# How Is a Heat Pump Sized? Manual J Explained > Manual J heat pump sizing in Seattle: heat loss at 24°F, cooling at 83°F, why tonnage is not sizing and why oversizing short-cycles. Ask for the numbers. URL: https://seattleheatpumpco.com/guide/heat-pump-sizing-manual-j/ Updated: 2026-10-21 *What a load calculation measures, why the old furnace's nameplate misleads, and how Seattle's energy code turns a room-by-room heat loss into the right heat pump.* Manual J heat pump sizing is a room-by-room calculation of how much heat your house loses on a design cold day and how much heat it gains on a design hot day. The heat pump is then chosen to match those loads, not the size of the old furnace. In Seattle, the energy code requires it: loads by ACCA Manual J, equipment selected by Manual S. [The Seattle Heat Pump Company](https://seattleheatpumpco.com/) plans ducted heat pumps across Seattle and King County starting from that calculation, with installation carried out by licensed and insured contractors. Within the wider [Seattle heat pump guide](https://seattleheatpumpco.com/guide/seattle-heat-pump-guide/), this page explains what the calculation measures, why it routinely disagrees with the nameplate on your basement furnace, and what to ask for when a quote lists a size. **Key takeaways** - Seattle's energy code requires Manual J loads and Manual S equipment selection, sized no larger than the smallest equipment that meets the load. - Seattle and Bellevue size for 24°F heating and 83°F cooling; Redmond's heating design temperature is 17°F. - Tonnage describes rated capacity at test conditions. It is an output of sizing, not a substitute for it. - Oversizing causes short cycling, poor summer humidity control and wasted energy. ## What Manual J actually calculates A Manual J calculation answers two questions for each room and for the whole house: how many BTUs per hour leave the house at the heating design temperature, and how many BTUs per hour enter it at the cooling design temperature. Those two numbers are the heating load and the cooling load. ### Heating load and building heat loss Heat leaves a house in two ways. It conducts through walls, windows, ceilings and floors, faster where insulation is thin and slower where it is thick. It also leaves with air: every cubic foot of warm air that leaks out through gaps around windows, rim joists, attic hatches and plumbing penetrations is replaced by cold outdoor air that has to be heated. The calculation adds both for every room, using the area of each surface, its insulation level, the window type and the measured or estimated air leakage. The temperature difference drives all of it. A house loses heat faster at 24°F than at 44°F, so the calculation uses a fixed design temperature that represents a cold but not record-breaking night. Seattle's energy code tables set that at 24°F for Seattle and Bellevue, 24°F for Edmonds, 25°F for Auburn and 17°F for Redmond. Redmond's colder figure is the reason houses on the [Kirkland and Redmond](https://seattleheatpumpco.com/service-areas/kirkland-redmond/) side of the lake can need more low-temperature capacity than an identical house in the city. ### Cooling load and building heat gain The cooling side runs the same logic in reverse, with one large difference: sunlight. A west-facing wall of glass can add more heat on a July afternoon than everything else in the room combined. Manual J accounts for window orientation, shading, the heat given off by people and appliances, and the moisture the system has to remove, using a cooling design temperature of 83°F for Seattle. Cooling used to be an afterthought here. It is not anymore. Sea-Tac's cooling degree days averaged about 176 a year in the 1990s and about 337 a year from 2016 to 2025, and the June 28, 2021 heat dome reached 108°F. A heat pump sized only for winter can leave upstairs bedrooms hot in August, so the calculation has to carry both loads. For houses that have never had air conditioning, [central air from a heat pump](https://seattleheatpumpco.com/central-air-conditioning-heat-pump/) is often what starts the project. ## Why tonnage and BTU ratings are not sizing One ton of capacity equals 12,000 BTU per hour. A "3-ton" heat pump is nominally a 36,000 BTU per hour machine. That label is where most sizing conversations start, and where many go wrong. Nominal tonnage describes rated capacity at standard test conditions. A heat pump's heating output falls as the outdoor temperature drops, and how fast it falls differs a great deal from one model to the next. Two units sold as 3-ton can deliver very different heat at 24°F. Seasonal ratings such as HSPF2 do not settle it either, as the [SEER2 and HSPF2 guide](https://seattleheatpumpco.com/guide/seer2-hspf2-ratings-explained/) explains. That is why Manual S exists: it matches a specific model's published capacity at your design conditions to your calculated load, rather than matching labels. The furnace nameplate misleads for a different reason. A furnace rated at 100,000 BTU per hour tells you what the furnace could produce, not what the house needs. Many Seattle furnaces were oversized when installed, and many houses have since gained attic insulation, new windows or air sealing that cut the load further. Matching a heat pump to the old furnace can bake that oversizing into a new system. The same applies to replacing an old heat pump: [heat pump replacement](https://seattleheatpumpco.com/heat-pump-replacement/) is a good moment to recalculate, not to copy. ## What goes wrong with an oversized heat pump Bigger feels safer. It is not, and the reasons are mechanical. ### Short cycling An oversized system reaches the thermostat setpoint quickly and shuts off, then starts again when the temperature drifts. Each start is the least efficient part of a run and the hardest on the compressor. Rooms far from the thermostat get uneven heat, because short runs do not push air to the ends of long duct runs for long enough. Short cycling has other possible causes, including thermostat location, control settings and airflow problems, so it is a symptom to investigate rather than proof of oversizing on its own. ### Humidity in summer A heat pump removes moisture in cooling mode by running air across a cold indoor coil, where water condenses and drains away. That takes time. A system that cools the air quickly and stops never runs long enough to pull much water out, so the house reaches 74°F and still feels clammy. Seattle summers are usually dry, but the warm, humid spells that do occur are when an oversized system shows it. ### Variable-speed equipment narrows the penalty A variable-speed compressor can slow down to a fraction of its full output, so a modest amount of extra capacity costs less comfort than it would with a single-stage unit. It does not make sizing optional. Below its minimum speed, a variable-speed unit cycles too. The [variable-speed versus single-stage guide](https://seattleheatpumpco.com/guide/variable-speed-vs-single-stage-heat-pump/) covers how the turndown range interacts with sizing. ## Undersizing has its own cost The opposite mistake shows up on cold mornings. An undersized heat pump runs flat out and still falls behind, so the backup heat makes up the difference. With electric resistance strips, every hour of backup costs roughly two to three times what the same heat costs from the heat pump, depending on conditions. Seattle's code helps by keeping supplemental heat off above 40°F and requiring the lockout set to 35°F or lower at final inspection, but it cannot fix a system that is too small for the house. How backup heat should be staged is covered in the [backup heat settings guide](https://seattleheatpumpco.com/guide/heat-pump-backup-heat-settings/). ## The thermal balance point, worked through Here is how load and capacity meet, using round illustrative numbers rather than any real house. Suppose a calculation shows a house loses 30,000 BTU per hour at Seattle's 24°F design temperature, and much less on milder nights, because heat loss scales with the indoor-outdoor temperature difference. Now picture a heat pump whose published capacity falls as it gets colder. Plot both lines against outdoor temperature and they cross somewhere. Above that crossing, the thermal balance point, the heat pump alone carries the house and cycles or slows down. Below it, the house needs more heat than the heat pump can give, and backup fills the gap. Good sizing puts the balance point low enough that backup runs only on a handful of hours. Sea-Tac averaged about 25 nights a year at or below freezing and about one night a year at 20°F or below over the 1991 to 2020 normals, with a coldest night of 14°F from 1991 through 2025. A heat pump whose capacity crosses the load near or below 24°F leaves very little work for the backup in a typical Seattle winter. The [Seattle winter guide](https://seattleheatpumpco.com/guide/do-heat-pumps-work-in-seattle-winter/) covers what happens on the rare colder nights. ## Insulation and air sealing change the answer Because Manual J adds up conduction and air leakage, anything that cuts either one shrinks the load. Attic insulation reduces conduction through the ceiling, often the largest single surface in a one-story house. Air sealing at the attic floor, rim joists and penetrations cuts infiltration, which in older, leakier houses can be a large share of the total. Better windows help both. That has two practical consequences. If envelope work is planned, it is worth doing first or at least including in the calculation, because it can drop the system a size and lower both the equipment price and the running cost. And if envelope work was done after the old furnace went in, the old nameplate overstates the load even more. Older houses with mixed eras of improvements are where this matters most; see [heat pumps for older homes](https://seattleheatpumpco.com/heat-pump-older-homes/). ## From load calculation to equipment and ducts Sizing is the first link in a chain. The load calculation sets the capacity needed. Manual S picks a specific model whose capacity at design conditions meets that load without exceeding it more than the code allows. The chosen model sets the airflow the ducts must carry, typically a few hundred cubic feet per minute per ton, and that airflow has to fit through the existing return and supply ducts at acceptable static pressure. If it does not, the duct work changes, or the equipment choice does. [Using a heat pump with existing ductwork](https://seattleheatpumpco.com/heat-pump-existing-ductwork/) covers that second step, and the [installation process guide](https://seattleheatpumpco.com/guide/heat-pump-installation-process/) follows the chain through commissioning. ## What to ask for on a quote A quote that names a tonnage without the numbers behind it leaves you guessing. Ask for: - the calculated heating load and cooling load for the whole house - the design temperatures used, which should match the energy code table for your city - the model's capacity at the heating design temperature, from the manufacturer's data - how air leakage was estimated or measured - whether planned insulation or window work was included If two quotes propose different sizes, those numbers usually explain why. The [quote comparison guide](https://seattleheatpumpco.com/guide/how-to-compare-heat-pump-quotes/) lists the other lines worth checking, and the [cost estimator](https://seattleheatpumpco.com/tools/heat-pump-cost-estimator/) gives a budgeting range before anyone visits. When you want the calculation done on your own house, the [ducted heat pump installation](https://seattleheatpumpco.com/ducted-heat-pump-installation/) assessment starts there. ## Questions ### Can I use an online heat pump size calculator instead of Manual J? An online calculator that asks for square footage and a climate zone gives a rough starting number, not a size. It cannot see your window area, insulation, air leakage or which rooms face south. Seattle's energy code requires a Manual J calculation and Manual S selection for the installed system, so the calculator is useful only for checking whether a quote is wildly off. ### My contractor quoted the same tonnage as my old system. Is that wrong? Not necessarily, but it should be a result, not a starting point. If the load calculation lands on the same size, fine. If the quote simply copies the old nameplate, ask for the heating and cooling loads and the design temperatures used. Many houses have gained insulation or new windows since the original equipment went in, which lowers the load. ### Is a slightly oversized heat pump safer for cold nights? Usually not. Extra capacity helps only on the few coldest hours, while the rest of the year an oversized system short-cycles, controls humidity poorly in summer and runs less efficiently. Seattle's code caps size at the smallest equipment that meets the load. Cold nights are better handled by choosing equipment with strong low-temperature output and a controlled backup.