How Much Cooling
Do I Need?
"More Is Worse" and "Less Is Better"

This is the second most popular page on our website. Interestingly, more than
half the viewers searched using "500 SF/Ton" in the search phrase. Why is that
interesting, you ask? Because very few homes require as much cooling as 500
SF/Ton yields. In fact, ACCA has only identified three conditions where 500 SF/Ton
MIGHT be required:
1. A home with inefficient construction (old windows, substandard insulation, high
air leakage to/from outdoors, etc.).
2. A home with a substandard duct system in an unconditioned space.
3. A home with a lot of windows (more than 30% of wall area) exposed to direct

Those homeowners who aren't in that group can think in terms of 700 SF/Ton and
up - to as much as 1,200 SF/Ton.

Those homeowners who are in that group should first look into ways to get out,
before spending money on additional capacity:
1. Improving the inefficient construction (scroll down to learn about the "Blower
Door Test").
2. Fixing that leaky duct system (ACCA tells us that can reduce a home's cooling
requirement by 100 to 400 SF/Ton!).
3. Investing in shading devices.

We've just added our comments on load calculations submitted by a contractor.
You might want to
read them. Here is the home in question.

We can't remember seeing an undersized air conditioner in a residence: The jobs
we see have too much cooling and too little ductwork; repairing the ductwork
DELIVERED CAPACITY, so if you think your air conditioner is too
small, always review your duct installation before increasing cooling capacity.


Your air conditioning must run for extended periods at peak times in hot, humid
weather to properly dehumidify your home
. You’re more comfortable in a dry,
warm atmosphere than in a cool, moist atmosphere; and there’s much less risk
of mold growth in a drier atmosphere. Those facts stress the need to properly size
air conditioners.

IMPORTANT NOTE: When it comes to air conditioning, less is better, and you must
resist suggestions to “go a little bigger” to be safe; the contractor is just
following his "Business Plan" (which is to sell as many tons as he can).

When a furnace is provided, it must be selected for cooling airflow (furnaces are
manufactured in various combinations of heating capacity and cooling airflow), so
upsizing the cooling may mean unnecessarily upsizing the heating as well. By
upsizing the cooling, then, you may be paying three ways: A larger air conditioner,
larger ducts (more airflow, remember) and a larger furnace; and that’s only
installed cost, wait till you start paying to operate the oversized equipment.

Oversizing furnaces jeopardizes heating comfort, and often generates the “blast
of heat” syndrome: The furnace only runs for a short time, even in very cold
weather; the resulting “swings” in indoor temperature are quite noticeable, and
objectionable to many. As with air conditioning in hot weather, you want the
furnace to run relatively continuously in very cold weather; here the issue is
comfort, there is no mold risk.

TRUE STORY: We recently reviewed a home in Rehoboth Beach, DE in which the
furnace was large enough to heat the home to -40F (Yes, that's 40 degrees F
BELOW zero); since design there is +18F, that home won't be real comfortable in
cold weather.

IMPORTANT NOTE: Service life for air conditioners and heat pumps is around 15
years; if it's time to replace an existing unit, you have a once in a lifetime
opportunity to correct oversizing. Whatever you do, don't arbitrarily go bigger
without first reviewing your duct system.


How To Tell If Your Air Conditioner Is Too Big

Time the operation of your outdoor unit on a hot day (Ideally the temperature
should be at least the "
1% value" - see below). Say it runs 20 minutes, is off five
minutes and then runs again. That means it runs 80% (20/25) of the time. If you
have a 3 ton unit, you could safely reduce to 2-1/2 tons; if you haven't fixed your
ductwork yet, you could probably reduce to 2 tons when you do.

" ASHRAE 1% Value"

We no longer design cooling systems for "Worst Case" temperatures; that's been
illegal for decades. Today, we use the 1% value: That temperature that historic
records indicate is exceeded 1% of the time (87.6 hours per 365 day year).


Well, think about it: So it gets hotter; it doesn't stay hotter 24/7, and the sun has a
way of moving around during the day and going away periodically
; you can't hurt
your equipment running it continuously for long periods of time, it won't get old and
tired. What we like to see is relatively continuous operation at peak times during
oppressively hot weather; we may see the indoor temperature "float" a degree or
. This is nowhere near having a heater that's too small and you're "FREEZING".
Beware: Unscrupulous contractors prey on this type of groundless fear.


It's July 21, 2011 and the outside is seething: 97 degrees dry bulb (temperature)
and 79 degrees wet bulb (a measure of moisture). ASHRAE 1% design is 90/76,
the AC's been running continuously for some time, and the indoor conditions are
ranging around 76 degrees with a 75 degree setpoint, so there's  some float.
Humidity is in the mid to low 50's, there's continuous air movement, and it feels
GOOD in here: None of that "clammy" sensation you often get with 500 SF/Ton  
(we're in excess of 700SF/Ton with the duct improvements and downsizing).

Our major "fenestration" (think windows) exposure is SW, which has just peaked
in terms of "solar gain". That means the indoor temperature will soon go down to
setpoint, and the AC will cycle off. In the interim, it's been very comfortable.


We can't remember any homes with insufficient capacity; we do see beaucoup
ship is a big problem, and if you have a "Flexible Duct System" you can forget
about adequate airflow. Duct leakage can also be a big problem: Suppose you're
drawing 130 degree air from your attic because the return ductwork is not sealed
as required by Code; or suppose your supply ductwork is not sealed and you're air
conditioning your attic. We also see a lot of undersized ductwork; can you imagine
having to breathe through a drinking straw? Much of what we do involves reducing
installed capacity
(in conjunction with ductwork improvements, of course).

Time to Add on to Your Home?

There's good news: You probably already have enough cooling to support a
15% or 20% increase in floor space
; you can use the money you would have
spent for an additional system improving your old system (repairing ductwork,
implementing zoning, giving the old 8 or 10 SEER the deep six, etc.); you may even
end up buying less energy every year.

Time to Replace Your Old Equipment?

Here's more good news: You probably won't have to buy as much capacity as
you now have
; you can use the money you would have spent for the additional
capacity improving your old system (repairing ductwork, implementing zoning,
etc.). And now's the time to improve the building envelope: Consider a "Blower
Door Test", improving your insulation, replacing windows, etc.


We were commissioned to review a proposal for air conditioning a new home,
and we called the builder to discuss his sub's bid. He volunteered his sub’s HVAC
design procedure, and here it is in its entirety:
1.    Determine tonnage
4,200 sf gross x 0.67 = 2,814 sf net
2,814 sf net ÷ 400 sf/ton = 7½ tons (note that they rounded up, not down)
Doing a little math, we see this contractor was using 597 square feet per ton,
based on the gross footage.
2.    Determine number of supplies
7½ tons x 400 cfm/ton = 3,000 cfm   (that’s Cubic Feet per Minute, a measure of
3,000 cfm ÷ 100 cfm each = 30 supplies required
3.    Determine number of returns
8 rooms @ 1 each = 8 total
4.    Summary: 7½ tons, 30 supplies, 8 returns

After studying the plans and "
Doing The Math", we recommended 4½ tons, 13
supplies and 5 returns. The homeowners saved about $4,000 on the installation,
and pocketed the difference between that and our fee; they were quite comfortable
during the ten years they lived there.

The design procedure the subcontractor used is good for business: It’s quick,
there’s no need to worry about how big the windows are or what direction they
face (or whether the lot is shaded or sunny, for that matter), there’s always
enough cooling, and plenty of cooling equipment gets sold. Because the
system's oversized, cycling a lot and running inefficiently, it'll need more
service; that too is good for business.

IMPORTANT NOTE: The fact that the square foot method is illegal today doesn't
seem to matter; there are still contractors out there using that as we speak. Watch
out for those "Dinosaurs", they'll sell you more than you need in a "New York

IMPORTANT NOTE: It is prohibited to use floor space as the determining factor
to select heating and cooling equipment; it can take many hours to properly
calculate heating, cooling and room airflow requirements

contractors only
know one way to
make more money:
To sell more tons; it
often seems that
expensive homes
have a way of
ending up with more
 tons per SF than
lesser homes, even
though more
expensive homes
usually have the
best windows
money can buy, and
are built better.

FYI: The cost per
ton ($2,500 to
$5,000 give or take)
take) doesn't seem
to vary for the same
type of work done in
different classes of

Contractors of
America, developers
of "Manual J") tells
us "Efficient single
family detached
homes with a normal
amount of well
distributed glass
typically fall in the
700 to 1,200 range"
(para. 2-4, page 9).
700 to 1,200
square feet per
ton of cooling
we're not sure if the
includes thicknesses
of outside walls,
closets, "air space"
beyond balconies on
upper floors, etc.

It's illegal to use
square feet of
floor space to
determine heating
and cooling

It's especially illegal
to use 400 or 500
square feet per ton
for every home.
Here's our plan to
22% of
installed cooling
capacity from a
large luxury home:
One unit was
completely, another
was downsized, and
several were moved

This equipment
manufacturer has
the following
requirements (to
facilitate airflow
through the units,
optimizing unit
48" above,
6" on one side and
12" on the other,
and 24" between
adjacent units

Those requirements
were not met, the
ambient air was
much hotter than it
should have been,
and that reduced
cooling capacity;
that belief drove our
decision making.

We also specified
two large fans to
circulate cooler air
through the space.

Using other than the
1% value.

Not assuming there
are drapes or blinds.

Not assuming drapes
or blinds will be closed
(an exception is
permitted for

Not evaluating shade
from roof overhangs.

Not taking benefit of
window screens.

Using excessively high
infiltration rates (ACCA
has accumulated a lot
of data in this area).
Here's a
Blower Door Test"
used to find and repair
air leaks in the
building envelope.

It's a good idea when
building a home, or
when considering
replacing old
equipment: Why buy
more than you need?
More from ACCA on the Square Foot Method
"Single family dwellings that are characterized by inefficient
construction and/or inefficient duct system (in an
unconditioned space) may be in the 500 to 700 SqFt/Ton range."
"Single family detached dwellings that have reasonably
efficient construction and a reasonably efficient duct system
may be in the 700 to 1,000 SqFt/Ton range."
"Efficient single family detached dwellings that have a normal
amount of sunlit glass areas may be in the 800 to 1,200 SqFt/Ton
"Efficient single family detached dwellings that have large
sunlit glass areas may be in the 500 to 800 SqFt/Ton range."
Manual J, page 73