This is a common question asked of HVAC contractors and through online forums, especially at this time of year. It seems like the answer should be simple, take the size of your home and multiply by some number and you have your answer. Some contractors will convince you of the truth of this and will explain that they have been putting in Air Conditioners for thirty years so they know what your home needs. Unfortunately I am not going to give you the answer to the title question, but I will help you to find out when you are calling HVAC contractors.
The truth of the matter is that the size of the A/C unit for your home is almost as unique as your fingerprint. It depends on many variables in the home including but not limited to attic insulation, wall insulation, leakiness of the home, windows, doors, refrigerator, lights, number of people in the home as well as a dozen other things. All of these variables are not information that a contractor can get over the phone or by spending 5 minutes looking at your current unit. The contractor should look at these aspects of your home and perform careful calculations to properly size your unit. There may be a few contractors that take the time to carefully consider your home before sizing a unit but most just want to get the sale and move on to the next job.
To show the difference that these factors make I modeled two homes for best and worse conditions considering all of the variables above. One is an upscale 4400 Sq. ft, two story with a basement the other is a 900 Sq. ft. ranch on a slab. The results surprised even me. For the high efficiency two story, the unit recommended is under 1.5 tons (A/C units are sized in tons which is equivalent to 12,000 BTUs/hr). This same home if it were poorly built and inefficient would require a 5.5 Ton unit. The efficient Ranch would require ¾ of a ton of cooling though you typically can’t get less than 1 ton while the inefficient ranch would need a 2.5 ton unit (40% bigger than the luxury home). If you follow one “Rule of thumb” that I found in another article that was actually trying to dispute the “Bigger is Better” philosophy the Two story home would need a 9 ton unit as compared to a best case scenario of 1.5 tons.
The big question after reading all of this is why does it matter if I get too big a unit, it will just cool the home a little quicker. The reality is that all kinds of bad things can happen when you get too big of an AC unit. First of all you will be cycling the unit more frequently which causes greater wear and tear. A properly sized unit should be running almost constantly during the hottest part of a hot summer day. You will also pay more for a bigger unit on the front end but also for the energy to run it. The biggest issue though is the health and safety of both your family and your home. An air conditioners job is to cool the air and also to take moisture out of the air. A home with an oversized unit will feel cold and clammy because the unit does not run long enough at any given time to begin removing moisture at the condenser as it is designed to do but rather the cold blast of air produced by the unit will begin causing condensation in your home. When this occurs you have the potential for mold growth, as well as the formation of other allergens but this moisture can also start attacking the structural part of your home and can cause walls to rot which is when you start paying the big money.
How then do you make sure you get the right size unit? Start with a rule that if they size your unit over the phone they probably are not taking all of the factors in to play. You should ask the contractor to show you his load calculations though probably 80% of them will look at you with a blank stare or explain that they don’t do load calculations or it will cost you a lot more. I always wonder why it should cost more for the professional you are hiring to provide you with correct answers. Would you expect a heart surgeon to do some testing before he gave you a heart transplant or just take his word that he has been doing this for 30 years and he’s sure your heart is no good (he also has a good deal on a heart they’ve had laying around for awhile). If the contractor will provide load calculations ask him what kind of “fudge” factors they have included. HVAC contractors never want to be called back because the unit is not producing enough heating or cooling so they will add in a little, but in the meantime the program that they are using is already adding in a little and the modeling is based on keeping the house cool on the hottest part of the hottest day of the year so the other 364 days it is already oversized. The other option which is always a good idea is to hire a third party to help you decide which unit to get such an HVAC consultant or an energy auditor. It’s won’t be cheap, perhaps $300-$700 depending on the size and complexity of your home but that is little more than a ton of excess cooling will cost you and you will be have greater comfort, safety and well being. Should you have any questions regarding this article or other energy efficiency measures feel free to ask me at firstname.lastname@example.org.