Air conditioning 101

It's that time of the year....summer.   Air conditioners are starting to hum.  Some aren't.  They're broken.

Let me share with you some of what us homebuilders....er...."building scientists" *snicker* ....have learned regarding the proper a/c size and efficiency ratings you'll need to know if yours goes kaput.

Logic says buy the highest SEER (Seasonal Energy Efficiency Ratio) rated unit you can afford, the higher the number the better.  These days they begin at 13 SEER....that's rock bottom....up to 23 or 24 SEER. That is not necessarily a smart plan.  There are other variables that need to be considered.

If your home is relatively new and has a tight "envelope" (very good wall and ceiling insulation, good windows and doors, proper caulking, well sealed a/c ducts, etc), you can probably get by nicely with a 16/18 SEER unit.  

That's because the a/c won't come on that often or run for very long at a time due to the home's draft-free tightness.  If the a/c isn't running a lot, a 20 SEER won't save enough to justify the premium price over a 16 SEER unit.  Save the money and just buy the 16 SEER model.

On the other hand, if you have an older, under-insulated, drafty house, your a/c unit will probably come on often and stay on for a long time at a stretch. Then you should strongly consider a 20 SEER system.  

It will run enough, and therefore save enough, to justify the premium over a 16 SEER unit.  Of course, there are also other upgrades you should consider doing if your house is that deficient.

Logic also says, "If a 4-ton unit is good, a 5-ton unit is better", right?

Not really.  Just as a car needs to warm up and be driven a few miles and get up to speed before you begin to realize optimum mileage efficiency, an a/c unit needs to run at least 10 minutes at a time before it reaches peak performance.  

An over-sized a/c unit that comes on and then cycles off after only 5 minutes or so isn't even getting warmed up.  Your fancy 20 SEER system is just spinning it's wheels.  Don't "over-size".

So how do you know how "tight" your home is?  If you've been in your home a while you probably know if it feels drafty or not, and of course your utility bills can tell you a lot, too.  But the best way is to hire an energy auditor to come to your home and do a "blower-door test".  Spend a little, save a lot.

They'll temporarily seal up a door opening and, using a fan, depressurize your home.  They'll then compare the pressure inside and outside, and this will tell them how tight your house is.  Using this information a competent HVAC contractor can calculate the proper SEER rating and capacity your home needs.

Yeah, I know.  Boring post.  I just hate to see my friends get ripped off by slick talking salespeople yapping non-stop on TV/radio.  Be careful out there.

S