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How to Size Your Air Conditioning System

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Save energy save money. An improperly sized AC unit can be a real energy waster, so what size should my AC be? So lets look at sizing air conditioning systems.

My air conditioning was working, but I feared that something was wrong. As the heat has become relentless in Houston these past few weeks, I noticed that the system was struggling to keep up. It was constantly on. I knew that my electricity bill was going to be high; my wife was concerned about my daughter’s birthday party. Would our guests be comfortable? What size was my ac unit, and did I know how to size my air conditioning?

I knew that my ac was sized properly for the house, so I thought that maybe the refrigerant had a problem. HVAC units come sized to provide so many tons of air conditioned air. They are either at the ton mark or a half ton mark. For example, many smaller homes may have a 3 ton unit or a 3 and a half ton unit. You will not have a 3.75 ton unit. To determine the number of tons in principle is fairly basic formula: square footage of the home divided by 500 equals the tons needed for your system. That is how to size an air conditioning unit. However, let us take a 1700 sq ft home; this gives us the result of 3.4. The next step is where HVAC service people may disagree. Our result is close to 3.5, so most installers would go with a 3.5 ton unit. There is an argument against this though. A few installers may tell you that a unit has to be undersized by a bit to function well, so they would suggest a 3 ton unit. There is a case to be made for this statement, but my HVAC technician who came out this weekend made quite a different argument that I wish to share.

Hubert is a great guy, and I trust him to always have my back when it comes to working on my air conditioning. His client base is small, because he does the HVAC work as a side line business, but he did receive training and is qualified. He checked my system out, and he said that it was working fine for what it was. He then suggested his course of action. He said that because the heat has been so bad lately, the unit just could not keep up. First, it may need cleaning. The coils on the compressor and evaporator coil should be cleaned every so often. I rinse them down with water, but he said that using an acid solution may be in order to really clean the coils. Then he mentioned the size of the unit. His thoughts about size is that ac units should be a bit larger, because during a heat emergency like what we are facing now, the air conditioning systems just cannot keep up. In our 1700 sq ft example, he would go with a 4 ton unit. The smaller units would run all day, and they would never be able to cool the house below 80F. This was actually what I was experiencing. By having a larger unit, you may use more electricity while it was in use, but you would not use more than the smaller units, because once it did its job, it would shut down, while the other units continued to run all day long. This running constantly was happening to me too. He was making sense.

I am going to start with the acid cleaning to see where that gets me. I wanted to share this story with you, because I know that you may be facing a similar situation. We home inspectors will frequently comment on our reports about the size of the HVAC system. Typically, we fall into the same categories as the installers: most of us would go with the 3.5; and a few of us would say the 3 ton unit (which is where I have always fallen). However, if an ac technician is suggesting a larger unit (the 4 ton in our example), there may be a case that he is right, given our climate. I am going to check into this some more.

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4 Responses to “How to Size Your Air Conditioning System”

  1. Frank –

    Save your money on both ends… The acid cleaning is only going to remove the debris the water didnt! In most cases, brushing it with a soft bristle brush does just as good as the acid cleaner.

    As far as your sizing method of 500sf/ton… this is a good “Rule of thumb” principle, but very far from accurate! For instance, I have a 1800 sf upstairs on my home, but cool it sufficiently with a 2.5 ton unit in Charlotte NC. On the extreme hot days of 95 degrees, it does struggle a bit, but nothing that closing the blinds early in the day wont cure! We can generally maintain 71 degrees all day long since we planted shade trees on the side of the home in direct sunlight all day. The shade helps lower the cooling load.

    Do a search for reviews of coolnsave. Its a 1 gal water/day product that will not only increase system efficiency, but also make your system work better during those high load days.

    Upsizing your unit is not the answer. On days when its not 95 outside, your system will do nothing but shortcycle on and off, and never properly cool your home.

    Load test…


  2. Thank you for commenting Mark.
    The 500 sq ft rule is one that I have heard from various installers, but each home is a special case. I appreciate your opinion about the upsizing of the unit. I am getting mixed reviews from a few installers, so I am not sure where I land on the topic. In most cases I have dismissed technicians who have suggested larger units, but as I said, Hubert is someone I have trusted, so it is hard to discount him out of hand.

  3. Jason Says:

    I would be very cautious of anyone claiming to be a HVAC professional that does NOT do a proper heat load calculation using manual J. Anything else is a guess. Just because the old unit worked does not mean the new one will using the same size. Rules of thumb calculations should be avoided when investing in one of the most important part of your home.

  4. Jason, I do agree with you that the house has to be analyzed better when determining the best system, and performing a proper heat load calculation using manual J should be part of what is done. This post was written before I had seen most HVAC technicians take that step. I wanted the consumer to be aware what size they may need in general, and the arguments about what size may be needed. In the end, a better informed public can make smarter choices.

© Frank Schulte-Ladbeck Professional Home Inspector Houston, Texas
Frank Theodor Schulte-Ladbeck
home inspector, TREC# 9073
Houston , Texas , 77063 United States

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