or roof which is not insulated
+ 4,000 BTUs for a home or residential kitchen included in the cooled area
+ 1,500 BTUs for each window which receives significant daily sunshine
+ 1,500 BTUs for a room over a kitchen or boiler room IF the kitchen or boiler room is actively producing heat during the cooling period
+ 600 BTUs per person over two, if more than two occupants will be occupying the room during the cooling period
Subtract BTUs from the total required if these factors are present:
- 1,000 BTUs if the room is on the shaded side of the building
Calculate the final total BTUh needed from the above steps. This should place you in the right range of cooling capacity
needed. Review the warning below about buying an oversized air conditioner.
The table below gives recommended air conditioning BTU's necessary to cool a single room. The data in the table assumes
that the ceiling over the room is insulated and that the room is not over or is not itself a special heat-producing
area such as a kitchen or boiler room.
Table 1: Base BTUs - Recommended Air Conditioner BTUs
Number of Rooms Cooled
Room Area MINimum sq.ft.
Room Area MAXimum sq.ft.
BTUs Needed
One
.
100
4,000 - 6,000
One
100
180
6,000 - 7,500
One
180
270
7,500 - 9,000
One
270
400
9,000 - 10,500
Several
.
400
10,500 - 12,000
Several
400
500
12,000 - 13,500
Several
500
700
13,500 - 15,000
Several
700
800
15,000 - 16,500
Large Areas
.
900
16,500 - 18,000
Large Areas
900
1000
18,000 - 19,500
Large Areas
1,000
1,100
19,500 - 21,000
Large Areas
1,000
1,100
19,500 - 21,000
Large Areas
1,100
1,200
12,000 - 22,500
Large Areas
1,200
1,500
22,500 - 24,000
Large Areas
1,500
1,700
24,000 - 25,500
Large Areas
1,700
1,900
25,500 - 27,000
Large Areas
1,900
2,200
27,000 - 28,500
Do not buy an air conditioner which is oversized (too many BTUh) for the area you need to cool. You may think that bigger is better, but not in the case
of air conditioning.
To make a room comfortable the air conditioner needs to both cool the room air AND dehumidify the room air.
If the air conditioner is too large for the space to be cooled, the temperature will drop quickly and the A/C unit will shut off before
the air has become adequately dry.
The room will be either too cold or too humid for comfort.
Sketch courtesy of Carson Dunlop.
How Much Cooling Capacity do we need Per Square Foot of Building Area? How Much Space can a Ton of Cooling Capacity Serve?
Maybe 450 sq.ft. to 1000 sq.ft. of a typical home can be cooled per ton of cooling capacity: that is, one ton (or 12,000 btuh) of air conditioning can cool about 500 sq.ft. of space. Sketch courtesy of Carson Dunlop
But the real answer is, it depends. Some of the factors that affect the ability of an air conditioner to cool a space need to be considered besides just the number of square feet. These include at least the following questions about air conditioning load and cooling requirements:
What are the sun and heat characteristics of the geographic area where the building is located (southern U.S. vs. northern U.S. or Canada, for example)?
How much direct sunlight is falling on the building?
Is it bright hot sun or only partly sunny?
What are the exterior colors of surfaces on which sunlight is falling?
How well the building is insulated?
How drafty is the building?
How many occupants are in the building?
What other heat sources (or cooling sources) are in the building?
How high are the interior ceilings?
How does air circulate within the occupied spaces?
What defects in the air conditioning system need to be overcome, such as duct system errors or damage, dirty filters, blocked cooling coils, etc. ?
Other types of portable or individual-area air conditioners
Heating & Cooling units - capable of both cooling or heating a room using electricity. Basically these units are small heat pumps
that are mounted in a building window or wall. Heating/Cooling units will give two different BTUh figures, one for cooling and one for heating.
These figures will differ, for example, producing 18,000 BTUh in cooling mode but only 12,000 BTUh in heating mode. The difference between
heating and cooling, and the amount of heat actually available will depend also on the outdoor temperatures when in heating mode (as with
any heat pump system, the unit cannot provide heat below certain temperatures.)
Slider or Casement Window units - narrow tall cooling systems which are designed to fit into the narrow space provided by
casement or slider windows.
Through-wall air conditioners - air conditioning units which are designed to be installed into a metal sleeve which is then
itself installed in an opening cut into the building wall, leaving windows unobstructed, or perhaps for use in a room without
a suitable window in which an air conditioner could be placed. BTU output is typically a bit more than the smallest window
air conditioners but otherwise is similar in range.
Portable air conditioners - units on wheels which are plugged into an outlet but can be moved room-to-room and do not require a window for their exhaust. These
cooling units are of modest cooling ability, typically around 10,000 BTUh.
How to Determine BTUs or Tons of Cooling Capacity of an Air Conditioner from its Data Tags
See RATED COOLING CAPACITY for an explanation of how to determine the cooling capacity of an air conditioner that is already installed at a building or go directly to these links within that article:
FROM MODEL # - how to determine the BTU capacity or Tons of cooling capacity of an air conditioner from model number FROM EQUIPMENT RLA # - how to determine the BTU capacity or Tons of cooling capacity of an air conditioner from the RLA number COOLING RULES OF THUMB - how to guesstimate how many tons or BTUs of cooling a building needs
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Additional technical contributors & reference sources for this article are listed below.
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Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.
Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to various industry and air conditioner sales publications and consumer pamphlets including Sears Kenmore(R) air conditioning sales
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
RATED COOLING CAPACITY How to determine the cooling capacity of air conditioning equipment
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