Sizing A/C Units and Ducts

[James H. Bushart]

Interesting video.

Notice how "square footage" is never mentioned.

[Bob Elliott]

He does a good job of explaining oversizing but I stay away from calculation.
opps watched the wrong one first but Similar.
http://www.bestofbuildingscience.com/WM_proctor1.html

[James H. Bushart]

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.

[Eric C. Van De Ven]

Originally Posted by jbushart

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!

[James H. Bushart]

Originally Posted by evandeven

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.

[Eric C. Van De Ven]

Originally Posted by jbushart

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.

[James H. Bushart]

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".

[James E. Braun, CMI]

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.

[Eric C. Van De Ven]

Originally Posted by jbraun

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.

[jhugenroth]

Originally Posted by jbushart

Interesting video.

Notice how "square footage" is never mentioned.

Yep.

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

[James H. Bushart]

Originally Posted by jhugenroth

Yep.

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

Yep.

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.

[Roy D. Cooke, Sr]

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

[Charley L. Bottger]

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

[James H. Bushart]

Originally Posted by cbottger

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

Sure you will, but you will still be wrong, Chucky.

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.

[Charley L. Bottger]

Originally Posted by jbushart

But you will still be wrong, Chucky.

Scared