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BAYHEAT
G-003 · REV 2 · CHECKED 2026-10-09 · MODEL v1.0.2 ▾

Drafted with AI assistance. Numbers computed by model v1.0.2 or sourced. Human review: pending (BayHeat editorial desk). Licensed review: not applicable.

Technical review: no page-specific licensed approval recorded.

M measured · C computed · S spec · R reference · E estimate

Methodology

Every number the planner shows traces back to one of the formulas on this page. There's no sales pressure here — no buy links, no product picks — just the load calculation, the warm-up model, the grade formula and the circuit-sizing rule, worked on BayHeat's standard example: a 24×24 ft attached two-car garage in Chicago, R-13 walls, one uninsulated steel door, average drafts, held at 55°F.

The load calculation: conduction

Walls, garage doors, windows and the service door each lose heat by conduction: assembly U-value (BTU/h per ft² per °F) times area times the indoor/outdoor temperature difference (ΔT). At Chicago's 99%-design temperature and a 55°F target, ΔT is . The garage door alone, at U-1.15 for an uninsulated steel door, accounts for of the design load.

Ceiling
34%
Air leaks
24%
Big door
23%
Slab edge
9%
Walls
8%
Windows
3%
House wall
-3%
Service door
2%
FIG. 1 Where your heat goes, sorted largest first.

Infiltration

Air leaking through gaps around doors, windows and the building envelope follows Q = 0.018 × V × ACH × ΔT, where V is the garage's volume in ft³ and ACH is the design-condition air changes per hour for its tightness class ( tight to very leaky). The constant also carries an altitude correction, since colder, denser air at sea level infiltrates at a different rate than the same volumetric flow at elevation — Chicago's gives a factor of (1.0 at sea level; Denver's 5,414 ft works out to about 0.82). At "average" tightness, this garage's infiltration load comes to .

The ceiling: attic as a series resistance

A vented attic doesn't sit at either the indoor or the outdoor temperature. It floats in between, set by the ceiling's insulation on one side and the roof deck plus attic ventilation on the other. The model treats this as two resistances in series: the ceiling's own UA (insulation) and the attic-to- outdoors UA (roof deck conduction plus of attic air changes per hour), combined as UA_ceiling_eff = 1 / (1/UA_ceiling + 1/UA_attic-out). That combined UA times ΔT gives the ceiling load — for this garage, , the single largest line on the chart above. The same calculation yields an intermediate attic temperature — on this garage's design day — that a flat, non-series model can't produce, and that a blown-in ceiling upgrade changes directly.

Slab-edge loss

Heat leaving through an uninsulated slab doesn't scale with floor area. It scales with the exposed perimeter, because the loss concentrates at the edge where the slab meets outdoor air. The model uses an F-factor (BTU/h per linear ft of edge per °F), not a U-value times area: Q = F × L_ext × ΔT. L_ext is the exterior perimeter, with any common (house) wall excluded. An uninsulated edge runs about F = ; edge insulation (R-10 to R-20, 24-48 in deep) roughly cuts that F-factor in half or better.

Warm-up: thermal capacitance and a 1-minute simulation

"How long until it hits 55°F" is a different question from the design load. It needs a different model: a lightweight thermal-capacitance simulation, not a steady-state formula. The garage's light mass — framing, drywall, the air itself, plus whatever's stored inside — is lumped into one capacitance figure — for this garage — and the slab is modeled separately as a semi-infinite solid that only slowly gives up or absorbs heat at its exposed surface. The simulation steps forward in 1-minute increments (an explicit Euler step), each minute adding the heater's output and subtracting the current heat loss, divided by the capacitance, to get the next minute's temperature. Starting from this garage's free-float temperature on an average January day — — a 10 kW class heater reaches 55°F in about .

FIG. 2 Modeled warm-up, an average January day (71 min to target).

BayGrade: a climate-independent envelope score

BayGrade answers a different question than the design load does: not "how much heat does this garage lose in Chicago," but "how leaky is this envelope, independent of climate." It's the exterior-coupled UA (every term except the house-wall coupling) divided by floor area — UA_ext / A_floor — evaluated at any single ΔT, since UA itself doesn't depend on temperature. This garage comes out to , which lands in grade .

Heat escapesYour garage: DHolds heat
  • Grade A: UA_ext/A_floor ≤ 0.38
  • Grade B: ≤ 0.62
  • Grade C: ≤ 0.90
  • Grade D: ≤ 1.25
  • Grade F: above 1.25

Sizing margin

The recommended capacity (qSize) is the design load (qDesign) times a margin — becomes () — so the heater isn't sized to its exact worst-case load with zero headroom.

Circuit sizing (NEC)

A fixed electric heater is a continuous load under , so the conductors and breaker are sized at 125% of the heater's running current, not the bare nameplate current. A 10 kW class heater at 240V draws about running, at 125%, which rounds up to the next standard breaker: with 4 AWG copper (NM) or 6 AWG copper (THHN in conduit). A cord-and-plug heater on a shared receptacle circuit follows a different rule instead — caps it at 80% of the branch rating. Your electrician and your local code edition govern.

Try it on your garage

Every formula above runs the same way on your own dimensions, insulation and local design temperature — not just the worked example on this page.

Try it on your garage →

Related

Sources

  1. [1] NFPA 70, National Electrical Code, 2023, NFPA. Retrieved 2026-09-25.
Size my garage — 60 s