How does a heat pump cool?

Heat pumps can also be used to cool living spaces if the necessary technical requirements are met. Whether air/water, brine/water or water/water heat pump – unlike classic air-conditioning systems, the heat pump cools via the water pipes of the underfloor or wall heating without any unpleasant draught. A distinction is made between two modes of operation, active cooling and passive cooling. The principle is the same for both cooling variants: Instead of extracting heat from the environment and releasing it to the building to be heated, as is done in heating mode, the excess room heat is absorbed in summer and dissipated to the outside.

More about the way the heat pump works and its principle

Kermi x-change dynamic heat pumps

Passive cooling with a heat pump

Passive (natural) cooling is purely based on the principle of heat transfer. Thermal energy is extracted from the building via the respective heating circuits and fed directly to the primary circuit of the heat pump with an additional heat exchanger. The heat energy is transferred to the ground or groundwater via the primary circuit. During passive cooling, the heat pump is not operating – remains switched off “passive”. Underfloor heating or wall heating is also suitable as a cooling surface in this case.

Kermi x-change brine/water heat pumps

Active cooling with a heat pump

The heat pump needs to be reversible for active cooling. This means: It must be possible to reverse the cooling circuit process . Because for active cooling, it runs in the opposite direction. When cooling, the water circulating in the respective heating circuits or separate cooling circuits extracts thermal energy from the building and is fed directly to the heat pump In the heat pump, the reversed cooling circuit process  ensures that the heat in the room is dissipated to the outside air. The heat pump remains “active”. Underfloor heating or wall heating is suitable as a cooling surface for  active cooling.

Kermi heat pump and storage

Difference between active and passive cooling

Passive cooling

Active cooling

With passive cooling, the water in the heat distribution system is cooled exclusively by heat exchange with the ground. Cold water flows through the heating or cooling circuits at about 18 degrees Celsius and extracts the heat from the living space.

With active cooling, the operating principle of the heat pump is reversed. So on hot days, the heat pump absorbs room heat and cools it down via the compressor. The building becomes a “refrigerator”, so to speak.

The heat is dissipated via an additional heat exchanger so that the excess heat is conducted into the ground. The heat pump is not actively working while this happens. Only the heat source and the heating system pump are in operation and ensure circulation and the necessary heat exchange.

The heat pump is actively working. The coolant cycle of evaporation, compression, and condensation runs in the opposite direction to heating mode: The original evaporator becomes the condenser and the condenser becomes the evaporator.


If  cooling is realised via underfloor or wall heating, the temperature should not fall below the dew point temperature. Otherwise moisture could settle on or in the floor and cause damage. Safety is provided by special sensors that reduce the cooling capacity if the temperature falls below the dew point (depending on temperature and relative humidity).


General requirements

General requirements for cooling with the heat pump


An important component of a building with cooling is the reasonable shading of the window surfaces. In order for the effect of cooling to be noticeable in the building, the solar gains from the sun must be reduced to a minimum in the cooling case, otherwise more heat energy is brought into the building through the windows on warm summer days than cooling energy can provide.

Cooling in the room

A suitable cooling system must be installed in the room so that the cold water supplied by the heat pump can also be used. The simplest system is to convert the installed underfloor heating into underfloor cooling with the help of suitable room controllers and dew point monitoring. However, the effect of underfloor cooling in the room is highly dependent on the installation distance between the underfloor heating elements, the nature of the floor (e.g. tiled or wooden floor) and also the construction of the building. Depending on these factors, floor cooling can only have a limited effect on the room temperature. Furthermore, there may possibly be a loss of comfort due to a “cold floor”. Possible other or additional cooling systems such as ceiling cooling elements or systems with fan convectors in various designs can increase the cooling capacity in the room many times over in most cases.

Dew point

A very important point when cooling a building is the dew point. Depending on the ambient temperature, flow temperature in the cooling system, humidity, and surface properties, undesirable moisture and water formation can occur when cooling in buildings by falling below the dew point. This must be avoided at all costs, as major damage to the building and the system can occur if unwanted moisture and water form. When using the cooling function, all piping and fittings must be made of corrosion-resistant material. Furthermore, the relevant pipes and buffer tanks / hydraulic switches within the building must be insulated to be vapour diffusion-tight.

Depending on the cooling system, either dew point monitoring must be ensured and, if the temperature falls below the dew point before moisture accumulates, the cooling of the system in question must be deactivated, and/or the condensate accumulating must be discharged in a controlled manner (e.g. with fan convectors).

Dew point monitoring in an underfloor heating and underfloor cooling system

A dew point monitor is recommended directly on the storage tank, after the respective heating/cooling circuit pump. In addition, a dew point monitor must be installed and connected in each underfloor heating manifold.

Switching from heating to cooling – Change/Over signal (C/O)

So that all the heating/cooling technology in a building knows whether the system is in heating or cooling mode, there is the so-called Change/Over signal (C/O). A higher-level controller decides, usually on the basis of the outside temperature, whether heating or cooling is required; this is normally done by the heat pump. To do this, a contact is closed or opened when switching from heating to cooling. This contact informs the other participants in the system (e.g. the underfloor heating/cooling) that cooling is now taking place.

Planning and design of a cooling system

To ensure that the cooling works, we also recommend designing each cooling system using a cooling load calculation. The general cooling load, the required cooling capacity of the systems in the room, and all other required data must be calculated and planned. Don't forget things like solar gains from solar radiation, internal gains from electrical appliances or people, air exchange through manual or controlled ventilation of the building, etc. should be included in the calculation.