The right AC size for a Lynnwood home comes from a load calculation, not from square footage alone or the size of the old unit. A proper calculation, often called Manual J, looks at insulation, windows, orientation, air leakage and ducts, and two houses of the same size can need quite different systems.
Sizing is the single decision that most affects comfort, running costs and how long a new system lasts. It is also the step most often skipped. This guide explains how sizing works, what goes wrong when it is off, and what to ask so the system you buy actually fits your home.
What does AC size actually mean?
Air conditioner size is cooling capacity, measured in BTUs per hour or in tons. One ton equals 12,000 BTUs per hour, so a three-ton system removes about 36,000 BTUs of heat each hour at its rated conditions.
It has nothing to do with the physical size of the box. A compact modern unit can have more capacity than a bulky old one. When a provider says your home needs a certain tonnage, they mean the amount of heat it must remove on a hot day to hold a comfortable indoor temperature.
Why is square footage not enough?
Because heat enters a house through its windows, walls, roof and gaps, and those vary far more than floor area does.
| Factor | Why it changes the size |
|---|---|
| Windows | Large west- or south-facing glass adds a lot of afternoon heat. |
| Insulation | Attic and wall insulation slow heat coming in from outside. |
| Air leakage | Drafty older homes pull in warm outdoor air. |
| Orientation and shade | Trees and neighboring buildings can cut solar heat significantly. |
| Ducts | Ducts in a hot attic or leaky crawlspace lose cooling before it reaches rooms. |
| Occupants and appliances | People, cooking and electronics all add heat. |
In the Lynnwood area, the mix of older ramblers, split-levels and newer tight-built townhomes means rules of thumb fail often. A well-insulated new townhome may need less capacity than an older house half its size with single-pane windows.
What goes wrong when an AC is oversized?
An oversized system cools the air quickly and shuts off before it has run long enough to pull moisture out. The house reaches the set temperature but feels damp and clammy.
- Short cycling: frequent starts and stops that wear out parts faster.
- Clammy air, because short run times remove less humidity.
- Uneven temperatures, with some rooms overshooting while others lag.
- Higher running costs from repeated start-ups.
Oversizing is common because it feels safe. It is not. Bigger is only better when the calculation says the house really needs it.
And when it is undersized?
An undersized system runs constantly on hot afternoons and may never reach the set temperature. It works hard for long periods, which raises bills and strains the compressor. In a mild climate, slight undersizing is less harmful than oversizing, but on the hottest days it shows.
Pro tipIf your current system short cycles or the house feels clammy, mention it. It is a clue the old unit may have been oversized, and the new one should not repeat the mistake.
How do heat pumps change the sizing question?
A heat pump must handle heating as well as cooling, and in western Washington the heating load is usually the larger of the two. The provider should calculate both and explain how backup heat covers the coldest days. The heat pump guide explains this in more detail.
What should you ask the provider?
- Did you do a load calculation, and can I see the result?
- What capacity did it recommend, and why is it different from my old unit, if it is?
- Are my ducts big enough to carry the airflow this system needs?
- Would a two-stage or variable-speed system handle our mild days better?
A provider who can answer those clearly is doing the job properly. For the full process, read the AC replacement guide, or see the Lynnwood AC replacement guide for how to get connected with a local provider.
Frequently asked questions
Can I just replace my AC with the same size?
You can, but only if the old size was right. Many older systems were sized by rule of thumb, so a load calculation is worth doing.
Does a bigger AC cool faster?
It cools the air faster but often removes less humidity and short cycles, so the house can feel less comfortable.
Do I need a load calculation for a mini-split?
Yes, room by room. Oversized ductless heads in small bedrooms are a common mistake.
About this articleGeneral guidance from Lynnwood Air Comfort, a free referral service. It is not a substitute for an on-site inspection by a qualified technician.