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.