BH-001: The 4× problem

Search "how many BTU to heat a 2-car garage" and every published answer is a single number or a narrow range, with no envelope attached to it. Run the same 24×24 ft garage through a model that accounts for climate, the ceiling, air leaks and the attic, and the honest answer spans a wide range too — because it genuinely depends on the garage.

What publishers say

Four rules we could verify against a live page, quoted verbatim, each converted to the BTU/h a 24×24 (576 ft²) garage implies:

Two more publishers that carry a garage-heater sizing rule could not be confirmed against a reachable page as of this report's retrieval date, so they're left out rather than guessed at.

Our model's answer: four real envelopes, one garage

None of the rules above ask what the garage is actually made of. Ours does. Same 24×24×9 ft footprint, same Chicago design day, same 55°F target — only the walls, ceiling, door, window and draftiness change:

That's a 0.2 ×CComputed by our engineheatLossDesign() high/low across the four envelopes aboveReport a problem spread from bare-and-leaky to well-insulated-and-tight — on one garage, in one city, computed the same way each time. Every publisher figure above sits somewhere inside that range, because a bare garage and a sealed one are genuinely different heating problems, not the same problem with different opinions attached.

Why the rules disagree

A flat BTU-per-square-foot rule can only be right for one envelope. On our standard as-is example, the ceiling and attic alone account for 34 %CComputed by our engineheatLossDesign(EXAMPLE_A_INPUT).items — ceiling_roof.pctReport a problem of the load, and air leaks another 24 %CComputed by our engineheatLossDesign(EXAMPLE_A_INPUT).items — infiltration.pctReport a problem. A rule that only counts floor area treats a leaky attic and a tight, insulated one as the same garage. Climate matters too: the same envelope in a colder city needs a bigger heater, and no per-square-foot rule carries a design temperature with it.