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

Frequently Asked Questions

What is a typical design temperature difference?

Use the outdoor design temperature for your area (available from ASHRAE data) subtracted from your desired indoor temperature (usually 70 degF). For example, if outdoor design is 5 degF, DeltaT = 65.

Should windows be calculated separately?

For accuracy, yes. Windows have much lower R-values (R-1 for single pane, R-3 for double). Calculate window and wall areas separately and sum the losses.

Learn More

Guide

Understanding HVAC Sizing: How to Calculate Heating and Cooling Loads

Learn how to size HVAC systems using Manual J load calculations. Covers BTU requirements, climate factors, insulation values, ductwork sizing, and equipment selection.

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