Calculation guide for temperature reduction factors to temperature zone "X" in the PHPP "Areas" worksheet
1. Zones that are heated to a lower temperature than the main building:
For temperatures above 10 °C these spaces should be included in the energy balance. Please consult your certifier for detailed instructions.
For lower temperatures, e.g. the rooms are only kept frost-free, a reduction factor can be calculated using the MS Excel tool provided by PHI. The tool uses a conservative approach and is only recommended for buildings, for which the influence on the total specific heating demand is small or moderate.
2. Unheated buffer zones:
In most cases the reduction factor will be close to 1. Thus we recommend ignoring the buffer zones and entering all envelope areas as adjacent to ambient.
If a more accurate calculation is desired, the reduction factor can be calculated from the relation of the conductance of the area between the two zones and the conductance of the exterior envelope of the buffer zone. Ventilation heat losses should also be considered, if relevant, thus it may be easiest to determine the conductance of the buffer zone by setting up an extra PHPP.
For unheated staircases there are standard values in the PHPP manual.
For unheated conservatories/winter gardens/solariums, a calculation procedure is described in EN ISO 13789.
For unheated garages attached to buildings, investigations by PHI have shown that the reduction factor is very close to 1. The areas should be considered in PHPP as adjacent to ambient air.
3. Zones heated only occasionally, but then with 20 °C :
If heated only sporadically, e.g. once a month: ignore heating and calculate as in 2)
If heated regularly, e.g. once a week: In this case it may already make sense to include it in the thermal envelope. If not, a reduction factor of e.g. 6/7 can be used (for 1 day per week). This would be conservative as it ignores the fact that it takes some time until the space has cooled down again, especially if the partially heated space has good insulation against the ambient air. In order to also take the cool-down period into account, a dynamic simulation would be needed.