Room air flow characteristics

When heating is achieved with the ventilation system, the temperature of the supply air will vary widely over the heating season. The maximum heat air temperatures should not exceed 50°C at the supply air diffuser. At the other extreme, the supply air temperature can be below room temperature during sunny hours in winter, when the heat demand can be completely covered by solar and internal gains. Even so, the room temperature can easily rise - for example, to 24°C - and exceed the set-point temperature. In this case, the air heater will shut off and the temperature of the supply air may drop to as low as 16°C (Ta = -10°C; Tr = 25°C; nHE = 0.75) Thus, at the lower end, the supply air temperature difference between room air and supply air can be up to about -8K. Figure 11.1.4 presents simulation results for a high-performance house in Davos and Zurich. Note that heat exchangers with low efficiencies should not be used.

-4 0 4 8 12 16 20 24 28 32 >32 <-8 -4 0 4 3 12 16 20 24 23 32 >32

Temperature difference (K) Temperature difference [K>

Source: Dorer and Haas (2003)

Figure 11.1.4 Cumulative frequency of temperature differences between supply air and living room in a high-performance house for the period of October-March: (left) for the cold and sunny climate (Davos); (right) for the moderate climate (Zurich)

-4 0 4 8 12 16 20 24 28 32 >32 <-8 -4 0 4 3 12 16 20 24 23 32 >32

Temperature difference (K) Temperature difference [K>

Source: Dorer and Haas (2003)

Figure 11.1.4 Cumulative frequency of temperature differences between supply air and living room in a high-performance house for the period of October-March: (left) for the cold and sunny climate (Davos); (right) for the moderate climate (Zurich)

Since the difference between supply air temperature and room air temperature varies considerably, air movement within the room can vary greatly. Experience with existing houses and laboratory measurements indicate that thermal comfort and ventilation efficiency are well within acceptable limits. An overview is given in the following section. For strategy B, the critical point is not the air flow characteristic produced by the ventilation/heating system. Rather, the radiation asymmetry and the risk of cold air down draught at the windows and external walls are the limiting factors for air heating.

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