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How to size a wine cellar cooling unit

Every sizing mistake has a symptom. Too small and it never reaches 55°F. Too large and it short-cycles, fails to pull humidity down, and dies early. Neither is a warranty claim you will win.

The rating that decides everything

Cooling units are rated for a cellar temperature and an ambient temperature — for example "55°F cellar, 90°F ambient". Those two numbers are the whole specification, because a compressor that removes 3,000 BTU/h at a 20°F temperature difference removes far less at 35°F of difference.

Ambient vs cellar difference	Practical derating
~20°F (winter, mild climate)	Rated capacity
~35°F (55 cellar / 90 ambient)	Rated capacity
~45°F (55 cellar / 100 ambient)	About 15% less
~55°F (55 cellar / 110 ambient, garage in Arizona)	About 30% less

This is why a unit chosen from a brochure figure fails in a hot garage. Use the BTU calculator: it takes your real ambient, insulation and glass and returns the capacity you need at that ambient.

What goes into the heat load

Heat arrives through four routes, and most calculators only count one:

Conduction through walls, ceiling and floor — driven by area, insulation value and the temperature difference.
Glass — a single-pane window gains far more than its area suggests, especially with sun on it.
Internal loads — lights, people, and the small heat each bottle releases while it comes down to cellar temperature.
Door openings — a cellar entered twice a day is a different problem from a tasting room open all evening. Add 10-25% for regular access.

The oversizing trap

A unit that is 50% oversized reaches setpoint quickly, then cycles on and off. Each start pulls locked-rotor current, the compressor wears, and — critically — short run times do not remove enough moisture, so the room settles too dry and your corks shrink over years. Bigger is not safer.

The right target is a unit that runs long, steady cycles. Aim for capacity about 10-20% above your calculated load, not double it.

Placement rules that are not optional

Waste heat goes outside the cellar. Always. Venting into the same room recycles heat and the unit never stops.
Clearances as specified. Usually 6-12 inches of free air around intake and exhaust.
Never into a sealed garage in a hot climate without ventilation — that is how you cook a compressor.
Dedicated circuit. Startup current is several times running current.
Condensate plan. Gravity drain if possible; a condensate pump is one more thing to fail.

If you are between two sizes

Change a variable rather than buying bigger: increase ceiling insulation to R-30, add an insulated door, or shade exposed glass. Any one of those often removes more load than the next size up in unit. Then run the calculator again — the difference in unit price can be several hundred dollars, and the running cost difference lasts the life of the cellar.