According an article called Bigger isn’t Better in the July 04 issue of Fine Homebuilding (which sadly, you can’t read unless you pay good money), more than 70% of newly installed central air installations are either improperly sized or installed. And 95% of all new air conditioning installations fail in regards to energy efficiency.
Now the Bigger isn’t Better never says where those figures came from, so I just have to take them on faith. But from what I have seen while watching the trades, I have no doubt that they are basically accurate. Almost nobody installs central air correctly. Bad trade practices abound in the HVAC industry and most of time the consumer never even notices.
A cynic might say “So what else is new? All of the trades are full of bad practices. They all are full of people who like to cut corners at the expense of doing a good job.”
I am not inclined to argue with the cynics. In fact, one of the reasons I started this blog is that I wanted a place to complain about all the bad practices that occur in the trades. But I would argue that number one cause of bad practices in the HVAC trade is different than in all other trades.
From what I have seen, most bad practices in the trades occur because the tradesmen in question are lazy or are trying to save a buck. In other words, they know better, but they just to choose to do things the wrong way.
But in the HVAC industry things are different. The HVAC industry is the only trade that I know of where guys who have a clean conscience before God and a sincere desire to do right for their customers are routinely ripping off their customers. And they do this in spite of the fact that refrigeration engineers have been telling them for the last hundred years that they are doing things the wrong way.
Why do so many honest contractors in the HVAC trades ignore official advice? Because it is almost impossible for men (and HVAC contractors are overwhelmingly men) to truly believe that bigger is not better. But when it comes to installing air conditioning, bigger is definitely not better.
In fact, when it comes to installing air conditioning it is better to err on the side of installing equipment that is undersized. As hard as it is for most men to believe, there is absolutely no upside to getting an air conditioner that is any bigger than absolutely necessary.
This is because an oversized air conditioner will short cycle. In other words, it will turn on and off quite rapidly because it can satisfy the load so quickly. This is bad for the compressor and will shorten the life span of your air conditioner. Think of turning on and off your car all the time without driving anywhere and you get the idea.
But the effects of short cycling go beyond just shortening compressor life. It is also the reason that an oversized air conditioner will not reduce humidity properly, leaving you less comfortable than if you had a properly sized air conditioner. I suspect this a factor that causes many people to claim that they don’t like air conditioned air.
Furthermore, an oversized air conditioner is an energy hog and will cause your electric bills to be far higher than they need to be. This also goes back to the short cycling. An electric motor has almost double the draw when it starts up as opposed to when it gets running. So the more you short cycle, the closer you come to doubling the energy consumption of your air conditioner.
An important thing to remember when we talk abut why oversizing is such a serious problem is that when you figure your air conditioning loads, you are figuring the peak load that the air conditioner is going to have to handle. But most of the time an air conditioning system does not have to deal with peak load. This is why it is better to undersize by 20% rather then oversize by 20%. If the unit is undersized by around 20% it will perform just fine most of time. It will only struggle those few times that you approach peak load. On the other hand, if you oversize a unit by 20% it will tend to short cycle the vast majority of time when you are not at peak load.
As you can see, there is no upside to having an oversized air conditioner. But according to Bigger isn’t Better, most of units being installed today are oversized. Even people who should know enough to properly size an air conditioner still install oversized units.
A good example of this can be found in this Home Energy Magazine article titled “Bigger is not better: sizing air conditioners properly…..
A colleague of ours (we will call him Bill) approached us at a conference seeking advice on selecting an air conditioner for his renovated home. Our recommendations included, “Be sure that the cooling load is calculated and that the air conditioner is sized to that load.” When Bill attempted to follow these instructions, only one of the four contractors would submit a sizing calculation (two others just wanted to know how many square feet there were in the house). Bill hired the contractor who did the calculation and installed a high-efficiency four-ton unit. Is this a success story? Not really.
The contractor calculated a total cooling load of 37,580 Btus per hour at 105°F outside and 70°F inside. While the cooling load he calculated could have been met by a three-and-a-half ton air conditioner, the contractor convinced Bill to buy a four-ton unit “because then you will always have plenty of cooling.”
Bill’s air conditioner short-cycles (cycles on and off more often and for shorter periods of time than it should) even during the hottest weather and removes very little moisture from the air. What went wrong? Four things:
The design temperature for the area is 97°F. The contractor increased the outside design temperature by 8°F.
The recommended design indoor temperature is 75°F. The indoor temperature was lowered by 5°F. The temperature “fudges” increased the inside to outside differential by 59%.
The contractor increased the calculated load by 20% as a safety factor.
The equipment selected was a half-ton larger than the next highest available size to meet the load he calculated.
A three-and-a-half ton air conditioner would have been perfect for Bill’s house. Instead he paid more for an extra half-ton of cooling. In addition to costing more to buy, Bill’s air conditioner will use more energy than a properly sized system, raising his utility bills. It won’t dehumidify the air as well as a smaller system would, and chances are that Bill will be less comfortable. The utility, which gave Bill a rebate for his purchase, will also lose, since the oversized unit aggravates summer peak-load requirements.
Between mistakes in calculations and deliberate over sizing, the contractor in the above story oversized the system by about 40% above what would have been required to meet peak loads. To need so much extra cooling capacity, temperatures would have had to risen so much that they would have taken the entire electrical grid in “Bill’s” area down.
Yet I can almost guarantee you that the contractor that “Bill” hired was not trying to rip him off. It speaks well of him that he actually tried figure the load. An awful lot of contractors want to take the lazy man’s way out and figure off square footage and outdated rules of thumb (Full disclosure: that is how I was taught to do before I read up on the matter on my own). If he had been doing his own house, I bet this contractor would have bought the same size machine.
But even though the contractor in this story figured the load as best as he was able (though he still got them wrong by almost 20%), he still felt like he had to add an extra 20% “just to be on the safe side.” This shows how little he really understood about what he was doing.
In once sense, though, “Bill” got off lucky. According to the Fine Homebuilding article, it is quite common for contractors to install units that are 1.5 to 2 times too big. And the funny thing is that most people never even realize what is going on. The air conditioner is in some out of the way place were they can’t hear it short cycle. They don’t realize how much better comfort they could be getting from a properly installed system because almost no one has a properly installed system. And when the system breaks down prematurely, they blame the manufacturer for not making things the way they used to.
Such is the price of ignorance.