How to determine the cooling capacity of air conditioning or heat pump equipment
- COOLING CAPACITY, RATED - 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
- Questions & Answers about how to find out the cooling capability of an air conditioner or heat pump from translating its data tag, label, or other information.
- References
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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. Also see LOST COOLING CAPACITY for diagnosis of poor air conditioner performance, and see AIR CONDITIONER BTU CHART for guidance in purchasing an air conditioner of the right size or capacity in Tons or BTUs. This website answers all questions about air conditioning and heat pump systems. Also see How to Measure Central Air Conditioning Temperatures for
a discussion of what temperatures to expect at different points in the air conditioning system.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
RATED COOLING CAPACITY - How to Determine Air Conditioning or Heat Pump Equipment Rated Cooling Capacity
If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS. See How to determine the cooling capacity of air conditioning equipment if the system seems to be working but is inadequate to cool your building.
The 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.
"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
Serial 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 A/C DATA TAGS 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:
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.
Do not buy an air conditioner that has more tons or BTUs of capacity than you actually need.
Oversized Air Conditioning Systems: Can an air conditioning system have too much capacity?
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.
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:
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
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.
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Technical Reviewers & References
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Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
- 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
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
- Thanks to Scott at SJM Inspect for suggesting this EPA document and for technical editing remarks regarding our air conditioning website,
SJM Inspection Service LLC, serves the entire state of CT, sjminspect.com 203-543-0447 or 203-877-4774
5/16/07
- Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
- Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
- "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]
- Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
- 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
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard- Asbestos HVAC Ducts and Flues field identification photos and guide
- Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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