What Size Furnace Does Your Washington Home Actually Need?

Homeowners almost always ask it the same way: "What size furnace do I need for a 2,000 square foot house?" It's a fair question with an unsatisfying answer. Square footage is one input out of a dozen, and it isn't the most important one.

Here's how sizing actually works in Western Washington, why the rules of thumb you find online overshoot our climate, and what an honest estimate should show you before anyone quotes a number.

The short answer — and why it's only a starting point

Most Puget Sound homes end up with a gas furnace somewhere between 40,000 and 80,000 BTU of input capacity. Small, tight ramblers sit at the low end. Big, drafty two-stories in colder pockets sit at the top.

That range is useful for setting expectations. It is not a specification. Two 1,900 sq ft houses can need furnaces 20,000 BTU apart based on nothing more than attic insulation, window area and which city they sit in.

Why square footage rules break down here

The classic internet rule is "30 to 60 BTU per square foot." Look at that spread for a second. On a 2,000 sq ft home, it produces an answer between 60,000 and 120,000 BTU. That's not a calculation, that's a shrug.

What actually drives the heating load:

• Insulation. A 1962 rambler with R-11 in the attic and none in the walls loses heat several times faster than a 2015 build to modern code.
• Windows. Single-pane aluminum versus modern double-pane low-E changes the load dramatically, and a wall of west-facing glass in Des Moines behaves differently than the same wall in Enumclaw.
• Air leakage. Old crawl-space homes with unsealed rim joists and leaky can lights lose heat continuously, and it doesn't show up in square footage at all.
• Ceiling height and layout. Vaulted great rooms hold far more air volume than the floor plan suggests.
• Ductwork location. Ducts running through an uninsulated crawl space or a cold attic lose heat before it ever reaches a register.
• Orientation and shading. A house tucked under Douglas firs on the north side of a hill sees less solar gain than the one across the street.

Your city's design temperature does real work

Washington's energy code assigns each area a winter outdoor design temperature — the number every load calculation gets run against. Across the 45 cities we serve, it varies by 12 degrees:

This is why a contractor who sizes the same way in Olympia as in Tacoma is guessing. Twelve degrees of design temperature is a meaningful chunk of load on an identical house.

What a Manual J load calculation measures

Manual J is the ACCA industry-standard method for calculating how much heat a specific house loses on a design-temperature day. Done properly, it's a room-by-room accounting of:

1. Wall, ceiling and floor assemblies with their actual insulation values
2. Window and door area, glazing type and orientation
3. Estimated infiltration based on age, construction and condition
4. Duct location and losses
5. The local design temperature and your target indoor temperature

The output is a BTU-per-hour heat loss number. The furnace is then selected to cover that load with a modest margin — not double it.

It takes a technician measuring your house, not a phone call and a zip code. Every Varsity estimate includes a real load calculation at no extra cost, because sizing off the old nameplate is how bad systems get replicated for another 20 years.

Why oversizing is the expensive mistake

Bigger feels safer. It isn't, and this is the single most common sizing error we find in Puget Sound homes.

An oversized furnace blasts the house past setpoint in a few minutes, shuts off, then fires again a short while later. That's short cycling, and it costs you in four ways:

• Comfort. Big temperature swings, blasts of hot air, then a chill. Rooms furthest from the furnace never get their turn because the burner shuts down before air distribution evens out.
• Wear. Ignition is the hardest thing a furnace does. Ignitors, inducer motors and flame sensors fail sooner on a unit that starts 12 times an hour instead of four.
• Noise. Frequent startups in a hallway closet or a garage wall are impossible to ignore.
• Airflow strain. A furnace sized well beyond the ductwork's capacity pushes more air than the ducts can carry, raising static pressure and noise.

Undersizing is real too, just rarer — the furnace runs continuously on a cold snap and still drifts below setpoint. In our marine climate, though, we see three oversized systems for every undersized one.

Signs your current furnace is the wrong size

• Cycles on and off every few minutes in cold weather
• Rooms near the furnace roast while bedrooms at the end of the run stay cold
• Loud, abrupt startups followed by short runs
• Repeated ignitor or flame sensor failures on a relatively young unit
• The nameplate says 100,000 BTU on a 1,400 sq ft house

Sizing isn't only about the furnace

A correctly sized furnace still underperforms if the ductwork can't carry the air. Undersized returns are the most common issue we find in 1960s–1980s Puget Sound homes: one central return grille trying to feed a system that needs two. Symptoms look identical to a sizing problem — uneven rooms, whistling registers, high static pressure.

When ducts genuinely need work, replacement runs $3,000 to $8,000 in standard markets and $4,000 to $10,000 for larger or more complex layouts. Often it's a targeted modification rather than a full replacement, and it gets bundled into the install day. We measure before we recommend it.

What size means for price

Capacity affects price less than people expect. Standard-efficiency gas furnace replacement runs $4,000 to $8,000 installed in standard markets; high-efficiency condensing units, larger homes and premium markets run $5,000 to $10,000. Replacing a furnace and central AC together runs $10,000 to $16,000.

The bigger cost swings come from efficiency tier, venting requirements and site conditions — not from the difference between a 60,000 and an 80,000 BTU cabinet. Which means there is no financial reason to "round up for safety," and plenty of comfort reasons not to.

Want a starting number before anyone visits? The instant estimate tool asks seven questions about your home and shows Good, Better and Best starting prices in about a minute. The final number gets confirmed at the in-home visit, where the load calculation happens.

Before you size a furnace, price the alternative

If your furnace is 12+ years old and you also want air conditioning, run the comparison honestly. A ducted heat pump runs $12,000 to $18,000 in standard markets and covers heating and cooling in one system, and qualifying installations can access up to $4,400 in PSE rebates when replacing an existing electric or fossil-fuel system — the exact amount depends on what you're replacing and what you install. Programs change, so we confirm your current eligibility and handle the paperwork. From April 2, 2026, PSE rebates require a PSE-certified installer, which Varsity is.

If the furnace is young and healthy and gas is cheap for you, a straight furnace replacement is still the right call. We've written the full comparison with the actual math.

How we size it

1. Measure the house. Rooms, ceiling heights, window area, insulation where it's visible.
2. Inspect the ducts. Supply and return sizing, condition, location, obvious leakage.
3. Run the load calculation against your city's design temperature.
4. Present Good, Better and Best options in writing, with the size and the reasoning shown.
5. Install in one day in most cases, pull the permit, register the 10-year parts and labor warranty.

If a contractor quotes you a furnace size before walking your house, that number came from a habit, not your home.

Get the number that fits your house

Varsity Heating and Cooling is owner-operated out of Federal Way, serving 45 cities across King, Pierce, Thurston and Kitsap counties. Every furnace estimate includes a real load calculation, written Good/Better/Best pricing and no manufactured urgency.

Request your free estimate or call (253) 785-4622, Monday through Friday, 8 to 6.