Transmission heat loss calculator: inputs, result and limits
Which inputs are required?
For every envelope component enter heat-transfer area A, U-value, and a project-specific temperature adjustment factor Fx for adjoining unheated spaces where applicable. Enter length l and psi value for each thermal bridge from a detail calculation or applicable catalogue; add point chi values as one sum. Use one consistent internal or external dimension system throughout.
What is returned?
Results are HT in W/K, area-specific H'T, heat flow at the selected temperature difference, and the share due to linear and point thermal bridges. Individual contributions support plausibility checks.
Method and interpretation
HT = Σ(Fx·Ui·Ai) + Σ(psij·lj) + Σ(chik). Heat flow equals HT·ΔT. A factor of 1 applies to direct transfer to outside air; obtain other factors from the applicable balance method. Negative psi values are permissible with a consistent dimensional convention.
Calculator limits
A steady-state transmission balance, not a complete design heat load. Ventilation and infiltration, ground dynamics, solar and internal gains, and heat-up allowances are excluded. The calculator neither supplies a blanket bridge allowance nor selects psi values.
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Source: ISO 13789:2017 and DIN EN ISO 13789:2018-04; Lohmeyer Practical Building Physics, 10th ed., section 2.4.5, equation (2.27). Psi terminology and the consistent dimensional convention were additionally checked against the locally held DIN EN ISO 14683:2018-03.