A few years ago, a student of mine told a funny story in a home energy rater class. He was an HVAC contractor and said he was installing a new air conditioner for an elderly woman. As he was explaining things to her, he mentioned that they would be installing a 4 ton unit. "Oh, my," she said. "How are you going to get something so big into my backyard?" The confusion here is completely natural. HVAC and home energy pros find this story funny because when you say an air conditioner is 4 tons, we know it's not weight. It's a number that tells how much heat the air conditioner can remove from the house in an hour. (Let's ignore the issues of nominal vs. actual capacity and AHRI de-rating.) A 4 ton air conditioner is one that can remove 48,000 BTUs of heat per hour from the house. For most people, though, 4 tons means 8000 pounds. (A BTU is a British Thermal Unit, approximately the amount of heat you get from burning one kitchen match all the way down.) Most pros also know how such a common term as 'ton' turned into a bit of HVAC jargon.

Before Willis Carrier invented the modern air conditioner, people used to cool buildings in the summertime with ice harvested from rivers and lakes in the wintertime. A Green Homes America article quotes ice production figures from the 19th century Ice and Refrigeration journal, indicating that the 1890 crop from the Hudson River was about 4 million tons. OK, so people used to cool and refrigerate with ice. How does that equate to air conditioning capacity in BTUs per hour, you ask? Well, let's get quantitative and find out. When ice is below freezing and it absorbs heat, the temperature increases. When ice is at its melting point, 32° F, and it absorbs heat, its temperature doesn't change. If you've had a physics or chemistry class, you may recall that the amount of heat needed to melt ice is called the latent heat of fusion. In Imperial units, that number is 143 BTUs per pound. That's actually a lot of heat to pump into a pound frozen water. Once the ice is melted into liquid water, it takes only 1 BTU per pound to raise the temperature 1 degree.

So if you've got a pound of ice at 32° F, you put 143 BTUs into it to melt it completely. Then it takes only 180 more BTUs to raise the temperature of that pound of water from 32° F to 212° F, the boiling point.air conditioning unit for computer room Anyway, getting back to our main discussion, if you have a ton of ice, it takes (143 BTU/lb) x (2000 lbs) = 286,000 BTUs to melt it completely. home ac unit making noiseYou could do that in one hour or 10 hours or a year, depending on how quickly you pump heat into it. ac unit 1 pkSomewhere along the line, though, someone decided to use 1 day—24 hours—as the standard time reference here. If the ice melts uniformly over the 24 hours, it absorbs heat at the rate of 286,000 / 24 hrs = 11,917 BTU/hr.

Rounding that number up makes it a nice, round 12,000 BTU/hr. In air conditioning jargon, then, a ton of AC capacity is equal to 12,000 BTU/hr. If you're wondering how this term got institutionalized, it was probably the usual way. People in the industry start using it, and then the professional organizations make it official. An architecture website has a quote from 1912 that claims the American Society of Mechanical Engineers standardized it. It sounds likely, but their numbers don't work out, so I'm gonna go with Honest Abe on this one and remain skeptical. For the fearless: If you want to read some funny HVAC banter on this topic, check out this thread in the HVAC-Talk forum. And if you figure out what 'heat of zaporization' is, let me know! The Magic of Cold, Part 1 - How Your Air Conditioner Works It's Called an Air Conditioner — Not an Air Cooler! 5 Questions to Ask When Replacing Your Air Conditioner , used under a Creative Commons license. Photo of ice harvest by Robert N. Dennis, in the public domain, from Wikimedia Commons.

Abe Lincoln photo and quote from The Canterbury Tales by Chaucer. heating & cooling distribution,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.© , All Rights Reserved. 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, 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: 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 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 the More Reading links or topic ARTICLE INDEX shown below.

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. - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice. Green link shows where you are in this article series. OR use the Search Box found below at Ask a Question or Search InspectApedia Click to Show or Hide FAQs Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly. Publisher's Google+ Page by Daniel Friedman