Climate and heating
ALF and building location and how the occupants heat the building.
Selecting the location determines:
- the coldness of the climate (ALF-values);
- the length of the heating season (ALF-values);
- the windiness of the location (air leakage zone);
- the climate zone (1, 2 or 3) from Appendix B of NZS 4218:2009 Thermal insulation – Housing and small buildings.
The designer selects:
- the time when the occupants apply heating (the "heating schedule" – evening-only, morning and evening, morning to evening or continuous);
- the temperature that the occupants heat the building to (the "heating level" – 18°C or 20°C).
ALF does not consider heater efficiency or responsiveness. It assumes that the selected heating level is instantly achieved with whatever heating appliances are installed. If the evening-only heating schedule is used, it does not therefore take account of any energy needed before 5:00pm.
Climate and heating background
The thermal performance of a design depends largely on the behaviour and requirements of its occupants. A particular design might have benefits with intermittent heating but disadvantages with 24-hour heating. In particular, the benefits of thermal mass largely depend on the applied heating schedule. Intermittent heating generally favours lightweight buildings while continuous heating favours heavy thermal mass buildings.
Winter is defined from long-term average monthly temperatures. The threshold temperature was decided from a small survey asking subjects which months they used heating, and this data was compared with long-term average monthly temperatures from NIWA. This led to a threshold average monthly temperature of 11.5°C.
ALF therefore defines "winter" as all the months with an average monthly temperature of 11.5°C or less. In theory there are very warm areas that may not have a "winter", but because cold spells can occur anywhere, ALF applies a minimum ALF-value of 10% of the Wellington (Kelburn) ALF-value.
For more information on ALF-values, see information about the ALF-value calculation in the Manual.
Interpolation and extrapolation
Other temperature levels can be interpolated or extrapolated if required.
Example of interpolation:
- Choose the temperature which you are interested in, such as 17.5°C
- Calculate the heating energy for the heating level below (16°C) and the one above (18°C): E16 and E18.
The heating energy for 17.5°C is given by:
Example of extrapolation:
- Choose the temperature that you want to know the heating energy for, such as 22.5°C.
- Calculate the heating energy for the two closest available heating levels (18°C and 20°C): E18 and E20.
- The heating energy for 22.5°C is then given by:
Extrapolating the energy to levels lower than 16°C in warm climates may result in unreliable results because the energy consumption becomes very sensitive to small changes in temperature levels.
The three different heating levels were selected based on monitored internal temperatures in New Zealand houses. New Zealand houses are generally heated to comparatively low temperatures. A winter indoor temperature of 20°C is very uncommon. Temperatures under 18°C, on the other extreme, are not recommended for health reasons.