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Key measure of heat loss
The space heating demand, or heat load, of a home is the amount of heat energy needed within the home to maintain the rooms at the desired temperature.
Traditionally, heating demand calculations assumed an internal temperature of 21°C and a fixed external temperature of typically -1°C or -3°C (known as a ‘design day’).
More modern methods, including calculations such as the SAP, take account of variations in external temperature, wind speed, internal temperature rises due to sunshine and electrical appliances, etc.
When used to set targets, space heating demand is usually expressed per square metre of floor area so that it is independent of property size.
It’s a useful target when setting retrofit standards since it is a function of the fabric, infiltration, ventilation and internal gains (sun as solar gains) of the property, independent of the efficiency of the space heating system.
Units: kilowatt-hours per year, kWh/yr, or kilowatt-hours
(Definition is based on one from “Carbon Jargon”, an NHBC Foundation report, researched and written by Neil Cutland of Sava).