What size mini split does a 600 sq ft bedroom in Federal Way, WA need?
600 ft² bedroom in Federal Way, WA
9,000BTU/hr
8,779 BTU/hr of load at 14.6 BTU/ft² — the figure for 351 cooling degree days
| Head size to buy | 9,000BTU/hr |
|---|---|
| Calculated load600 ft² × 14.6 BTU/ft² | 8,779BTU/hr |
| BTU per square foot hereinterpolated from Federal Way's 351 cooling degree days | 14.6BTU/ft² |
| What the 20 BTU/ft² rule givesthe rule overshoots by 27% here | 12,000BTU/hr |
| Summer design temperature | 86°F |
| Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone | 25°F |
| Summer day–night swinga fairly dry climate | 20°F |
| Days at or above 90°F | 6days/yr |
| Oversize from the size stepwithin the normal step | 3% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Federal Way runs 351 cooling degree days, which works out to 14.6 BTU/ft² — so for this bedroom the rule overshoots by 27%. A single national figure has to be wrong somewhere, and this is where.
- Bedroom: the room where oversizing hurts most — quiet, low load, long runtimes wanted.
- 9,000 BTU/hr sits close to the load, which is where a mini split works best. Inverter equipment is happiest running long and slow at part load; it is the short, hard cycles of an oversized head that leave a room cold and damp.
- Federal Way is fairly dry — summer nights stay within 20°F of the day's high, the signature of humid air. A meaningful share of this head's work is removing moisture, and it only removes moisture while it is running. This is the climate where an oversized head produces a room that reads cold and feels clammy.
- Do not forget the winter side. Federal Way designs to 25°F. A cold-climate rated head will hold most of its capacity there, but a standard one will not — ask for the capacity at 25°F in writing.
- Where these numbers come from. Seattle-Tacoma Airport, 14 km from downtown Federal Way, daily records 2015–2024. Design temperatures are calibrated against published ASHRAE values at eight airports and land within about 2°F of them.
The short answer
A 600 square foot bedroom in Federal Way, WA needs a head of about 9,000 BTU per hour. The calculated load is 8,779 BTU/hr.
Where that number comes from
Federal Way, WA runs 351 cooling degree days a year, which puts the room at 14.6 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 12,000 BTU/hr.
The winter half of the question
Federal Way, WA designs to 25°F. A mini split is a heat pump, and its heating capacity falls as the outdoor temperature drops — the nameplate is measured at 47°F, not at the temperature you actually need it.
Sizing a single room is not sizing a house
There is no other room to absorb a mistake. An oversized head short-cycles, never runs long enough to dehumidify, and leaves the room cold and damp.
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Read more
- Mini Split Blowing Cold Air in Heat Mode — Defrost, UsuallyEvery air-source heat pump does this. Knowing what a defrost cycle looks like is the difference between waiting ten minutes and calling somebody out.
- Mini Split Leaking Water Indoors — Four Causes, One CommonA cooling head makes a couple of litres a day. It leaves through a small pipe with no pump behind it, and that pipe is where nearly every leak starts.
- Multi-Zone Mini Split Not Cooling Every Room — The Minimum Output ProblemThe system is not underpowered. It is oversized for the one room that is calling, and multi-zone units handle that far worse than single-zone ones.