Sizing Central Air Conditioning for a Pacific Northwest Home

Most of the Pacific Northwest spent generations without central air conditioning. That changed after a string of brutal summer heat events. Today, many Portland-area homeowners are installing AC for the first time, and the single most consequential decision they will make is not which brand to buy — it's what size system to put in. Get sizing wrong and the equipment will underperform, wear out faster, and leave the house less comfortable than you hoped.

Why Bigger Is Not Safer

Air conditioning is sold in tons of cooling capacity. A common assumption is that a larger unit will simply cool the house faster. In practice, the opposite is true.

A well-sized system runs in longer, steadier cycles. Those cycles give the equipment time to do something just as important as lowering temperature: removing humidity from indoor air. When a system is oversized, it blasts the house to setpoint quickly and shuts off before completing a full humidity-removal cycle. The air feels cold but damp. That clammy, sticky quality at a perfectly reasonable thermostat reading is the hallmark of a short-cycling oversized system.

Short-cycling also batters the compressor. Starting up is the hardest moment in any refrigeration cycle, and a unit that starts and stops dozens of times a day accumulates wear far faster than its engineering intended. Oversized equipment often fails earlier than a properly sized unit, and costs more to run in the meantime. The efficiency rating discussed here is explained in how the SEER efficiency rating works.

What a Load Calculation Actually Is

The industry standard method for sizing residential cooling is called a Manual J load calculation. It is not a square-footage chart. It is an actual physics calculation.

A proper Manual J accounts for the orientation of the house and how much direct sun each wall and roof surface receives across a typical summer day. It accounts for the thermal resistance of your wall, attic, and floor insulation. It factors in window area, window type, and which direction each window faces. It considers how much air leaks through the building envelope — every gap around a pipe penetration, every poorly sealed attic hatch. It accounts for how many people typically occupy the house, because people generate heat.

The result is a number in BTUs per hour that represents the peak cooling demand the house places on a system under the hottest local conditions. That number tells you what size equipment to install. Any contractor who proposes a size based on square footage alone, or on what a neighbor installed, is skipping this step.

How Older Portland Housing Stock Changes the Picture

The Portland metropolitan area has a large inventory of homes built before modern energy codes. Many have thin wall insulation, original single-pane windows, and attic insulation that has settled over the years. Some have no mechanical ventilation at all, relying on natural air leakage as the de facto exhaust strategy.

All of those characteristics increase cooling load. A leaky house in full summer sun continuously pulls in warm outdoor air. Single-pane glass lets radiant heat pour through as though the window were open. An under-insulated attic can push the temperature of the ceiling surface well above room temperature, adding a steady heat load the AC has to fight.

On the other hand, the Pacific Northwest has qualities that work in your favor. Mature tree cover is unusually common here, and afternoon shade from Douglas firs or big-leaf maples can dramatically reduce solar gain on west-facing walls and windows. A good load calculation captures both sides of that ledger — it doesn't assume your older house is a worst case, and it doesn't ignore real deficiencies in the envelope.

SEER2 in Plain Terms

New central air conditioning systems carry an efficiency rating called SEER2 — Seasonal Energy Efficiency Ratio, second edition. The "2" reflects an updated testing standard that more realistically models how equipment performs across a full cooling season rather than at a single ideal operating point.

The ratio is straightforward: it expresses how much cooling a system delivers relative to the electrical energy it consumes. A higher SEER2 number means more cooling per unit of electricity, which translates to lower operating costs per hour of runtime. Equipment sold today must meet a federal minimum efficiency floor. Beyond that floor, you are choosing how much additional efficiency to pay for upfront in exchange for lower utility costs over the life of the system. A contractor can walk through that trade-off using your expected runtime hours and local electricity rates.

What to Ask Before You Sign Anything

Ask your contractor for a written Manual J load calculation for your home. If they propose a size from square footage alone or from the size of a neighbor's system, that is a red flag. Ask what assumptions they made about your insulation and windows, and whether they physically inspected your attic and crawl space. A calculation done from the street is guesswork dressed up in professional language.

If your home has known envelope problems — drafty windows, minimal attic insulation, gaps around penetrations — ask whether addressing those before installation would change the recommended equipment size. In many cases, modest improvements to the building envelope allow a smaller, less expensive system to handle the load comfortably. The insulation investment and the equipment savings sometimes offset each other, and you end up with a tighter house and a right-sized system.

The goal of every one of these questions is the same: to ensure that what goes into your home is matched to what your home actually needs, not to a rule of thumb or whatever happened to be on the truck. Federal guidance on central air conditioning is published at https://www.energy.gov/energysaver/central-air-conditioning.