What size mini split does a 400 sq ft living room in Hillsboro, OR need?
400 ft² living room in Hillsboro, OR
9,000BTU/hr
8,068 BTU/hr of load at 18.3 BTU/ft² — the figure for 699 cooling degree days
| Head size to buy | 9,000BTU/hr |
|---|---|
| Calculated load400 ft² × 18.3 BTU/ft² × 1.1 for a living room | 8,068BTU/hr |
| BTU per square foot hereinterpolated from Hillsboro's 699 cooling degree days | 18.3BTU/ft² |
| What the 20 BTU/ft² rule givesthe rule overshoots by 8% here | 8,800BTU/hr |
| Summer design temperature | 92°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 | 23°F |
| Days at or above 90°F | 20days/yr |
| Oversize from the size stepwithin the normal step | 12% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Hillsboro runs 699 cooling degree days, which works out to 18.3 BTU/ft² — so for this living room the rule overshoots by 8%. A single national figure has to be wrong somewhere, and this is where.
- Living room: more people, more glazing, more electronics. The 1.1× factor accounts for that.
- 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.
- Hillsboro is dry — nights fall 23°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. Hillsboro 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. Portland International Airport, 31 km from downtown Hillsboro, 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 400 square foot living room in Hillsboro, OR needs a head of about 9,000 BTU per hour. The calculated load is 8,068 BTU/hr.
Where that number comes from
Hillsboro, OR runs 699 cooling degree days a year, which puts the room at 18.3 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 8,800 BTU/hr.
The winter half of the question
Hillsboro, OR 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.