DEAR TIM: Help! We recently had a replacement air conditioning unit installed. It can't seem to get the house cool enough and it always seems humid as well. The factory representative came out with the installer and verified that there were no coolant leaks and that the unit is functioning properly. I'm really "hot" under the collar about this. What could be wrong? J. K.

DEAR J.K.: Believe it or not, it is quite possible that you may have had the wrong sized unit installed. Based upon everything that you sent to me, I feel that this is the only explanation.

Most homeowners are unaware that air-conditioning units come in a wide variety of sizes. They can range in size anywhere from 1 ton to 7 tons, often in half ton increments. Don't confuse these tons with weight. One ton of cooling is equivalent to 12,000 BTU's (British Thermal Unit). It takes roughly 12,000 BTU's to melt a ton of ice in a 24 hour period. Bear with me, I'm about to explain all of this.

Air conditioning equipment removes heat and humidity from your home. The size requirements of this equipment depend upon how much heat is in your home and how quickly it is generated. This heat generation is commonly referred to as "Heat Gain" and is measured in BTU's. There are two types of heat gain, external and internal. External heat gain is that heat which comes from sources outside of the house (sunlight, hot air). Internal heat gain is that heat which comes from sources inside of the house (people, lights, cooking, appliances, etc.).

These sources of heat must be measured accurately. Tables have been developed for these specific purposes. These tables are based upon many variables, all of which must be taken into consideration. The proper use of these tables, in almost all cases, requires that the heating contractor gather specific data concerning your house.

The data which must be gathered includes, but is not limited to, many of the following variables: thickness of wall and ceiling insulation, square foot area of insulated ceilings and floors, square foot area of exterior walls and the compass direction they face, square foot area of windows in each of these wall and efficiency of these windows, shading of windows, average number of people occupying the house, and geographic location of the house.

When all of these measurements are made and the mathematics completed, the heating contractor will have arrived at a figure for "Heat Gain" for your particular home. The calculations are so specific, that two identical houses on the same street may have two different heat gain calculations. For example, the house with the greater number of windows facing west will have a larger heat gain.

The importance of performing the calculations should not be underestimated. An air conditioner that is too small will be unable to remove all of the heat and humidity. An air conditioner which is too large will "short cycle." It will not run long enough to adequately remove excess humidity. Air conditioning units should be sized for the exact heat gain or slightly larger. Properly sized equipment runs at peak efficiency.

I suggest that you call your heating contractor and ask him or her to produce the complete set of calculations. Make the individual prove to you that in fact the proper sized equipment was installed. The air conditioning unit outside of your house will have a factory applied sticker or plate indicating how many BTU's it is rated for. Check it yourself!

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Comments: Welcome! I, Tim Carter, don't answer questions here. If you post a question here in the Comments Area, perhaps another visitor will help you. You need to go to the Ask Tim page if you want a question answered. Once there, look closely at how many weeks behind we are. Please be patient as you use this free service. If you have an emergency and need to talk to me, there is an option there for you. Heather 06 Jun 2008, 14:27 Can I downsize from a 3 ton to a 2 ton, The square footage of my house is far less than I thought, It is 1350. Thanks John Montesinos 04 Jul 2008, 20:34 Hi, can you please help me ..I am running a/c ducts for my older house( never got central air). I would like to pre run ducts ready for full system and I woyld like to place right ducts on rooms ... what size of flexible ducts and how many specially on bigger room ..I may run a 3 ton unit for a 1200 s/f house room 150 s/f ,10" celing room 200 s/f , 10" celing bathroom 40 s/f 10 celing living room 810 s/f w/ kitchen 10 ' celing, Thanks... mike a 23 Aug 2009, 14:31 would a 3 ton unit heat and cool a 1650 square foot home? Gis4Gangsta 24 Sep 2009, 17:30 In my experience, you should calculate 600 sq feet per ton. A 3 ton system sounds just right. roman 01 Mar 2010, 07:13 We are one of the biggest garments manufacturing company in Bangladesh. For our upcoming factory we will need central air cooling system for 200000 square feet and height of the walls will be 14 feet. Please tell us how many tons AC will be required for this type of project. We intend to keep our temperature on 25 degree. Your suggestion in this scenario will be appreciated. Duane 17 Apr 2010, 15:36 I have a 3 ton unit to cool the house. The inside is 1600 sq ft. However, since the house was built back in the 60s before the building codes were updated, it's not big enough for this house. We would need 4 ton, seer 13 to cool it down better. khehoi 20 Oct 2011, 04:56 we need to install airconditioner in our church hall area of 6000 sqft and height of ceiling 25 ft how many nos of window ac of 1.5 ton required      View all comments *Name: Email: Notify me about new comments on this page Hide my email *Text: Security Image: Security Image Text:   Remember, Tim Carter doesn't answer questions here in the comments. You need to go to the Ask Tim page.