Are there any constraints to the installation of heat pumps?

Heat pumps offer good energy performance, but their implementation comes up against several types of constraints: technical (space requirement, noise), environmental (risk of water pollution) and above all economic (cost of the electricity required for their operation).

Heat pumps for ambient heating extract heat from a low-temperature source in the environment and transfer it to a building to be heated [→ Q59]. For this technology to be effective, it must obey a basic condition imposed by the laws of physics: the temperature difference between the cold source (ground, ambient air, lake, etc.) and the heating system must be as small as possible. The greater this temperature difference, the more electricity the heat pump consumes to operate, making it increasingly uneconomical to use.

This constraint naturally works against systems that use outdoor ambient air as a heat source. As the outside temperature is significantly lower in winter than in summer, the performance of this type of heat pump drops sharply in winter, precisely when the heat demand in buildings is greatest.

As a general rule, it is not economical to use heat pumps for heating temperatures above 65°C for individual buildings or above 80°C for remote heating networks. The use of a heat pump is particularly suitable when combined with relatively low-temperature underfloor heating systems (water network at approx. 35°C). However, the installation of underfloor heating can be expensive, especially in existing buildings. This configuration may also be relevant for suitable warm air heating systems with a supply air temperature of up to 40°C (conventional warm air heaters blow air at much higher temperatures).

As far as the installation constraints are concerned, they differ depending on whether the heat extracted by the heat pump comes from the ground, air or water.

If the heat is extracted from the ground, a distinction must be made between the constraints associated with shallow systems and those associated with geothermal probes. Shallow-depth systems, which are designed to recover the heat supplied to the ground by the sun (or that stored by the building itself [→ Q77]), have a collector that consists of a network of horizontal coils buried one metre below the external surface of the building (garden, square). Another approach is to use the structure of the building itself. In this case, the main constraint is the need to install the so-called “energy geostructure” system when the building’s foundations are built. Geothermal probe heat pumps have two vertical tubes that descend to a depth of 50 to 200 metres. Depending on the nature of the subsoil, it may be too difficult (and expensive) to drill at such depths.

For heat pumps that extract heat from a water source (ground water, lake, river), the risk of pollution must be taken into account if the geothermal probes in which the heat-carrying fluid circulates (often a mixture of water and glycol) break. These installations therefore require prior authorisation.

Heat pumps that use the outside ambient air as a heat source are easy to install (especially outside existing buildings) and significantly cheaper than in-floor systems. However, they have two drawbacks: noise and space. Despite considerable technical progress in recent years to make them quieter, the fans generate noise that can disturb the neighbourhood, especially at night. In addition, in indoor installation, these heat pumps require large-diameter air supply pipes, which makes them relatively bulky.