What size mini split does a 800 sq ft living room in Great Falls, MT need?
800 ft² living room in Great Falls, MT
15,000BTU/hr
13,060 BTU/hr of load at 14.8 BTU/ft² — the figure for 365 cooling degree days
| Head size to buy | 15,000BTU/hr |
|---|---|
| Calculated load800 ft² × 14.8 BTU/ft² × 1.1 for a living room | 13,060BTU/hr |
| BTU per square foot hereinterpolated from Great Falls's 365 cooling degree days | 14.8BTU/ft² |
| What the 20 BTU/ft² rule givesthe rule overshoots by 26% here | 17,600BTU/hr |
| Summer design temperature | 93°F |
| Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone | -19°F |
| Summer day–night swinga dry climate | 32°F |
| Days at or above 90°F | 23days/yr |
| Oversize from the size stepwithin the normal step | 15% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Great Falls runs 365 cooling degree days, which works out to 14.8 BTU/ft² — so for this living room the rule overshoots by 26%. 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.
- 15,000 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.
- Great Falls is dry — nights fall 32°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. Great Falls designs to -19°F. That is cold enough that a standard head will have lost a large part of its rated capacity by the time it is needed most — check the manufacturer's capacity table at -19°F, not the nameplate, and look at hyper-heat rated equipment.
- Where these numbers come from. Great Falls International, 7 km from downtown Great Falls, 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 800 square foot living room in Great Falls, MT needs a head of about 15,000 BTU per hour. The calculated load is 13,060 BTU/hr.
Where that number comes from
Great Falls, MT runs 365 cooling degree days a year, which puts the room at 14.8 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 17,600 BTU/hr.
The winter half of the question
Great Falls, MT designs to -19°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.