What size mini split does a 400 sq ft bedroom in Boise, ID need?
400 ft² bedroom in Boise, ID
9,000BTU/hr
8,521 BTU/hr of load at 21.3 BTU/ft² — the figure for 1,213 cooling degree days
| Head size to buy | 9,000BTU/hr |
|---|---|
| Calculated load400 ft² × 21.3 BTU/ft² | 8,521BTU/hr |
| BTU per square foot hereinterpolated from Boise's 1,213 cooling degree days | 21.3BTU/ft² |
| What the 20 BTU/ft² rule givesthis room needs 7% more than the rule says | 8,000BTU/hr |
| Summer design temperature | 99°F |
| Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone | 10°F |
| Summer day–night swinga dry climate | 29°F |
| Days at or above 90°F | 56days/yr |
| Oversize from the size stepwithin the normal step | 6% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Boise runs 1,213 cooling degree days, which works out to 21.3 BTU/ft² — so for this bedroom the rule is 7% short. 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.
- Boise is dry — nights fall 29°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. Boise designs to 10°F. A cold-climate rated head will hold most of its capacity there, but a standard one will not — ask for the capacity at 10°F in writing.
- Where these numbers come from. Boise Air Terminal, 6 km from downtown Boise, 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 bedroom in Boise, ID needs a head of about 9,000 BTU per hour. The calculated load is 8,521 BTU/hr.
Where that number comes from
Boise, ID runs 1,213 cooling degree days a year, which puts the room at 21.3 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 8,000 BTU/hr.
The winter half of the question
Boise, ID designs to 10°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.