Sizing A/C Units and Ducts

Interesting video.

Notice how “square footage” is never mentioned.

He does a good job of explaining oversizing but I stay away from calculation.
opps watched the wrong one first but Similar.

I avoid them as well in a home inspection. I am reading where some guys are using the square footage of a home in order to report to their clients that their A/C service is insufficient in size. When I was getting bids on the new system that I installed in my own home last month, I had contractors who came in and measured my floor space and gave me a bid on the spot that I promptly dismissed and threw in the trash. As this expert explains, the sizing of the equipment is a result of a computer calculation that considers much, much more than this.

If you are designing a new system, that video applies. When you are just changing the condensing unit/air handler, it is a different matter. The existing duct work will dictate what sized unit can be used efficiently.

A home built in 1970 with with inefficient windows may require a larger, 1 ton larger according to the guy in the video, won’t work as the duct work was not designed for it.

The square foot method is a basic guesstimate for certain regions and when it is all said and done, in most instances, once you add in all the variables, isn’t that far off.

I recall doing an inspection on a 900 sq ft condo. The guy installed a 5 ton unit! I told my clients that it should be replaced to a properly sized unit. During the inspection, the unit came on for five minutes and shut off. It did this all through the inspection. That is what is known as short cycling!

The dynamics of heat loss as opposed to the loss of cooled air are quite different. What might be the rule of thumb in Florida is folly in Maine.

The square foot method is a basic guesstimate for **certain regions **and when it is all said and done, in most instances, once you add in all the variables, isn’t that far off.

To further prove my point, our home is 1,700 sq ft. The rule of thumb is 1 ton per 600 sq ft. That is roughly a 3 ton unit.
When you factor in all the variables, the total cfm required comes to 1200, which is a 3 ton unit. Our duct work was designed for a 2 ton unit, which is why it was replaced.
The 3 ton unit was installed to compensate for the, at the time, insufficient attic ventilation and insulation, no “drops” in the bathrooms or laundry room,etc.

You are still sizing equipment based upon assumptions…such as the assumption that the original duct design was correct, the assumption that the present ducts are operating at the capacity in which they were originally designed, that the air leakage rate is at (neither greator or less) than what was originally calculated, that the landscaping (particularly the shading) is at the same level as the original calculations…and so on, ad nauseum.

In other words…without the Manual J calculation…you are guessing and in that regard you would be correct that your guess is as accurate as anyone else’s guess based upon square footage or any other “rule of thumb”.

Usually if you just insulate the attic on a single story house, you can easily cut a half of a ton off your A/C unit. I have seen this many times, back when I owned a heating and cooling company. And of course, like the video says, there is usually not enough returns.

We did that on our home along with installing a ridge vent and a radiant barrier,along with the new duct work. The ridge vent/radiant barrier brought the attic temperature down significantly, and then, by planning out the duct work properly, it is now balanced. If I were to put a flow meter in front of the vents, it would probably be very close to equal on every vent.


A 1000 sq ft home with lots of single pane windows and little insulation may need a larger sized unit than a well-insulated home twice that size


Take two homes with 1500 sq ft on the same street.

One sits on a hilltop with no trees or wind barrier, 150 sq ft of single paned windows facing north and west, 75 sq ft facing south and east, R-19 attic with 80% of the ductwork outside of the thermal boundary (no mastic at the seams and joints and uninsulated).

Second sits in a valley, fully shadded on the south and east, Low-E windows facing south and east (150 sq ft) and regular double paned facing north and west (75 sq ft), R-38 attic with all of ductwork inside of the thermal boundary (no mastic at the seams and joints and uninsulated).

How many contractors (and home inspectors) would walk off the 1500 sq ft and apply their “rule of thumb” to determine the adequacy of the cooling capacity of the a/c? I think most, and for at least one of these two units…they would be wrong.

Air Conditioner or Heat Pump Sizing Chart

Preparing your sizing chart:

  1. Print out this page.
  2. Leaving the page intact, carefully cut out the holes on the dotted lines.
  3. For operating instructions see bottom of page.

1 1/2 to 2 ton

2 1/2 to 3 1/2 ton

4 to 5 ton

Sizing Chart Operating Instructions

  1. Stand on the curb.*
  2. Hold the sizing chart approximately one foot from your face.
  3. Look at the house through each hole.
  4. If the house fits in a hole, that’s the size unit to use.

*If the curb is not available - ask the homeowner where a curb would be if there was one.

Would you like a little more accuracy? Or see our Table of Contents](

Un like some that has never accomplished anything but sit behind a desk its hard for them to comprehend that when someone has done that same thing day in and day out for most of their adult life that one simply has the ability to walk through a home view the windows, doors and insulation based on SQ footage that is typical for the local area and determine the exact tonnage required . There are always unusually designed homes that will require the J program but by all means not every home as some would lead to believe.

Oh BTW Bushy tell me the foot print of your house SQ ft the type of windows and age how much insulation and type in the attic and I will tell you how many tons of A/C your contractor is going to install and I have never seen your house ;-);-):wink:

Sure you will, but you will still be wrong, Chucky.;-):wink:

You represent the contractor who every prudent inspector needs to be wary of before agreeing with the equipment he installed as being “adequate” or not. Like you, many feel confident that they can correctly guess the proper system…often oversizing the unit to compensate for error and adding to its inefficiency.


Actually “Chucky” is correct–after thirty-five or so years it becomes easier just by looking. I’ve proved that on many an occasion…my observations closely matched calculations of the Manual J.

But even today, if called upon, I would refer to Manual J for assurance only. Fortunately, I don’t get called so much for anything other than consultation–so much easier that way. (I just tell 'em to refer to the Manual J.)

Good advice, since nobody…and I mean nobody…has 35 years of experience in the high efficiency homes that are being constructed to meet today’s efficiency standards.

What was correct in 1980 is not correct, today, but that doesn’t seem to sit too well with “experts” who think they can size a unit from a message board post. LOL.

For example…look at all of the things that Chucky didn’t ask before declaring his abilities to size a unit on an unseen structure.

“…tell me the foot print of your house SQ ft the type of windows and age how much insulation and type in the attic…” Would it affect the sizing of the unit if there were vaulted ceilings or six inch exterior walls? What if the house was a stone structure with plaster walls? Or a hundred year old structure with a plaster and lath exterior walls? And ductwork…is it inside or outside of the thermal boundary? Is it sealed or visibly leaking conditioned air into unconditioned areas? Is it insulated? What is the natural ACH? What if it were on an uninsulated concrete slab and completely shaded on the east and south exposures with 40’ tall Blue Spruce…or there wasn’t any shade on the house at all and more than half of the southern and eastern exposure was glass? Could any of this affect the efficiency and sizing of the unit?

Oversized units resulting from wild-azz guesses from some contractors have a whole lot to do with the mold issues found in newer homes where short cycling units do not run long enough to remove humidity — yet those “experts” with all of their 35 to 50 years of experience in houses built in the 30s through 90s still want to add 1/2 of a ton as a fudge factor.

Without meaning to, Chucky just demonstrated why…no matter how much of an “expert” a home inspector might consider himself to be…he is best to leave the sizing of an A/C unit to a licensed HVAC contractor who uses Manual J calculations.

No Bushy I just demonstrated how a person with no practical knowledge of HVAC can sit in a BPI class room for a couple of days and become a instant expert. Oops Sorry lost my head for a minute you are a expert on everything.

Are you afraid to give me the info I asked for Put up or shut up:D:D:D

I’m not interested in “guesstimates” from self-proclaimed experts. Thanks anyway.:D:D:D

I thought so not hard to see through your smoke and mirrors. In the military we used to have these 90 day wonder boys straight out of school I see now we have 2 day wonder boys or did you get trained on line. You know John has a two day IR class he would probably take you under his wing then you would be an IR/HVAC instant expert WOW:D:D:D