Photograph of  this unusual attic air conditioning system is an example of the range of human creativity observed during a career of building inspections How to determine the cooling capacity of air conditioning or heat pump equipment

  • COOLING CAPACITY, RATED - CONTENTS: How to determine the rated cooling capacity of an air conditioner. Decoding air conditioner model number can tell its BTUs or tons. How to translate an air conditioner RLA to cooling capacity in BTUs or Tons. Air conditioning system cooling capacity rules of thumb
  • POST a QUESTION or READ FAQs about how to find out the cooling capability of an air conditioner or heat pump from translating its data tag, label, or other information.
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Rated cooling capacity of an air conditioner or heat pump:

Here we explain exactly how to estimate the rated cooling capacity of an air conditioning system by examining various data tags and components.

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RATED COOLING CAPACITY - How to Determine Air Conditioning or Heat Pump Equipment Rated Cooling Capacity

Explanation of a ton of cooling capacity (C) Carson Dunlop AssociatesThe cooling capacity of an air conditioning system is expressed in BTU's or tons. One ton of cooling capacity equals 12,000 BTU's/hour of cooling capacity.

[Click to enlarge any image]

"One ton" of cooling capacity, historically, referred to the cooling capacity of a ton of ice. Tons of ice does not explain a key ingredient in the comfort produced by air conditioning systems, dehumidification of indoor air - that is, taking water out of the air.

Cool air can hold less water (in the form of water molecules or gaseous form of H2O) than warm air. Think of the warmer air as having more space between the gas molecules for the water molecules to remain suspended.

When we cool the air, we in effect are squeezing the water molecules out of the air. When an air conditioner blows warm humid building air across an evaporator coil in the air handler unit, it is not only cooling the air, it's squeezing out some of the water in that air. Both of these effects, cooler air and drier air, increase the comfort for building occupants. Sketch courtesy of Carson Dunlop Associates.

There are several ways to determine the rated cooling capacity of an air conditioning system's equipment:

Air Conditioning Equipment Age and Capacity from Equipment Numbers

Sketch on estimating the size or capacity of an air conditioner (C) Carson Dunlop AssociatesSerial number formats vary by range of years over which equipment was manufactured, and may vary among countries of manufacture for a given company's equipment, for example between the U.S. and Canada for Carrier air conditioning equipment.

Example: a Carrier Compressor/Condenser Serial# 1389E54894 on a compressor unit.

Air conditioning equipment age from serial number for the example above, the equipment was made after 1980. The first four digits of the serial number are week and year of manufacture, in this case, week 13 of 1989.

See AIR CONDITIONER BTU CHART for a guide to selecting an air conditioning system with the proper cooling capacity in tons or in BTUs.

Sketch courtesy of Carson Dunlop Associates.

FROM MODEL # - Air conditioning equipment capacity from model number

Example: Carrier A/C Compressor Condenser Model# 38XD12400 (same unit as used for the serial number example above), there is variation in how Carrier assigned these numbers but typically the numbers indicate either tonnage or MBTUH. This example has digits in the 4th and 5th positions (right hand 5 digits), so the rating is in MBTUH for this number and "24" signifies 24 MBTUH or 2 tons of capacity.

Be sure to review our article on how to read the data in
for a guide to reading the system cooling capacity either directly off of the sticker on the equipment, or for examples of how to find them model number which can be de-coded into cooling capacity and other features.

A Reference Guide to Heating and Air Conditioning Equipment model numbers, serial numbers, age, and capacity: at Carson Dunlop's  -  Technical Reference Guide, published by Carson Dunlop Weldon & Associates, Ltd., Toronto, 2006 for a $69.00 book which translates air conditioning equipment model numbers and serial numbers into date of equipment manufacture and rated BTUH capacity.

FROM EQUIPMENT RLA # - Air Conditioning Equipment Capacity from Equipment RLA Numbers

RLA Rule of Thumb: RLA, Rated Load Amps, or in some older texts, mis-named as "Running Load Amps" is the manufacturer's specified rated current draw when the equipment is operating, excluding the current draw during startup, but when the compressor is under load.

On a single-phase 240V circuit feeding an A/C compressor/condenser unit, the equipment will draw typically 5 to 6, (7 in some cases) RLA per ton of cooling capacity. So if the data tag on a compressor shows its RLA rating=21.2 I would rate the system as 21.2/7=3 Tons. Translating Tons into BTUH, 3tons x 12 MBTUH/ton = 36,000 BTUH estimated Cooling Capacity. Details of this and related calculations are in the "Guide" book cited above.

COOLING RULES OF THUMB - determining Cooling Capacity Requirements - Rules of Thumb

A home inspection does not involve the calculations of heat gain necessary to decide if the cooling capacity on a building is adequate, but the inspector is expected to examine and report on the rated system capacity (such as "36,000 BTUH") and on the presence or absence of cooling sources in the habitable rooms of the building.

Air conditioning capacity requirement:

Sketch explaining how many square feet of building can be cooled with one ton of air conditioning power (C) Carson Dunlop Associates

A simple rule of thumb for relatively cool climates such as the Northeastern United States: one ton per 400 sq .ft. (Commercial) or one ton per 500 to 1000 sq .ft. (Residential)

Or we estimate an air conditioning requirement of one ton per 400 to 800 sq .ft. for Space Pak Systems.

Or a 3000 sq .ft. house may require a 5-ton unit. Or count the supply outlets: 10 outlets @ 100 cfm (estimated) = 1,000 cfm = 2.5 tons needed.

Oversized Air Conditioning Systems: Can an air conditioning system have too much capacity?

Sketch explaining that oversized air conditioners are a mistake (C) Carson Dunlop Associates

Yes. If a system is over sized for a building it may be able to drop the indoor temperature so rapidly that the cooling cycle is too short to permit adequate reduction in the humidity level. Remember that indoor comfort is a function of both temperature and relative humidity.

Also, since an oversized air conditioning system will be cycling on and off more frequently, not only is the building actually less comfortable (temperatures are swinging up and down unnecessarily quickly and frequently) but it may also be harder on the equipment, thus shortening its life.

Turning electric motors on and off is hard on them.

If the "on cycle" of the A/C system seems unusually brief, or if the indoor humidity is not dropping this question merits further investigation. Sketch courtesy of Carson Dunlop Associates.

Do not buy an air conditioner that has more tons or BTUs of capacity than you actually need.

How to Determine BTUs or Tons of Cooling Capacity of an Air Conditioner from its Data Tags

In addtion to reading the present article (here) see


or determine rated cooling capacity

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.

How Big Should My Air Conditioner Be in BTUs or Tons?

In addition to reviewing
our COOLING RULES OF THUMB shown above,

see AIR CONDITIONER BTU CHART for a guide to selecting an air conditioning system with the proper cooling capacity in tons or in BTUs.

What Temperature Should an Air Conditioner Be Able to Maintain

Reader Question: 8/27/2014 Anonymous said: hat temperature should my new central A/C be able to hold

Regardless of all the sizing, energy usage and environmental considerations, what temperature should my new central A/C be able to hold at the hottest time of day? It seems like this should by irrespective of any particular climate, just need more capacity the hotter it can become.


I'm not sure I agree that the performance of air conditioning systems - the ability to hold a specific temperature at the hottest time of the day - is nor can be independent of the system design. For example, if we have a poorly insulated building, a building with high heat gains, a building where people regularly leave windows and doors open or come in and out constantly, it could be misleading and specious to only blame the air conditioning system for complaints that the building interior is too arm.

Wouldn't we first close open windows and doors, before installing a new, larger cooling system?

However you are quite right to ask how we should measure whether an AC system is performing as it should.
Some helpful references that answer that question are at

ANSI/AHRI Standard 210/240 with Addenda 1 and 2 (formerly ARI Standard 210/240) titled in more detail "2008 Standard for Performance Rating of Unitary Air - Conditioning & Air-Source Heat Pump Equipmet" (available online from AHRI and ANSI)

You might also want to refer to TSI's "Hvac Assessment Handbook A practical guide to performance Measurements in mechanical heating, Ventilating, and air conditioning systems" available from tsi [dot] com

In Australia and New Zealand or the South Pacific we refer readers to "Performance Standards Of Hvac Equipment (HVAC Equipment Performance Consultancy)" available from the Australian government abcb [dot] gov [dot] au


Continue reading at SEER RATINGS & OTHER DEFINITIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see AIR CONDITIONER TEMPERATURE MEASUREMENT for a discussion of what temperatures to expect at different points in the air conditioning system.

Or see AIR CONDITIONER BTU CHART - table of BTU requirements by cooling area size & other parameters

Also see LOST COOLING CAPACITY for diagnosis of poor air conditioner performance, and see This website answers all questions about air conditioning and heat pump systems.

Suggested citation for this web page

RATED COOLING CAPACITY at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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