Heat Loss Calculator Formula

Understand the math behind the heat loss calculator. Each variable explained with a worked example.

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Formulas Used

Conductive Loss

conductive_loss = r_value > 0 ? envelope_area / r_value * delta_t : 0

Total Heat Loss (with infiltration)

total_loss = r_value > 0 ? envelope_area / r_value * delta_t * infiltration_factor : 0

Estimated Furnace Size

furnace_size = r_value > 0 ? ceil(envelope_area / r_value * delta_t * infiltration_factor / 10000) * 10000 : 0

Variables

VariableDescriptionDefault
envelope_areaEnvelope Area (walls + ceiling)(sq ft)2500
r_valueAverage R-Value13
delta_tIndoor-Outdoor Temp Difference(degF)50
infiltration_factorInfiltration Factor1.2

How It Works

Building Heat Loss

Q = (A / R) x DeltaT x Infiltration Factor

A is the total building envelope area through which heat escapes, R is the insulation R-value, and DeltaT is the temperature difference between inside and outside. The infiltration factor (1.1 to 1.5) adds losses from air leakage.

Worked Example

2,500 sq ft envelope, R-13, 50 degF difference, 1.2 infiltration.

envelope_area = 2500r_value = 13delta_t = 50infiltration_factor = 1.2
  1. 01Conductive = 2500 / 13 x 50 = 9,615 BTU/hr
  2. 02With infiltration = 9,615 x 1.2 = 11,538 BTU/hr
  3. 03Furnace size = round up to 20,000 BTU/hr

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