The Problem
The tighter a home is, the less AC tonnage it requires. However, as homes have gotten tighter, ACs have not gotten smaller. HVAC contractors are there to fix your unit and sell you a new one. Analyzing your entire home's building envelope is not their job, and most are not trained or equipped to do it. It is easy to convince a customer that bigger is better and that the brute force of a big AC will solve their heat problems.
The Data
| Finding | Source |
|---|---|
| A study of over 400 Florida homes found more than 50% had cooling equipment oversized more than 120% of their actual Manual J load | Florida Solar Energy Center (FSEC) |
| Over 70% of residential HVAC systems in the U.S. are improperly sized or installed, leading to comfort complaints and energy inefficiency | National Comfort Institute, 2023 |
| Oversized AC systems waste 20-30% more energy and can cut equipment lifespan in half — from 15-20 years down to 8-10 years | DOE / ACCA research |
| Air leakage accounts for 30-70% of a home's total heating and cooling load — making it the single biggest variable in proper AC sizing, yet most contractors never measure it | Nate Adams, "The House Whisperer," The Home Comfort Book |
| An oversized AC satisfies the thermostat before the evaporator coil gets cold enough to remove humidity — the air cools, but the home never does | U.S. Department of Energy |
The Explanation
We love our HVAC contractors. They are essential to keeping Florida livable, their work is tough, and refrigeration circuits are complex. However, because there is so much complexity with the mechanical units themselves, the big-picture reality of building science is often neglected by the typical HVAC contractor. Their focus is keeping the units working and hopefully selling you a new one — not so much on the interplay between air, heat, moisture, your building materials, your home's position relative to the sun, how you live in your home, and your building's envelope.
This is not because HVAC contractors do not mean well or have malicious intent. It is simply a result of the inertia of tradition, tribal knowledge, and educational standards that have not evolved as they should have by now. The residential HVAC industry in Florida alone is estimated to be over $12 billion a year. Getting an industry that size, which already functions like a well-oiled machine, to change how it does things is no easy feat. It also takes a lot more time to properly size an air conditioner, and requires insulation modifications that most HVAC companies are not equipped to handle.
While 500 square feet per ton is a rule of thumb in the industry, properly insulated homes typically need far less cooling — often one ton per 700 to 1,200 square feet according to ACCA. In a properly spray-foamed home that number can be even higher. Of course, simply considering the square footage is not how you size an air conditioner regardless. What you need is the airtightness of your home as measured by ACH50 (air changes per hour at 50 pascals, measured using a blower door machine), a full Manual J calculation that measures the total heat gain on your home in BTUs (British thermal units, the amount of heat energy it takes to raise one pound of water one degree), and a Manual D calculation to properly size your ductwork. Then of course it all needs to be installed properly. Ductwork is a whole other story.
As you can see, this all gets complicated very quickly. It should come as no surprise that doing a perfect job is typically not profitable or feasible for most companies. However, since air leakage alone accounts for 30-70% of your home's total heating and cooling load, sealing up your home and then measuring its airtightness is the single biggest variable in sizing your next AC unit correctly. Also, cleaning up and sealing your attic makes duct leaks a non-issue.
Air conditioners that are too big do not run long enough to pull out all the humidity that is making you uncomfortable, and do not run long enough to pull the heat out of your walls, floors, ceilings, furniture, and so on. As soon as your oversized AC pulls heat out of your air, the thermostat is satisfied and the AC turns off, and then all that heat stored in your building materials pours right back into the air and the cycle continues.
What This Means for You
A homeowner will never be more comfortable than in a home with a properly spray-foamed attic paired with a variable-speed air conditioner installed at the right size. Unfortunately most homeowners never get there, because they are left confused by well-intentioned HVAC professionals installing oversized equipment that increases out-of-pocket costs for the bigger unit, increases energy costs every month, increases humidity, and does not pull heat out of building materials. Because these professionals seem to have a monopoly on comfort authority, homeowners end up confused when the "expert" gave them the "right" unit and they are still uncomfortable. There is a much better way: build it tight, ventilate it right! Get quality spray foam profesionally installed, have your airtightness measured, and then get a proper manual J and D calculation done on your home to get the correct size air conditioner.
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