Internal gains

ALF calculations and internal heat gains from hot water, household appliances and occupants.

Internal heat gains from hot water, household appliances and lighting and the metabolisms of the occupants contribute to space heating when a house is occupied. Like all other occupant effects, internal heat gains vary greatly in practice.

ALF calculations take the following simplified approach:

  • For hot water, a base internal gain rate of 100W is taken. This mainly takes account of the heat losses of the hot water cylinder and the hot water used in the building for showers, laundry etc. Whether the cylinder is inside or outside of the house influences the amount of gains. Instant gas cylinders have no standing losses while older hot water cylinders can have losses up to three times as high.
  • The heat release of lighting and appliances is linked to the total floor area of the building – bigger houses generally have more lights and appliances.
  • The metabolic heat release of occupants in the building is considered by a contribution linked to the number of occupants. ALF assumes:
    • three occupants for buildings under 120m2;
    • four occupants for buildings 120–180m2;
    • five occupants for buildings >180m2.

Both the length of the heating season and the timing of internal gains in respect to the heating schedule have to be considered when calculating the seasonal metabolic heat release. If, for example, most of the gains occur during the morning hours and the 'evening heating only' schedule is applied, few of the gains would be useful in terms of reducing the required evening heating energy.

In developing the internal gain figures, two time of day periods – day and night – have therefore been taken into account. The assumptions are:

  • Appliance gains are 16W/m2 with 25% available during the day and 5% during the night.
  • Lighting gains are 8.5W/m2 with 15% available during the day and 0% during the night.
  • Per person gains of 75W with 60% availability during the day (7:00-23:00) and 100% at night. (23:00-7:00).

The different timings and contributions of internal gain components were then approximated through an average power and an Internal Gain Factor. The Internal Gain Factors therefore reflect the relation of the time of internal gains in respect to the heating schedule and the number of heating months determined by the climate of the location.