Are you building a new home and deciding on air conditioning? Are you thinking of installing air conditioning in your current home? There are several pitfalls you should know about that can reduce the efficiency of even the best units by half, resulting in warm, humid spots and in higher bills than necessary.
The first problem to guard against is oversizing an air conditioner. On first thought, this would seem like a good idea - how can you have too much cold? There are three reasons:
First is that an air conditioner is most efficient when it has been running a while. If it is too big, it will run for a couple of minutes at a shot, whereas, if it is the right size, it will run much longer - on the hottest afternoons of the year, it should run all the time. This delivers the cold at the lowest cost.
The second is that the air conditioner is also a dehumidifier. Here on the West Coast we spend about 20% of our air conditioning money on taking moisture out of the air. If the unit is too big and spends more time off than on, the coils won't get cold enough, for a long enough period of time to condense enough moisture to be effective in removing moisture from the air.
The third problem with oversizing the air conditioner is that if you buy too big a unit, you are simply wasting money. A properly sized unit has a fudge factor built-in, so that once a proper heat loss/gain calculation has been done, buying a larger unit than called for is unnecessary.
In addition to oversizing the unit, there are other factors that can spoil an installation.
You can't have a high efficiency system with low efficiency ducts. Ducts that run outside the thermal envelope, in attics and crawlspaces, typically have R-5 insulation installed. Compare this with your R-13 walls and R-30 attics! Any ducts that run in unconditioned spaces should be sealed tight and insulated to at least the level of the surrounding areas - R-40 would be desirable. It would be best to keep those ducts out of these unconditioned spaces to begin with.
The location of the air handler is also very critical. If the air conditioner is trying to make 55° air, but the air handler is in a 130° attic, there will be big reductions in the efficiency of the system. Your air conditioner will be fighting an uphill battle, right from the start. System design is critical to run at peak efficiency. Keep those air handlers out of attics and crawlspaces!
If ducts are in unconditioned spaces, duct leakage can introduce hot, humid air to the system if it is on the return side. Supply side leakage just blows expensive cold air to the outside. So sealing these duct systems as tight as possible is a very good idea. This is another reason to try to keep the ducts out of unconditioned spaces to begin with.
Cold air is harder to push to the second floor than warm air is. This is why so many houses that heat properly have uncomfortable second floors in the summer. There are a couple of low cost duct modifications that you make to be sure that the right amount of air is making it upstairs.
Just as in winter when house air leakage brings in cold dry air from the outdoors, in summer that air leakage brings in hot, humid air. This causes the unit to work harder and longer, and if this leakage has not been properly accounted for in design, it can cause great discomfort. In a very leaky home, you will be able to buy a smaller unit if you properly seal and measure the home.
Remember that the only way to get a "right sized" air conditioner is to have your installer complete a heat loss calculation on your home. In addition, an air infiltration measurement will allow him to avoid using the default rates, and therefore perhaps offer you a smaller unit. You can see how important it is to consider all aspects of your air conditioner installations. If you are installing new systems or replacing existing units, call us for details on these low-cost, no-cost considerations.