What size mini split does a 300 sq ft bedroom in Tri-Cities, WA need?
300 ft² bedroom in Tri-Cities, WA
6,000BTU/hr
5,753 BTU/hr of load at 19.2 BTU/ft² — the figure for 817 cooling degree days
| Head size to buy | 6,000BTU/hr |
|---|---|
| Calculated load300 ft² × 19.2 BTU/ft² | 5,753BTU/hr |
| BTU per square foot hereinterpolated from Tri-Cities's 817 cooling degree days | 19.2BTU/ft² |
| What the 20 BTU/ft² rule givesthe rule overshoots by 4% here | 6,000BTU/hr |
| Summer design temperature | 98°F |
| Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone | 7°F |
| Summer day–night swinga dry climate | 31°F |
| Days at or above 90°F | 40days/yr |
| Oversize from the size stepwithin the normal step | 4% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Tri-Cities runs 817 cooling degree days, which works out to 19.2 BTU/ft² — so for this bedroom the rule overshoots by 4%. 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.
- 6,000 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.
- Tri-Cities is dry — nights fall 31°F below the afternoon high, which only happens when there is little water in the air. Latent load is small, so nearly all of the head's capacity goes into temperature. A slightly undersized head is forgiving here: it simply runs longer.
- Do not forget the winter side. Tri-Cities designs to 7°F. A cold-climate rated head will hold most of its capacity there, but a standard one will not — ask for the capacity at 7°F in writing.
- Where these numbers come from. Pendleton Municipal Airport, 17 km from downtown Tri-Cities, 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 300 square foot bedroom in Tri-Cities, WA needs a head of about 6,000 BTU per hour. The calculated load is 5,753 BTU/hr.
Where that number comes from
Tri-Cities, WA runs 817 cooling degree days a year, which puts the room at 19.2 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 6,000 BTU/hr.
The winter half of the question
Tri-Cities, WA designs to 7°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.