Sizing methodology

How the BTU estimate is calculated

The calculator is a screening estimate based on public room-sizing guidance. It is not a substitute for a room-by-room Manual J load calculation.

Professional sizing caveat

ACCA Manual J is the residential load-calculation standard to use when the room is large, cold-climate heating matters, windows are unusual, or comfort risk is high.

Open ACCA Manual J source
Model-page notes

How we compute fit notes

Model pages put certified numbers into context without changing them. These notes use certified fields only. We do not substitute manufacturer specifications when a certified input is missing, and we leave a note out when the calculation cannot be made.

Room-size starting point

Inputs: the model's certified heating capacity at 47°F and the public ENERGY STAR room air conditioner sizing table.

We compare the certified 47°F capacity with every BTU row in the ENERGY STAR table and use the row with the smallest absolute BTU difference. The model page then describes that row's square-foot range as a starting point. Models above the table's 34,000 BTU/hr maximum do not get a mapped room range.

This is a rough comparison between a heat pump's certified heating capacity and ENERGY STAR's room air conditioner cooling guidance. It is not a claim that the model will heat or cool a room of that size. Insulation, design temperature, windows, air leakage, and layout still belong in a room-by-room load calculation.

ENERGY STAR room air conditioner sizing guidance ↗

5°F heating-output retention

Inputs: certified heating capacity at 5°F and certified heating capacity at 47°F from the same model record.

We divide the 5°F capacity by the 47°F capacity, multiply by 100, and round to the nearest whole percent. For example, the displayed percentage answers one narrow question: how much of the model's published 47°F heating output remains at 5°F?

The result describes output retention, not efficiency, energy use, operating cost, or the ability to meet a particular building's heat loss. We calculate it only when both certified capacity fields are numeric and the 47°F value is greater than zero.

HSPF2 position within a capacity class

Inputs: each model's certified HSPF2 and certified heating capacity at 47°F.

We round 47°F heating capacity to the nearest 3,000 BTU/hr to form a MiniSplitFinder comparison group. This grouping is our catalog method, not a certification class. Within that group, we take the arithmetic mean of the certified HSPF2 values available in the current catalog. A model is shown as above, below, or equal to that mean, which is displayed to one decimal place.

This comparison is not a percentile or a rank. It changes when the certified catalog changes, and models without a numeric HSPF2 are excluded from the mean.

Constants and source mapping

Cooling table14 rows from ENERGY STAR room sizing guidance, from 5,000 BTU/hr through 34,000 BTU/hr.
https://www.energystar.gov/products/room_air_conditioners
Ceiling heightThe table is treated as an 8-foot-ceiling table. The calculator scales entered floor area by ceiling height divided by 8 before table lookup.
https://www.energystar.gov/products/room_air_conditioners
Sun exposureHeavily shaded: 0.9x. Very sunny: 1.1x.
https://www.energystar.gov/sites/default/files/tools/ES_RoomAC_FactSheet_23.pdf
Kitchen room typeKitchen: add 4,000 BTU/hr. Other room types do not add BTUs.
https://www.energystar.gov/sites/default/files/tools/ES_RoomAC_FactSheet_23.pdf
Climate zone lookupZIP lookup uses representative static ZIP-to-county entries from HUD-USPS crosswalk concepts, then county-to-zone entries from PNNL Building America / IECC climate-zone data. If your ZIP is not in our lookup table, pick your climate zone manually.
https://www.huduser.gov/portal/datasets/usps_crosswalk.html
https://basc.pnnl.gov/guide-determining-climate-zone-county-data-files
Capacity matchingWe match your estimate to models in the same size class.