What is the deal with A/C units and tonnage? How is a system sized properly?

Ok these are some quick and dirty rules of thumb when describing A/C tonnage, BTU's and how to size a unit for a home.

One ton of cooling = 12000 BTUS and is designed to cool approximately 400 square feet of living space.
So, if you have a 2000 square foot home you might have one 5 ton unit or two units a 2 and a 3 ton.
These are rough numbers so your results may vary.  When your home was built someone (hopefully) did a calculation to determine what size unit to put on that particular home.  If the home was made of glass then obviously the unit would be sized larger, for example.  I live in a 3200 square foot home but get by with a 3 ton split and a 3.5 split system.  A little undersized but I have returns that go under the concrete slab and 2x6 framing with good insulation.  My system is sized properly.

Now why not just upgrade the unit and stick a couple of 4 ton units at my place? I mean more is better right? NOOOOOOOO!  My units run often during the summer but they turn on and stay on.  If I upped the size of the units then they would 'short cycle'.  A fancy way of saying turn on and off all the time.  If I selected 77 degrees at the thermostat and the unit kicked on at 78 it would pull down to 77 in a hurry, no doubt.  But, and here is the kicker, it would not run long enough to dehumidify the air and thus run back up to 78 in short order and the cycle repeats, over and over again.

The hardest thing for any motor, and I don't care if it is your lawn mower, car, ceiling fan or A/C unit is to turn on.  The amp draw of an electric motor is higher at start up to overcome the torque necessary to get up to speed. Example, my compressor will momentarily draw 60+ amps at start up and then run at 14 amps.  That is a lot of heat in a short amount of time and motors HATE heat.  It is their Achilles heel.

Last how can you tell how many tons your unit is.  Look on the name plate on the unit.  You will find on the model number a few numbers and letters and then typically a 0 followed by a number divisible by 12.

Example:  Lennox model 13hpd 036 230

The 036 is the giveaway that this is a three ton unit - it moves 36000 BTU's of heat in cooling mode.  A five ton of the same model Lennox would be  Lennox model 13hpd 060 230.

You may have a unit that you think is undersized.  In Phoenix it runs a lot in the summer but the rest of the year it kicks on and off as needed.  This is ok as long as the unit is mechanically sound and in fact good for it.  Remember the starting and stopping is what is bad for the unit.  Airflow ect.  are totally different issues.

For more helpful tips and questions answered visit my free A/C help site at http://www.thermal-medics.com/ and please, call me or email if you have any questions..