Why Your A/C is Probably Letting You Down

Maybe you know it and it bothers you.  Maybe you are blissfully unaware.  Whichever it is, whether you just moved into your new home, just paid the most reputable guy you could find to install a new A/C system, or you've been in your house for years, your A/C system probably is under-performing in one or more ways, and the biggest way most A/C systems under-perform isn't intuitive.

Your A/C system "design" was likely based on the following criteria:

1) Be as cheap/profitable as possible,
2) Give some passable semblance of meeting code, and
2) Be good enough that nobody calls to complain.

Now, you may think that this is an indictment of your contractor, but it's mostly just a natural result of how much thought we put into our air conditioning systems.  We spend weeks deciding what fixture to install in our shower, but for A/C we basically don't want to be hot, don't want it to break, and don't want to pay more than we have to.  The first part seems simple enough: make sure it's big enough.  The second part we solve through using a reputable contractor who will recommend a reputable brand of equipment.  And the 3rd part involves shopping based on whatever knowledge we have.  The problem is, the very first part is probably wrong and prevents us from getting the other parts right.  Unfortunately it's not a problem most contractors address.

The Humidity Problem

Our fundamental nature with A/C is the first thing that gets us into trouble: most of us have systems that are simply too large, and that's actually a really bad thing for your home's comfort.  The last thing your contractor wants is a call about your house being hot.  As a result, residential A/C systems are almost always grossly oversized.  On the surface, this doesn't sound terrible, but in addition to cooling your house, your A/C system needs to effectively remove humidity, and oversized A/C systems don't do this well.

We've all experienced a time in the spring or fall when it was 75 degrees outside and we couldn't make our house comfortable.  The system isn't running, you're uncomfortable, you go to the thermostat and it says it's 74.  Hmmm.  You turn it down to 72.  20 minutes later you come back and turn it down to 70.  Then, in desperation, 68.  Another 10 minutes and you turn it back up to 72 because suddenly you're cold.  Meh.  Now you go put on something warmer and turn it back down to 70.  Why?  The humidity is too high in your house and there isn't a comfortable temperature.  Unless you invest in a secondary dehumidifier, there will always be some corner case days like the one described above, but there shouldn't be many.  Most of the time we should be able to be comfortable in our homes.

In a comfortable, room temperature environment, about half of the heat we give off is what engineers call "sensible" heat, or the heat we give off due to the temperature difference between the air and our skin.  The other half of the heat we give off is something called "latent" heat.  This is heat that is used to evaporate moisture from our breath and perspiration.  While we all know that people sweat and dogs pant to cool themselves, it's not something we generally think about a lot, but if the humidity is too high, this heat doesn't get out of our bodies, and we feel uncomfortable.

Psychometric Chart with Comfort Zones

The American Society of Heating and Refrigeration Engineers (ASHRAE) maintains a standard #55 called "Thermal Environmental Conditions for Human Occupancy."  I'm sure you don't want to read it, but they've done research on what's comfortable and provide the information summed up in charts of what's comfortable and what isn't, based on typical levels of clothing during winter and summer, etc.  If you look at their summer comfort psychometric chart, most homes spend a lot of time in the top corner, where humidity is high and temperature is low.  We're trying to be comfortable, but often slipping out of the comfort zone to the too humid zone or the too cold zone.

Wait, My A/C Doesn't Run ENOUGH?!?

Fundamentally, if your AC system isn't designed right (or isn't set up right), it won't remove humidity at an appropriate rate.  If you have leakage problems with your ducts, you need to fix them no matter what, but the most common problem is that the air conditioner is oversized and has short run times, which don't work well for condensing moisture from the air and running it out of your condensate drain to remove humidity.  Note that this isn't about efficiency, it's about capacity.  The problem can exists with a newer 16 SEER system or an older 10 SEER system.  You want a smaller system that runs longer duty cycles instead of a larger system with shorter duty cycles.  Keeping the math simple, if a 5 ton system can cool your house by running 40% of the time, a 2.5 ton system can cool your house by running 80% of the time, and you'll be much, much more comfortable.  Ideally your system would be designed and configured to RUN CONTINUOUSLY during peak load times, the afternoon and early evening of those hot August days, and is configured to run as much as is practical during off-peak periods.

The primary solution to this problem is to get the size right from the beginning.  Beyond that, depending on your furnace and system, other things may be adjustable to provide better comfort, but more than likely the solution is going to require spending some money on new equipment.  Most of us aren't going to spend that money until we absolutely have to, so this is most relevant if your system is on its last legs, but in future posts I'll discuss whether upgrading your existing system makes sense financially, where to best spend your money when you're replacing equipment, and how to know that it actually is sized correctly.