HVAC & Energy
Heat Loss Calculator
Build a transparent heating-loss estimate one envelope surface at a time. Enter whole-assembly U- or R-values, design temperatures and conditioned volume, then see transmission and sensible outdoor-air loss separately.
Change any input to update the calculation. Unit changes preserve the physical quantity.
Inputs
Use infiltration plus net ventilation after any verified heat recovery. Recirculated air does not count as outdoor air.
ACHDry-air planning scenario near room temperature; latent moisture load is excluded.
J/(kg·K)Envelope surfaces · net areas excluding openings
1 / 100 rows · each row keeps its own units
Calculated locally in your browser. No sign-up or input upload.
Results
Estimated total heating loss
0.853 kW2,910.557 BTU/hEnvelope transmission loss
0.35 kW1,194.25 BTU/hSensible outdoor-air loss
0.503 kW1,716.307 BTU/hEnvelope conductance
17.5 W/KIndoor–outdoor temperature difference
20 KOutdoor airflow
20.833 L/s44.143 CFMCalculation breakdown
- Exterior wall
- 50 m² × 0.35 W/(m²·K) × 20 K = 350 W
- Outdoor-air sensible loss
- 150 m³ × 0.5 ACH / 3600 × ρ × cp × ΔT
Assumptions & checks
This steady sensible-loss estimate is not Manual J, an energy simulation or a certified heating design. Ground losses, thermal bridges, wind effects and dynamic gains need application-specific treatment.
U-values, R-values and ACH are editable project inputs. Example values are scenarios, not universal material ratings or code-compliance presets.
Formula
Steady sensible heat balance. Whole-assembly thermal properties include relevant layers and surface effects; nominal insulation R alone is not a complete wall or roof assembly rating.
Worked example
Using the default values shown in the calculator, the same formula gives the following result. This is a quick sanity check for the calculation, not a design recommendation.
Inputs
- Indoor design temperature
- 20 °C
- Outdoor design temperature
- 0 °C
- Conditioned room / building volume
- 150 m³
- Effective outdoor-air changes per hour
- 0.5 ACH
- Assumed air density
- 1.2 kg/m³
- Air specific heat capacity
- 1006 J/(kg·K)
- Row 1 · Surface / assembly name
- Exterior wall
- Row 1 · Net surface area
- 50 m²
- Row 1 · Whole-assembly U-value
- 0.35 W/(m²·K)
Result
- Estimated total heating loss
- 0.853 kW
- Envelope transmission loss
- 0.35 kW
- Sensible outdoor-air loss
- 0.503 kW
- Envelope conductance
- 17.5 W/K
- Indoor–outdoor temperature difference
- 20 K
- Outdoor airflow
- 20.833 L/s
Related calculators
Continue your workflow with a related calculation.
HVAC & Energy
Mini-Split Size Calculator
Screen mini-split cooling capacity from room area and stated adjustments, or compare known heating and cooling loads with nominal BTU sizes.
HVAC & Energy
Heat Pump Running Cost Calculator
Estimate heat-pump kWh, annual and average monthly running cost from heat demand or runtime, editable COP/SCOP, tariff and auxiliary electricity.
HVAC & Energy
Blown-In Insulation Calculator
Calculate blown-in insulation bags, installed and settled depth, coverage and cost from a manufacturer label, with metric and imperial measurements.
HVAC & Energy
Duct Size Calculator
Size round or rectangular ducts by airflow, velocity or straight-duct friction rate; reverse known dimensions to CFM with editable air properties.
Frequently asked questions
Should window area be included in the wall row?
No. Subtract openings from wall area and enter windows and doors separately with their own whole-assembly thermal properties.
Why does ACH matter?
It sets the outdoor air volume that must be heated each hour. Leakage, ventilation and heat recovery change that value; recirculation alone does not create the modeled outdoor-air loss.
Does insulation R equal whole-wall R?
Usually not. Framing, layers, films and thermal bridges affect the assembly. Enter a verified whole-assembly value or document your approximation.
Reference data & method sources
- NIST: Technical guidelines for energy consumption
Steady transmission and sensible infiltration heat balance. Inputs must represent the whole assembly and actual outdoor-air rate; example U-values are user scenarios, not construction standards.