Heating Load

Heating load describes the required size of a heating system for providing comfortable indoor temperatures even on the coldest design day of the year. As this is sometimes confused with heating demand, one way of differentiating the two with an analogy might be thinking of the heating load as the power of a motor in horse power or kW, and heating demand as the energy you need to feed this motor for it to do what you want it to do, e.g. taking you from A to B. In PHPP the heating load is assessed for two weather variations: a cold, clear day and a moderate, overcast day. On a cold, clear winter day you can expect temperatures to be in the lowest range, as nothing (no cloud cover) stops the earth's surface from radiating back heat to the sky. On the plus side, solar radiation will be higher than on an overcast day. Whatever scenario turns out to result in the highest heating load will be applied for further considerations. For a Passive House, the heating load should not exceed 10W/m2 treated floor area. Example: with a treated floor area of 100 m2, the heating load should be no more than 1,000 W or 1 kW. How is the maximum heating load explained? By multiplying the specific fresh air requirement (30W/person) with the specific heat capacity of air at 20℃ and the difference of maximum supply air temperature (at about 52℃) and the minimum temperature at which air leaves the heat exchanger (16.5℃). The approximate result of this calculation is 350W/person.

\frac{30m^3}{h*person} * \frac{0.33Wh}{m^3*K} * (52-16.5)K \approx \frac{350W}{person}

Divided by the average space requirement per person (assumed  as 35m²) a maximum heating load of 10W/m² results:

\frac{350W}{person} * \frac{person}{35m^2} = \frac{10W}{m^2}

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