Heating Load Calculator (Envelope Transmission + Infiltration)

Heating Load Calculator (Envelope Transmission + Infiltration) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Math & Numbers
Input types: textarea, select, number
Output type: json
Sample coverage: 4
API ready: Yes

Overview

Estimate a room’s heating load from envelope transmission and cold-air infiltration. Enter each surface as an area and U-value pair, provide indoor and outdoor design temperatures, room volume, ACH, and an optional safety factor to calculate the total load in watts.

When to use

Estimate heat loss for a room during preliminary heating-system sizing.
Compare the impact of surface areas, U-values, and air-change rates on heating demand.
Apply a consistent safety factor to a simplified steady-state heat-load estimate.

How it works

1Enter one envelope surface per line using the format area,U-value, such as 50, 0.5.
2Set the area unit, temperature unit, indoor design temperature, and outdoor design temperature.
3Enter room volume and air changes per hour to calculate cold-air infiltration loss.
4The calculator adds transmission and infiltration losses, then applies the optional safety factor and returns the result as JSON.

Use cases

Preliminary heating-load estimates for residential rooms and conditioned spaces.
Building-envelope comparisons using different U-values or surface areas.
Checking how infiltration assumptions affect total heat-loss estimates.

Examples

1. Room with mixed envelope surfaces and infiltration

Building designer

Background

A designer wants a preliminary heating-load estimate for a room with 50 m² of envelope at U=0.5 and 10 m² at U=1.8.

Problem

The room must be evaluated at 20°C indoors and -5°C outdoors, including infiltration and a 1.1 safety factor.

How to use

Enter the two surfaces, select square meters and Celsius, then enter a room volume of 120 m³, ACH of 0.5, and a safety factor of 1.1.

surfaces: 50, 0.5
10, 1.8; areaUnit: m2; indoorTemp: 20; outdoorTemp: -5; tempUnit: °C; volume: 120; ach: 0.5; safetyFactor: 1.1; decimalPlaces: 2

Outcome

The calculator finds a 25 K temperature difference, 1075 W of transmission loss, 27 W of infiltration loss, and a final estimated load of 1212.2 W after the safety factor.

2. Low-loss room with no infiltration allowance

Energy analyst

Background

An analyst is checking the transmission loss through a single 100 m² surface with a U-value of 0.3.

Problem

The estimate uses 22°C indoors, 0°C outdoors, zero ACH, and no safety margin.

How to use

FAQ

What does the heating load calculator measure?

It estimates room heat loss from envelope transmission and cold-air infiltration using a simplified steady-state method.

How should I enter envelope surfaces?

Enter one surface per line as area,U-value, for example 50, 0.5. The U-value is in W/(m²·K).

What is the temperature difference used in the calculation?

The calculator uses indoor design temperature minus outdoor design temperature. For heating, this difference should be greater than zero.

What does ACH mean?

ACH means air changes per hour. It represents the cold-air infiltration or ventilation rate used with room volume.

What is the safety factor?

It multiplies the calculated total load to add a design margin. The default is 1.0, which adds no margin.

surfaces: 100, 0.3; areaUnit: m2; indoorTemp: 22; outdoorTemp: 0; tempUnit: °C; volume: 200; ach: 0; safetyFactor: 1; decimalPlaces: 2

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Room with mixed envelope surfaces and infiltration

2. Low-loss room with no infiltration allowance

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