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. 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. [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,contactor outside ac unit increase the comfort for building occupants. hvac induction unitSketch courtesy of Carson Dunlop Associates.wall mounted air conditioning units for conservatories There are several ways to determine the rated cooling capacity of an air conditioning system's equipment: 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. 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. 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. 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. data tag on a compressor shows its RLA rating=21.2 I would rate the system asTranslating 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. 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. 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) 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.

supply outlets: 10 outlets @ 100 cfm (estimated) = 1,000 cfm = 2.5 tonsIf 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 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. Do not buy an air conditioner that has more tons or BTUs of capacity than you actually need. In addtion to reading the present article (here) see

COOLING COIL or EVAPORATOR COIL 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. 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. 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