Making Your Air Conditioner Condition Air
Are you replacing your old air conditioner? Installing air conditioning in your current home? Building a new home with air conditioning? 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 for 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 in the Mid-Atlantic, we spend about half 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 long enough to remove enough moisture.
- The third problem with oversizing the air conditioner is that if you buy too big a unit, you are wasting money. A properly-sized unit has a fudge factor built in, so that once a proper calculation has been done, buying a bigger unit than called for is unnecessary.
In addition to over-sizing the unit, there are other things 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 crawl spaces, typically have R-4 insulation installed, if any. 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.
The location of the air handler/fan unit is also very critical. If it is trying to make 55° air but is in a 130° attic, then there will be big reductions in the temperature that finally makes it into the house. Keep those air handlers out of attics and crawl spaces!
If ducts are in unconditioned spaces, supply-side leakage just blows expensive cold air to the outside, and return side leakage will introduce hot, humid air to the system making the system work harder, longer. So sealing all of these duct systems as tight as possible is a very good idea.
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 may be able to make to be sure that the right amount of air is making it upstairs.
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 as we will make during a Home Energy Audit 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.