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Heating and cooling in a passive house

Heating a passive building

In a passive house it is not necessary to install radiators under windows. In general, the heat distribution through the air supply is sufficient. However, to continue to meet the Passive House criteria, the heating load must not exceed 10 W/m².

The heat provided by internal gains (human heat, cooking, domestic appliances) and solar energy must compensate for heat losses through transmission, infiltration and ventilation.

As long as the criteria of the Passive standard are met, the choice of heating system is free.

If the heat recovery is not sufficient to heat the dwelling (very cold days), it is possible to add a small heating source connected to the ventilation system. This can be supplied by a heat pump, a boiler or a compact unit.

Cooling my passive house

In some climates, the main problem is to keep the house cool in summer. The first rule to prevent excess heat is to limit solar gains as much as possible.

The first step should be to favor eaves, shading devices and cold colors of the envelope elements. The latter reflect the sun’s radiation and cool down the building by preventing heat from entering. 

If the nights are cool enough (ΔT > 5 Kelvin), opening the windows may be enough to cool the home.

However, if passive techniques are not sufficient,  active technique (air conditioning) may be considered. The active method is only necessary in countries where the air temperature regularly exceeds 25ºC, the humidity is around 12g of water per kg of air, and the soil temperature is around 27ºC.

Keeping a fresh interior in Portugal

The major challenge for passive construction in Portugal is to maintain an indoor temperature low enough not to exceed 25ºC for more than 37 days per year (10%).

Before considering active cooling techniques, the following solutions can first be implemented:

  • minimize the windows to the west;
  • increase the efficiency of shading devices;
  • take advantage of the surrounding vegetation to reduce air temperature;
  • install a climatic sink

The PHPP (Passive House Planning Package) software, which is a tool to assist in the design of passive houses, will indicate whether, despite the passive solutions mentioned above, active solutions are needed.

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The heat pump

A heat pump transfers heat from the coldest to the warmest medium in order to heat it up.

The heat pump captures calories either in the air, water or soil.

The heat taken from one of the three media mentioned above is sent to the evaporator (1) which transmits it to the heat transfer fluid of the heat pump. This fluid is then transformed into steam at low pressure. The compressor (2) compresses the steam and thus increases its temperature. It is at the condenser (3) that the superheated steam transfers its heat to the water or air circuit of the heating system. The heat transfer fluid, always compressed, becomes liquid again. The expansion valve (4) reduces the pressure of the heat transfer fluid. The temperature of the latter drops sharply, making it ready for a new heat absorption of the environment and the cycle can start again.

There are compact units that couple heat pumps, domestic hot water generation and ventilation. These models are often used in passive houses.

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