Last month I was in a house in Lake Forest Park where the homeowner had a 5-ton condenser sitting outside a 1,400 square foot rambler. The unit was maybe eight years old, and she kept asking why the house felt damp and clammy even though the air was “cold enough.” I didn’t need a load calculation to tell her what was wrong. That unit was cooling the house so fast it never ran long enough to pull moisture out of the air. She’d paid for capacity she didn’t need and got worse comfort for it.
A “ton” in HVAC has nothing to do with weight. It’s a unit of cooling capacity, and if you don’t know what it actually measures, you’re at the mercy of whoever quotes you a number. Let’s fix that.
TL;DR
- One ton of AC capacity equals 12,000 BTU per hour of heat removal — a term left over from ice-cooling history, not a measure of size or weight.
- Most Seattle-area homes need 1 ton per 500-700 sq ft, but insulation, windows, and sun exposure change that a lot.
- Oversized systems short-cycle, leave humidity behind, and wear out faster — bigger is not automatically better.
- A proper load calculation (Manual J) beats square-foot rules of thumb every time.
- Typical installed cost for residential AC or heat pump systems in King and Snohomish counties runs $4,500 to $12,000+ depending on tonnage and equipment type.
Why “Ton” Even Means BTU
Before mechanical refrigeration, ice houses cooled buildings with blocks of ice. One ton of ice melting over 24 hours absorbs about 288,000 BTU of heat — that works out to 12,000 BTU per hour. When compressor-based AC came along in the early 1900s, engineers borrowed the ice industry’s unit so customers could compare new equipment to what they already understood. The name stuck. Nobody at your local supply house is measuring literal tons of anything.
So when a tech says you need a “3-ton system,” they mean a unit rated to remove 36,000 BTU of heat from your home per hour. That’s it. No scale involved.
“The tonnage conversation is where good installs are won or lost. Our winter design temperature is mild, so Seattle-area homes need less capacity than most people expect — and the oversized unit short-cycles itself to an early death while dehumidifying nothing. Manual J first, equipment brand second.”
— Danil Sokolov, Heat Pump & Mini-Split Specialist at SPS Pro Services
How Much Tonnage Does a Seattle Home Actually Need
Here’s where a lot of quotes go wrong. Contractors in a hurry use a flat rule — usually 1 ton per 500 to 600 square feet — and call it sized. That rule was built for hot, humid climates like Texas or Georgia, not the Pacific Northwest.
King and Snohomish counties sit in a marine climate. Summers are mild, humidity is moderate, and our cooling load is driven more by solar gain through windows and heat from appliances than by outdoor temperature swings. I’ve measured homes in Bellevue with big south-facing glass that need noticeably more tonnage per square foot than a shaded 1970s rambler in Everett with small windows and a low roofline.
That’s why I don’t quote tonnage off a square footage chart. I run a Manual J load calculation — it factors in insulation levels, window area and orientation, ceiling height, duct location, and even how many people typically occupy the house. Two identical-looking 2,000 sq ft homes can come out half a ton apart once you run real numbers.
What Happens When You Oversize
Oversizing is the single most common sizing mistake I see on service calls, more common than undersizing. Here’s why it hurts:
Short cycling. A system too big for the load cools the house fast and shuts off before it’s run long enough to remove humidity. You get a house that feels cold and sticky at the same time.
Higher wear. Every start-up puts stress on the compressor and contactor. A right-sized system might run 10-15 minutes per cycle. An oversized one might run 4-5 minutes, dozens of times a day. More cycles, more wear, shorter equipment life.
Uneven temperatures. Short bursts of cold air don’t have time to mix through the ductwork, so rooms far from the return end up warmer than rooms near the air handler.
Wasted money up front. Bigger equipment costs more to buy and install. If you don’t need it, you’re financing capacity that actively makes your home less comfortable.
Undersizing has its own problems — the system runs constantly and still can’t keep up on the hottest days — but in this region, with our mild summer peaks, I see far more oversized systems than undersized ones.
Ton Size vs. Real-World Cost in Our Area
Here’s what I’m actually quoting homeowners in King and Snohomish counties this year, install labor and typical ductwork tie-in included. These are ranges, not fixed quotes — every house has its own duct condition and electrical situation.
| System Size | Typical Home Size | Central AC (add-on to existing furnace) | Ducted Heat Pump (heating + cooling) |
|---|---|---|---|
| 1.5 ton | 900-1,100 sq ft | $4,500 - $6,000 | $8,000 - $10,500 |
| 2 ton | 1,100-1,500 sq ft | $5,000 - $6,800 | $8,800 - $11,500 |
| 3 ton | 1,500-2,100 sq ft | $5,800 - $7,800 | $9,500 - $13,000 |
| 4 ton | 2,100-2,800 sq ft | $6,800 - $9,200 | $11,000 - $15,500 |
| 5 ton | 2,800-3,500 sq ft | $8,000 - $12,000 | $13,000 - $18,000 |
I put heat pumps next to straight AC on purpose. In this climate, most homeowners who call for heat pump installation end up choosing it over a standalone AC unit because it also replaces or backs up their heating system. If your furnace is already on its last legs, or you’re weighing furnace repair against replacement, a heat pump sized correctly for cooling often gets you heating capacity too — sometimes down to outdoor temps in the low 20s before backup heat kicks in, depending on the model.
One more cost factor people miss: electrical capacity. A larger heat pump or AC condenser draws more amps, and older panels — common in homes built before 1990 across Seattle and the surrounding suburbs — sometimes can’t handle the new load alongside an EV charger or electric range. If you’re adding a heat pump and planning an EV charger installation in the same year, get your panel checked first. I’ve had to pause installs mid-quote because the panel needed a electrical panel upgrade before we could even set the new condenser.
When Repair Beats Replacement
Not every sizing conversation ends with new equipment. If your current AC or heat pump is properly sized and under 10 years old, a failed capacitor or low refrigerant charge is usually a $200-$600 fix, not a $10,000 replacement. I’ll tell a homeowner straight: if the unit’s sized right and the compressor’s healthy, don’t let anyone talk you into replacing it just because it’s not ice cold. Check out AC repair options before assuming you need new tonnage. The math only points to replacement when the unit is oversized, undersized, past 12-15 years old, or the repair cost creeps past 40-50% of replacement cost.
Sizing Checklist Before You Buy
- Ask for a Manual J load calculation, not a square-footage estimate.
- Get your attic insulation R-value and window count/orientation on record — these swing the load more than floor area does.
- Have your electrical panel capacity checked if you’re installing a heat pump alongside an EV charger or upgraded range.
- Compare at least two tonnage quotes for the same house — if they’re off by more than half a ton, ask why.
- Confirm existing ductwork can handle the airflow (CFM) the new tonnage requires — undersized ducts choke even a perfectly sized unit.
- If your water heater is also aging, ask about bundling a water heater repair visit during the same appointment to save a second trip charge.
FAQ
Can I just size my AC based on my furnace’s BTU rating? No. Furnace BTU output measures heating capacity; AC tonnage measures heat removal in cooling mode. They’re sized independently, though a good contractor checks both when planning a combined system.
Is a bigger ton size ever the safer bet “just in case”? Not in this climate. Oversizing causes short cycling and humidity problems long before it ever helps on a hot day. Right-sizing based on load calculation, not worst-case guessing, gives better comfort and lower bills.
How do I know if my current system is the wrong size? Watch the run time. If your AC or heat pump shuts off in under 8-10 minutes on a warm day and the house still feels humid, it’s likely oversized. If it runs nearly nonstop and can’t hit the setpoint, it’s likely undersized.
Call SPS Pro Services at (206) 427-8907 or start from our AC installation page, and we’ll run the load calculation before we ever talk equipment size.