What size mini split does a 800 sq ft bedroom in Derry Village, NH need?
800 ft² bedroom in Derry Village, NH
15,000BTU/hr
14,112 BTU/hr of load at 17.6 BTU/ft² — the figure for 614 cooling degree days
| Head size to buy | 15,000BTU/hr |
|---|---|
| Calculated load800 ft² × 17.6 BTU/ft² | 14,112BTU/hr |
| BTU per square foot hereinterpolated from Derry Village's 614 cooling degree days | 17.6BTU/ft² |
| What the 20 BTU/ft² rule givesthe rule overshoots by 12% here | 16,000BTU/hr |
| Summer design temperature | 90°F |
| Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone | -5°F |
| Summer day–night swinga dry climate | 25°F |
| Days at or above 90°F | 14days/yr |
| Oversize from the size stepwithin the normal step | 6% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Derry Village runs 614 cooling degree days, which works out to 17.6 BTU/ft² — so for this bedroom the rule overshoots by 12%. 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.
- 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.
- Derry Village is dry — nights fall 25°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. Derry Village designs to -5°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 -5°F, not the nameplate, and look at hyper-heat rated equipment.
- Where these numbers come from. Concord, Concord Municipal Airport, 36 km from downtown Derry Village, 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 bedroom in Derry Village, NH needs a head of about 15,000 BTU per hour. The calculated load is 14,112 BTU/hr.
Where that number comes from
Derry Village, NH runs 614 cooling degree days a year, which puts the room at 17.6 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 16,000 BTU/hr.
The winter half of the question
Derry Village, NH designs to -5°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.