Reversing air-source heat pumps – Noise at defrost initiation and a noise reducing strategy

• A heat pump generates noise when it shifts to defrost mode.
• The noise level and character is dependent on the pressure differences in the system.
• The noise can be reduced by introduction of an idling time before the defrost.
• The noise reduction strategy causes small effect on the energy performance.

With the increasing use of air source heat pumps, noise disturbance can be a barrier for further market growth and acceptance. Both steady state noise level and noise events influence reported noise disturbance. In this study one of the transient noise events was investigated: the noise initiated when the heat pump shifts to defrost mode. The results show that noise from a heat pump at defrost initiation was strongly dependent on the pressure differences in the system at the time of the shift. A reduced pressure difference resulted in a lower noise level. A control strategy that adds an idling time for the heat pump just before the shift of the 4-way valve is therefore suggested. This will have a small negative effect (<3%) on the heat capacity of the heat pump but the effect upon the COP will be negligible.