What size AC does my house need — and why isn't bigger better?
The right AC size comes from a Manual J load calculation — a room-by-room accounting of your home's heat gain through windows, walls, attic, and air leaks, not a square-footage rule of thumb from a phone quote. Oversizing is the classic Seattle mistake: a too-big unit cools the thermostat quickly, shuts off before dehumidifying the air, and short-cycles itself toward an early replacement in our increasingly warm, humid Puget Sound summers.
The quick answer
- A 'ton' is cooling rate (12,000 BTU/hr), not weight — typical homes need 2–5 tons
- Manual J measures YOUR house: windows, orientation, insulation, leakage — not just square feet
- Oversized AC = short cycles, clammy air, higher bills, shorter equipment life
- Any bid quoting size without visiting (or measuring) your home is guessing
When sizing questions matter most
Every replacement and every first-time install. Replacements are the sneaky one: 'match the old unit' assumes the original was sized right (often it wasn't) and that nothing changed — but new windows, added insulation, or an attic upgrade all shrink the load. Homes routinely need smaller equipment than they had.
It matters double in Seattle because our cooling load is modest most of the summer with sharp peaks. Equipment sized for the peak by crude rules spends most of its life absurdly oversized for the ordinary 78°F day.
West-facing glass is the hidden driver in Puget Sound homes: a house with a wall of afternoon sun can need meaningfully more cooling capacity than an identical floor plan facing north — and square-foot rules never account for that. Manual J does.
What Manual J actually accounts for
The calculation walks your actual house: window area and orientation (west glass is Seattle's big afternoon-heat driver), insulation levels, ceiling heights, air-tightness, attic condition, occupancy, even shading from that big cedar. The output is a heat-gain number in BTU/hr that the equipment must match — room by room, which also exposes duct-distribution problems while it's at it.
Contrast the shortcut: '500–600 square feet per ton' style rules. Two identical-square-footage homes — one shaded mid-century with small windows, one new construction with a west-facing glass wall — can differ in cooling load by a factor of two.
Manual J also exposes duct problems while it's running: a room whose calculated load exceeds what the connected duct can deliver needs distribution work, not just a bigger outdoor unit. That's why load calculations belong on every serious Seattle-area bid, not only on custom homes.
Why oversizing specifically ruins comfort
Cooling comfort is temperature AND humidity. Dehumidification happens while air flows over the cold coil — it needs runtime. An oversized unit satisfies the thermostat in minutes and shuts down before pulling meaningful moisture, so the house reads 72°F and feels clammy. Then it restarts, over and over: short-cycling, with the same wear consequences it has on furnaces.
Right-sized (or slightly conservative) equipment runs longer, gentler cycles: steadier temperatures, real dehumidification, quieter operation, longer life. Variable-speed equipment widens the sweet spot further by modulating down on mild days.
In Puget Sound, dehumidification matters even when outdoor temps are moderate — smoke-plus-heat weeks seal houses tight, and an oversized AC that short-cycles leaves indoor air clammy at 72°F. Comfort is temperature and moisture, not just the number on the thermostat.
Sizing red flags in bids
A size quoted over the phone from square footage. 'We'll just go a half-ton bigger to be safe' — safety margin is the load calculation's job, not a vibe. And a replacement bid that never asks what's changed in the house since the last unit went in.
If two bids differ by a ton or more, someone skipped the math. That's precisely the situation our free second opinion on replacement quotes exists for — we'll run the numbers and show you which bid fits the house.
In Burien and South Seattle ramblers, we often find replacement bids matching the old 3-ton unit when attic insulation upgrades and window replacements have shrunk the load to 2.5 tons. Smaller equipment from honest math is good news — not a contractor cutting corners.
Questions Seattle homeowners ask
What does a 'ton' of cooling mean?
A rate: 12,000 BTU per hour of heat removal (historically, the melt rate of a ton of ice per day). Homes typically land between 2 and 5 tons — where yours lands is what the load calculation determines.
Is it bad to oversize 'just a little'?
A small margin above the calculated load is normal engineering. A half-ton-or-more 'to be safe' guess isn't margin, it's short cycles and clammy air. The calculation already contains the appropriate safety factors — trust the math over the vibe. In Puget Sound's moderate cooling season, right-sized equipment that runs longer cycles usually beats oversized equipment that hits setpoint in five minutes and never dehumidifies.
My old 3-ton unit worked fine. Why is the new bid 2.5 tons?
Probably because the math was finally done — or because your house changed (windows, insulation, air sealing all shrink the load). Newer equipment also delivers its capacity more effectively. A smaller number from a measured bid is usually good news for comfort and bills, not a contractor cutting corners. We provide free second opinions on replacement quotes when sizing differs between bidders.
Does Manual J apply to heat pumps and ductless too?
Yes — everything we install starts from the same load math, heating and cooling. Heat pumps add a heating-balance-point check; ductless adds per-room load matching so each head is sized to its zone. A free estimate on new installs includes this work — any bid that sizes from square footage alone skipped the engineering your house deserves.
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How we know this
Sizing practice per ACCA Manual J, the residential standard; comfort/short-cycle mechanics per standard psychrometrics. Bid-pattern observations from Glendale's second-opinion work on Greater Seattle replacement quotes.
Reviewed by the Glendale Heating & Air Conditioning team · Last updated 2026-07-15
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