Passive houses are able to reduce heat loss to the minimum, they do not need a large installation to achieve thermal comfort, even in the coldest climates. A Passive House is heated through the low-volume heat recovery ventilation system, rather than by a conventional hydronic or high-volume forced-air heating system.
Why don’t you have to heat in winter?
The sun shines through the windows every now and then in winter, and each person delivers 80 watts of heating power. In addition there is the heat emitted by electrical appliances, appliances for lighting and cooking emit plenty of heat. The reason we have to heat conventional buildings is because a lot of heat is lost from window ventilation and walls.
The solution for a home without a heating system is in theory very simple. The heat losses must be reduced to such a degree that less heat flows out of it than the heat supplied to it by sunlight and internal sources (people, stoves and appliances) free of charge. Every polar bear demonstrates that it is possible to live well with sufficient thermal insulation even in the arctic. In principle, buildings can be so well insulated that the solar radiation and the internal heat sources suffice for heating. An overall cost analysis shows, that it is cheaper to insulate a little more and heat less.
When a house needs to be heated costs occur for the heat generator, the heating surfaces and the piping. The key of the Passive House concept is to save the costs of heating surfaces and their piping by using the already present fresh air supply for heating.
The ventilation system required for this is already available in an energy-efficient house, to avoid the high window ventilation heat losses with the use of a heat-recovery system. In the passive house, the insulation thickness is now chosen so strongly that the necessary amount of required fresh air, for hygienic reasons, suffices to transport the needed heat to the rooms. The heating of the air is made possible via a small heating coil installed in the ventilation system, radiators are only required in the bathroom. For a fresh air heating system to be able to heat a building without having to circulate the air, the heating requirement must not exceed 15 kilowatt hours per square meter of living space per a year. This corresponds to the energy content of 1.5 liters of heating oil per square meter. The required heating power is then 10 watts per square meter. This means that a 150 sqm single family home does not need more than the equivalent of 225 liters of heating oil for space heating and a heating output of 1.5 kilowatts per year. This corresponds to the performance of an electric fan heater.
If a building requires more than 15 kilowatt hours of heating per square meter, the required amount of fresh air for heat transport is insufficient. Additional heating surfaces must then be installed and piped. The additional costs for that affect in connection with the higher heating bills, the building, making it ultimately more expensive than a passive house.
How does the passive house behave in winter?
- Due to low heat losses, passive houses only have to be actively heated in cold months November, December, January and February. If someone wishes to open the windows for a quick airing despite the plentiful fresh air supply through the ventilation, he can do that, but it is unnecessary, leads to dry air and increases heating costs.
- With properly installed insulation, unheated rooms are almost as warm as heated ones. This means that unheated bedrooms have pleasant temperatures even in the winter. This allows a year-round very comfortable sleeping with light bedding. In the winter if you want to sleep colder than in the summer, you can tilt the bedroom window at night. Thereby the air will not get fresher, but colder.
- Cold winter air contains very little moisture. The air in the house therefore gets drier the more air is released. This effect occurs regardless of whether it is ventilated via the windows or via a ventilation system, since the same external air is always supplied back in.
- In homes with little moisture sources, for example with few residents, few plants and occasional cooking the hygienically recommended fresh air supply can on cold winter days lead to unpleasantly dry feeling air. In these rare cases, higher humidity can be achieved by introducing additional moisture sources such as plants, laundry drying or humidifiers.
How does the passive house behave in summer?
Saving energy via the ventilation system is only possible in winter. From spring to autumn you can ventilate by opening the windows, just like in normal buildings. In summer, all insulated buildings behave the same. The insulation protects against external heat, but has also the effect that when the heat gets inside, for example by open or not shaded windows, it remains in the building, unless it is ventilated away at night. This effect occurs in every insulated building, not just in Passive Houses.
The Passive House ventilation is not powerful enough to ventilate away this heat. Heat that has penetrated inside can only be vented out through the windows. This works very well when two to three windows are not only tilted but fully open throughout the night. The long airing through the night stores the coolness inside the building. Then, during a hot day, if the windows are kept closed, which is easily done in a Passive House because of the ventilation system, and targeted windows shaded, the building remains pleasantly cool during the day. The building offers comfortable living occupied even on very hot summer days.