Where Does the Noise Come From?
Heat pumps produce noise while working due to their components. The noise level might be quite high, and this factor usually affects the decision when buying such devices. It is important to have an overview of how the system works, what usual noises are and what the government provides for in terms of noise regulation.
Air source heat pumps have four main elements when working: a compressor, a condenser, an expansion valve, and an evaporator. Of these elements, the ones making more noise when running are the compressor and the fan. Fan noises depend on different factors: the fan model and its speed, the airflow, and the pressure flow.
The airflow depends on the heat exchanger since air produces an aerodynamic noise when it passes through it. The noise intensity varies according to design and air velocity. Pressure flow, instead, is related indirectly to the noise since an increase in pressure makes the noise decrease and vice versa.
However, noises can derive also from the impact that the gap between the temperature of the air outside the refrigerator with the one where the refrigerating cycle occurs, creates. This causes the water in the air to become denser and freeze in the heat exchanger. In order to guarantee the correct working of the heat pump, it is necessary to run the compressor in reverse for some time in order to eliminate the frost accumulated. This can produce a disturbing noise that may combine with the fan noises.
Ground source heat pumps produce less noise, as they do not take the heat from the air, and they do not need a fan. Therefore, they can be placed inside and they are more silent. The picture below shows a scale of noises and their level in decibels.
Regulations and Government Provisions
In England, the government, in order to promote the use of micro generations and efficient systems, provided for Permitted Development Rights that do not require the installers to provide a planning application and noise impacts assessments. Some limits and regulations in order to control the noise level were also established. As reported by the UK Government, the noise should not be higher than 45 dB when being 1 meter away from the window of a neighbouring residential property.
The Microgeneration Installer Standard for ASHPs mandates that external fans should be set in a place that would reduce neighbours´disturbance as much as possible, while the noise of internal ones should be damped by using appropriate tools. Other regulations concern vibration and the need to place the heat pumps close to bedrooms or in places that produce vibrations, but appropriate materials should be used to reduce such effects.
Microgeneration Certification System (MCS) is a way to establish a certification of noise and energy efficiency. The micro wind turbine is the most developed part, and all the new units produced undergo an auditive test in order to be recognized. For these products, some labels are produced, and three colours are used to highlight the potential excess in noise emission (marked in red).