Apparently a heating element is standard with the ventilation system when it is supplied in Germany.
Exhaust air below freezing within the ventilation system is bad news, since it's going to be increasingly humid as the temperature drops and this will form frost when it drops below freezing, and could block the passage of air. This is not going to happen on the incoming air, since the temperature is rising and relative humidity falling, but it will very likely happen on the outgoing air.
Our system avoids this by blowing out more air than it sucks in. This makes the heat exchanger less efficient, so the outgoing air temperature drops less. It also drops the pressure in the house as a whole, and cold air is sucked in at two or three weak points in the airtight membrane. These points are the extractor fan in the kitchen, the gap at the bottom corner of the second and third leaves of our concertina door, and the front door.
In terms of a thermal system, a pressure imbalance is not a bad approach, if it is not used for long periods.
It's better having the house slightly under-pressure in the winter than over-pressure. Firstly, sucking cold air in probably means less heat loss than blowing hot air out, although every time I think about this, my head starts hurting. The cold air coming into the house will make the temperature drop a little, but the heat loss can be calculated by the lower efficiency of the heat exchanger. Secondly, an over-pressure house could lead to humidity building up in the wall structure, as air flows out and condensation occurs somewhere on the falling temperature gradient.
The alternative is heating the air coming into the heat exchanger so that it is closer to freezing and so that the air going out will not drop below the frost-point.
In this case, the temperature in the house will not drop much, and the heat loss can more directly be measured as the energy going into the heating element. This is likely to be electrical, and electrical heating is expensive. The main advantage is that it would stop the genkan area just inside the front door from getting so cold.
