Hydronic heating load

Paradise garage, Whitefish Bay

New Paradise, Michigan garage in IECC / Michigan Energy Code Zone 7. The north wall looks at the lake. Design outdoor air is -12°F from Sault Ste. Marie WSO (ASHRAE 99.6%). Infiltration is set for a tight new build. Pick underslab foam on Envelope before you pour.

Steady-state design load
41,911BTU/h

Boiler output to spec

60,000 BTU/h

15% pickup for door-open recovery

Intensity

26.2 BTU/h·ft²

1,600 ft² floor · ΔT 72°F

Hydronic flow

4.2 GPM

At 20°F water ΔT · 10°F needs twice that

Where the heat goes
Q = U × A × ΔT for the envelope, F × perimeter × ΔT for the slab, and 0.018 × ACH × volume × ΔT for air leakage.
NORTH · WHITEFISH BAY9×10 doorsservice50′ × 32′12′ walls · 19,200 ft³Uninsulated · leakage 21% of loadSOUTHEW24×36 double-hung on N, S, and E · R-11 doors on the west wall
  • Slab · Uninsulated (heated)15,941 BTU/h

    F = 1.35 Btu/h·ft·°F × 164 ft. Bare slab on grade in Zone 7.

  • Net walls (steel + R-12 + drywall)12,526 BTU/h

    Wood 2x4 @ 16" o.c. bridging included. Steel siding adds almost no R-value.

  • Infiltration (air leakage)8,709 BTU/h

    0.35 air changes/hour through 19,200 ft³. New construction with sealed doors sits near 0.30–0.45 ACH; tired weatherstrip on the lake runs 0.8–1.2.

  • Ceiling (metal roof, R-60)1,895 BTU/h

    R-60 dominates. This is a small slice of the load.

  • Windows (double hung)1,244 BTU/h

    Nominal 24×36 openings. Default U-0.48 is typical vinyl double-pane; aluminum frames run much worse.

  • Insulated garage doors1,178 BTU/h

    R-11 panels as specified. Perimeter leakage is counted in infiltration, not here.

  • Steel service door418 BTU/h

    Standard insulated steel entry, no sidelites assumed.

Hydronic package
Water-side sizing for a radiant floor system in this building.

Supply water

110–120°F

Load with pickup

48,198 BTU/h

Flow at 10°F ΔT

8.4 GPM

Suggested PEX

6 loops · 1,680 ft

Bare concrete is a strong emitter. At 26.2 BTU/h·ft² you want about 110–120°F mean water temperature with 12" PEX spacing. Put XPS under this pour — PEX in a bare Zone 7 slab is a ground heater.

At 60°F inside for 252 heating days (28°F average outside, 9,316 HDD65), expect about 1,367 gallons of propane ($3,348/yr at the price you entered, 90% efficient).

Vacant freeze-protect
Separate from the workshop load. If this garage sits empty for the winter, this is the heat and fuel to keep it from freezing.

Idle indoor setpoint

°F

Hold 40°F on the design night

30,269 BTU/h

ΔT 52°F vs -12°F outdoor. Not the occupied 41,911 BTU/h workshop load.

Heat for the vacant season

42.2 million BTU

3,024 degree-days at 40°F inside vs 28°F average outdoor.

propane vacant cost

$1,256

512 gallons at the price on System.

Unoccupied cost by fuel

Same vacant BTU, priced on each fuel. The selected System fuel uses the price you entered; the others use typical Chippewa prices so you can compare.

  • propane · selected

    512 gallons at $2.45/gallon

    $1,256

  • fuel oil

    339 gallons at $3.40/gallon

    $1,152

  • natural gas

    469 therms at $1.35/therm

    $634

  • electricity

    12,382 kWh at $0.18/kWh

    $2,229

What will actually bite you
  • This is a new pour in Climate Zone 7. Michigan wants R-10 slab-edge insulation 4 ft down; a heated slab needs another R-5 at the edge. Underslab foam is still cheap while the forms are open.
  • PEX without foam under the whole slab heats the ground. Put XPS under the pour and turn it up the edge before the concrete goes in.
If you spend money once
Same garage, same −12°F design day. Underslab foam is still cheap while the forms are open.

R-10 under the whole slab

2″ XPS under the pour, turned up the edge. Do this before the trucks arrive.

31,638 BTU/h after · saves 10,273 BTU/h (25%)

R-15 underslab (3″ XPS)

The better Zone 7 radiant floor. Still cheap compared to a bigger boiler.

30,457 BTU/h after · saves 11,454 BTU/h (27%)

R-20 underslab (4″ XPS)

Quiet floor, lower water temperature, less heat into Whitefish Bay soil.

29,749 BTU/h after · saves 12,162 BTU/h (29%)

R-15 underslab + PEX

The new-build hydronic package: foam under the pour, radiant in the slab.

30,457 BTU/h after · saves 11,454 BTU/h (27%)