Sizing 2 Mar 2026 6 min
Heat pump or furnace? The load calculation decides.
What a Manual J actually measures, why the size of your old furnace tells you almost nothing, and the three numbers that settle the question for a Portland house.
By Elliot Kwan
Every week somebody asks us which is better, a heat pump or a furnace, and every week the honest answer is the same: it depends on three numbers we do not have until we have been in your attic. This post is about those numbers.
The old furnace is not a measurement
The most common way a system gets sized is by reading the nameplate on the one being replaced. An 80,000 BTU furnace comes out, an 80,000 BTU furnace goes in. The trouble is that the first one was probably sized the same way, from the one before it, back to a builder in 1962 who picked the biggest unit that fit the closet.
In the 1,240 houses we have measured since 2017, the existing furnace was oversized in 71% of them, by a median of 40%. An oversized furnace short-cycles: it blasts, satisfies the thermostat in eight minutes, shuts off, and the far rooms never catch up. That is the “upstairs is always cold” complaint, and a bigger unit makes it worse.
What a Manual J measures
A load calculation is a room-by-room accounting of heat loss on the coldest design day and heat gain on the hottest. For each room we record:
- floor area and ceiling height
- every window — size, orientation, glazing, and whether a tree shades it in July
- wall construction and insulation, from the attic and the crawlspace, not from a guess
- air leakage, which we test with a blower door on request
- duct runs, and whether they pass through unconditioned space
The software (we use the ACCA-approved version) turns that into a heating load and a cooling load in BTU per hour, per room, and a total. For a typical 2,100 sq ft Alameda bungalow with original windows, that comes to about 42,000 heating and 25,000 cooling.
The three numbers that decide
1. The heating load at design temperature. Portland’s heating design temperature is 24°F. A cold-climate heat pump keeps most of its rated capacity down to 5°F, so if the load at 24°F fits inside the pump’s capacity at 5°F, you have margin. For the bungalow above a 3-ton unit does it with room to spare.
2. The ratio of heating to cooling load. A house with a heating load twice its cooling load — most of Portland — is a heat pump house: one machine sized for winter is only slightly generous in summer. A house with a huge west-facing glass wall and a small heating load can be the other way round.
3. What the ducts can carry. A 3-ton system wants about 1,200 CFM of airflow. If the existing ducts are undersized, the static pressure will be high and neither a furnace nor a heat pump will perform. We measure it before we quote, and the fix is on the estimate if it is needed.
Where a furnace still wins
A gas furnace is still the right call when the electrical service cannot support a heat pump without a panel upgrade nobody wants to pay for, when the house has a very high heating load and no realistic envelope fix, or when the owner plans to sell within two years and the rebate math does not close. We install both. The load calculation tells us which — and it is stapled to every estimate we write, so you can see it too.
The furnace nameplate tells you what somebody guessed in 1962. The load calculation tells you what the house needs today.