It makes perfect sense that an A/C can be too small for a building, but can an air conditioning unit be too large for a building? Well, let's explore this idea.
There are many people who approach a contractor, and say, "Just give me the next size bigger unit. I don't mind paying the money." In theory that sounds great, but any reputable contractor will not do that. Your HVAC contractor's job is to put the smallest unit in your home that will do its job and use the least energy to do so. Why? It's not a simple answer, but it does make perfect sense. There are a few reasons.
Fact is, an air conditioner serves another purpose other than to simply cool an area. The unit also draws humidity out of the air the same way a cold bottle of soda, or a glass of iced tea does when it sits on the counter. You'll notice that the bottle or glass appears to 'sweat'. It's not actually sweating though. The water that accumulates is called condensation. That's right. It's the same thing as the dew on the grass in the mornings.
Well, the evaporator coil located in your air handler, or with your furnace does the same thing to the air in your home. Because of this condensation, a drain is needed in your air conditioning system. Sometimes this drain gets clogged with debris or mildew and other things that grow in cool water environments. This is actually a common problem for homeowners. Air conditioning companies are called all the time to deal with 'leaky' units. Any size air conditioner will draw humidity out of the air. While a larger unit can cool a designated area more quickly, it won't pull condensation from the air as quickly as it cools.
The first problem one may encounter when having an oversized central air unit is short cycling. Short cycling is when the unit is so large that it cools the designated area too fast for the humidity to be pulled from the air. That leaves moisture in the air. Excess humidity makes the air feel stuffy, and warmer than it actually is. Also, since water is needed to breed micro-organisms, many who have excess humidity have higher occurrences of allergic reactions, possibly caused by mold that can grow anywhere in the home, including the air conditioning ductwork.
Another common problem having to do with humidity can be caused by vent placement and air flow direction as easily as by an oversized unit. This problem is 'wet spots'. Remember the glass of iced tea and the condensation it pulls from the air? Well, if you have a wall that has cold air constantly blown on it, the wall will become chilled all the way through. This chilled wall can draw moisture from the air on both the inside and outside surfaces of the wall. Both surfaces will become damp, and may become a breeding ground for micro-organisms.
The visible surface of the wall may be easy to clean, but inside the wall is where problems are more likely to exist. You can't see the mold and mildew on the inside surface of the wall. It can grow for years if unchecked. Members of the household can become sick, and no one will know why.
That's why it's a good idea to call an HVAC company to check your unit out before paying thousands of dollars on doctor's bills. Not all contractors are going to ask if people are getting sick often. Feel free to offer this information up to them. If they know it's happening, they should be smart enough to look around for possible signs of mold caused by a possible oversized unit.
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