# Heating Load Calculator (Envelope Transmission + Infiltration)

Estimate the heating (heat-loss) load of a room by the simplified steady-state method. Envelope transmission loss Q_trans = Σ(A_i·U_i·ΔT) over surfaces entered one per line as 'area,U' (U in W/(m²·K)). Cold-air infiltration loss Q_inf = 0.018·ACH·V·ΔT (W), where 0.018 W·h/(m³·K) ≈ ρ·c_p/3600. Total load = Q_trans + Q_inf, optionally multiplied by a safety factor (default 1.0). ΔT = indoor - outdoor (heating, >0). Temperature in °C/K/°F (only differences matter), area in m²/ft².

> Canonical page: https://elysiatools.com/en/tools/heating-load-calculator

- **Category:** Math & Numbers

- **Keywords:** heating load, heat loss, U-value, thermal transmittance, infiltration, cold air, ACH, design temp, safety factor

## 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.

## Inputs

- **Envelope Surfaces (area, U-value per line)** (textarea): One envelope surface per line as 'area,U-value'. U in W/(m²·K). Area in the selected Area Unit. Example line: 50, 0.5
- **Area Unit** (select)
- **Indoor Design Temp** (number): Indoor design (comfort) temperature, in the selected Temperature Unit.
- **Outdoor Design Temp** (number): Outdoor design (winter) temperature, in the selected Temperature Unit.
- **Temperature Unit** (select)
- **Room Volume V** (number): Room (conditioned space) volume V in m³.
- **Air Changes per Hour (ACH)** (number): Air-change rate ACH (1/h), cold-air infiltration/ventilation. Default 0.5.
- **Safety Factor** (number): Safety / pick-up factor applied to the total load. Default 1.0 (no margin); typical 1.1–1.2.
- **Decimal Places** (number)

## 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

- Enter one envelope surface per line using the format area,U-value, such as 50, 0.5.
- Set the area unit, temperature unit, indoor design temperature, and outdoor design temperature.
- Enter room volume and air changes per hour to calculate cold-air infiltration loss.
- The 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.

## Frequently asked questions

### 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.

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