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H

Heat

Heat is the least "noble" energy form. According to the second law of thermodynamics, heat cannot be transformed into higher forms of energy - like mechanical or electrical energy - without a loss, whereas the reciprocal process can happen loss free. While heat is a scalar and thus undirected, heat transfer always occurs from warm to cold, and never the other way around, unless work is added. As heat is at the end of a chain of energy transformations, it can be the tip of an iceberg of energy usage. Using "nobler" forms of energy to generate heat, e.g. using electricity to generate space heating, should be minimised, and waste heat from mechanical or electrical processes should be utilised to preserve energy sources.


Heat flow

Heat flow describes heat per unit of time in J/s or W; it is also a measure of power. Heat flow is a scalar, therefore not directed and not affected by changes of the co-ordinate system.

Applied to a building element, however, heat flow turns into a heat flow rate in W/m². It looses is undirectedness, becoming a vector. The heat flow rate is proportional to the temperature difference at both ends of the system, for example at building element surfaces.


Heating Demand

QH. What remains after usable heat gains (QS and QI) have been added to heat losses (QT and QV) in the energy balance of a building.


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}