What size AC for 1400 square feet in zone 3
| Nominal size to buy | 3tons |
|---|---|
| Calculated need1,400 ft² × 23 BTU/ft² | 2.68tons |
| Cooling load | 32,200BTU/hr |
| Square feet per tonthe number the rule of thumb quotes — and it moves with climate | 467ft²/ton |
| Oversize from rounding upwithin the normal rounding step | 12% |
| Same house in zone 1–2 | 3.1tons |
| Same house in zone 6–71.64× difference for the identical building | 1.9tons |
Notes
- "400 square feet per ton" is a Florida number being quoted everywhere. This house needs 3.1 tons in zone 1–2 and 1.9 tons in zone 6–7 — a factor of 1.64 for the identical building. Any single square-feet-per-ton figure is a climate assumption in disguise.
- Buying bigger "to be safe" is the classic mistake. An oversized unit reaches the thermostat setpoint quickly and shuts off before it has run long enough to pull moisture out of the air. The result is a house that is cold and clammy, and a compressor that wears out early from short cycling. Undersized is uncomfortable on the five hottest days; oversized is uncomfortable all summer.
- This is a rule of thumb, and the difference from a Manual J is money. The real calculation accounts for window area and orientation, insulation levels, air leakage, ceiling height, duct location, shading and how many people live there. A house with new triple glazing and a tight envelope can need a full ton less than this figure; a 1960s house with single glazing and an uninsulated attic can need a ton more.
- Get the Manual J before you buy, not after. It costs a few hundred dollars against a $8,000–15,000 installation, and it is the only thing that turns the number above into a specification. A contractor who sizes by square footage alone is guessing with your money.
- Heat pumps change the question. Sized for cooling, a heat pump may be short of capacity on the coldest days; sized for heating in a cold zone, it is badly oversized for cooling. Variable-capacity equipment or a modest backup heat source is the usual resolution, and it needs deciding at design time.
3 tons, and why the square footage alone did not tell you that
1400 ft² in zone 3 works out at 32,200 BTU/hr — that is 467 square feet per ton.
The figure people quote is 400 to 600 ft² per ton, and both ends of that range are real. They are just real in different places. The same 1400 ft² house needs 3.1 tons in zone 1–2 and 1.9 tons in zone 6–7.
Oversizing is the expensive mistake, not undersizing
An air conditioner has two jobs: drop the temperature and pull moisture out of the air. The second one only happens while it is running.
A unit that is too large hits the thermostat setpoint fast and shuts off. The air is cold and still damp, so the house feels clammy at 72°F and somebody turns it down further. Meanwhile the compressor starts and stops all day, which is what wears it out.
A unit that is slightly too small is uncomfortable on the five hottest afternoons of the year. That is the better failure.
What a Manual J adds
Window area and which way they face. Insulation in the walls and attic. How leaky the house is. Ceiling height. Whether the ducts run through an unconditioned attic. How many people live there.
Two identical-looking 1400 ft² houses on the same street can differ by a full ton once those are counted — a 1960s house with single glazing against a recent build with a tight envelope.
A Manual J costs a few hundred dollars against an installation of eight to fifteen thousand. It is the step that turns the number on this page into a specification.
If it is a heat pump
Then the sizing question has two answers, and they conflict. Sized for cooling, a heat pump may fall short on the coldest nights. Sized for heating in a cold zone, it is badly oversized for summer.
Variable-capacity equipment resolves most of it by modulating instead of cycling; a modest backup heat source resolves the rest. Either way it is a design decision, not something to discover after installation.
Nearby sizes
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- What size AC for 1600 square feet in zone 3
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- What size AC for 1400 square feet in zone 1–2
- What size AC for 1400 square feet in zone 4
- What size AC for 1400 square feet in zone 5
Read more
- AC Cannot Keep Up on the Hottest Days — By Design, UsuallyA system that keeps up on every day of the year is oversized for all the others. The question is how many days it should lose, not whether it should.
- AC Runs Constantly But Is Not Cooling — Six Causes, Cheapest FirstThe compressor running is not the problem. Something is stopping the heat it removes from leaving, and there are only six places that happens.
- Is 16 SEER Worth It Over 14? — The Payback, Worked ThroughSEER is a ratio, so the saving is a percentage of a bill you may not have. In Minnesota that percentage is small money; in Phoenix it is not.