Prerequisites for air heating

• A high-performance balanced ventilation system with heat recovery. General prerequisites for balanced mechanical ventilation also apply to ventilation with heating.

• Low heating power demand. The heating available for a room is limited by the volume of air supplied to the room and the allowable maximum supply air temperature. Assuming a nominal air change rate of 0.3 to 0.5 room volumes/h (about 1 m3/m2h) and a maximum supply air temperature after the air heater of about 50°C, the maximum heat delivery to rooms is approximately 10 W/m2 living area. Figure 11.1.2 shows the heating power available for different air flow rates and temperature differences between air supply (T_sup) and return (T_r). This is the basis for the guidelines for Passivhaus design and the requirement of the Swiss Minergie standard.

• A very well-insulated building envelope. This is essential if the limited heating power of the ventilation system is to provide comfort. Surface temperatures of walls and windows must be close to the room air temperature. Delivering heat under a window or at an exterior wall is no longer necessary with such insulation levels and window quality.

• No prolonged and low temperature setbacks. When the room temperature is set back for a prolonged absence (for example, a week), rooms will take a long time to heat back up, given the limited heating power of the ventilation system. When bedrooms are left to cool down at night, the very good insulation of such houses will minimize the temperature drop - for instance, to less than 2K. Reheating to day temperatures can be quickly achieved by the ventilation heating; but the energy benefit of such a small temperature decrease is negligible.

Source: Anne Haas, EMPA 175, Duebendorf, www.empa.ch/abt175

Figure 11.1.2 Temperature difference, flow rate and heat delivery to room

11.1.2 Heating technique

The supply air can be heated by:

Source: Anne Haas, EMPA 175, Duebendorf, www.empa.ch/abt175

Figure 11.1.2 Temperature difference, flow rate and heat delivery to room

11.1.2 Heating technique

The supply air can be heated by:

• hot water coil-supplied heat from a heat storage or a heat source - many different energy sources can be used;

• the condenser of an exhaust air heat pump - in this case, not only the power of the heat delivery, but also the power of heat production, may be limited; and

• direct electric heating - in this case, there is great flexibility concerning the position and subdivision of the air heater.

In single family houses, ventilation and heating equipment are usually in the same room. The air heater is a part of the ventilation unit and is located after the heat exchanger and the fan.

In multi-family houses, it is usually necessary that the room temperature and the air flow rate can be regulated independently and individually by dwelling.

Several configurations of the heating and ventilation equipment are possible:

• individual heating and ventilation systems for each dwelling (as in single family houses);

• an individual air heater connected to a central heating system with an individual ventilation system for each dwelling; and

• an individual air heater connected to a central heating system with a central ventilation system and individual flow rate control for each dwelling.

Individual air heaters for each room are not recommended. If the client demands it, however, direct electric heating is the most common solution. Providing a heat exchanger for each room is expensive and reduces the advantages of combining heating and ventilation.

Solar and ambient energy systems as heat sources perform better when the heat they supply can be at a lower temperature. Air heating requires a relatively high supply temperature. Hot water supplied to an air heat exchanger may need to be 50°C to 60°C to achieve the needed maximum heating power. Domestic hot water heating also requires this temperature range. The amount of energy required for room heating and for DHW is comparable in high-performance houses. Accordingly, if an active solar system meets this demand for supplying water at these temperatures, the collectors should be the high-efficiency type (selective coated absorber with solar glazing or, still better, vacuum tube collectors).

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