Evaluation of the theoretical and in-use performance of Exhaust Air Heat Pumps

Abstract

Integrated units that combine Exhaust Air Heat Pumps (EAHPs) with Heat Recovery Ventilation (HRV) and, in some instances, Domestic Hot Water Storage (DHW), are becoming increasingly popular in the domestic market across Europe with over 24,000 EAHPs purchased in the EU in 2017 alone. Early research into using EAHPs demonstrated energy savings being conservatively between 20% and 50% when compared to conventional systems. Recent research has suggested that, in reality, EAHPs in-use energy performance can be worse than that estimated by various standardised theoretical assessment methods (COP/SPF in the range of 0.4 to 6.0). More worryingly, published data on this in-use operation is effectively non-existent for NZEB type dwellings and few studies have stress tested the robustness of the EN standards in accounting for the effects of in-use operation. The study presented in this paper investigated whether the standard methods used to predict in-use energy performance are sufficiently robust and adequately capture operational performance for EAHP systems. The energy performance of two identical EAHP systems in Ireland (one rural/ one urban) were monitored for close to 12 months. During the live in-use monitoring period, the EAHPs had ‘heat-pump/heat recovery only’ operating mode ratios of 16%/84% and 22%/77% for rural and urban systems respectively. The average HRE in-use efficiency was 92% and 64% for the rural and urban systems respectively. While the manufacturers stated Seasonal performance factors (SPF) ranging from 2.2 (for DHW) to 5.8 (for Space Heating), the average in-use SPF was found to vary between 1.7 and 3.8 depending on the boundary reported and the location. More research is urgently required in order to bring much needed clarity for designers and energy assessors regarding which boundaries can be universally applied to EAHP systems. Given the range of SPF which could apply to the HP’s examined, the paper highlights the importance of ensuring that realistic indicators of in use performance are provided, aiding appropriate decision-making by policymakers, industry and end-users.