Air-to-air heat pumps: A wolf in sheep’s clothing?

Air-to-air heat pumps are increasingly promoted as a means for energy saving and a future component in a more flexible electricity demand (load management). At the same time, heat pumps potentially contribute to long-term changes in comfort behaviour and practices, which may undermine the energy saving potential. This paper sums up the findings from a Danish research project on air-to-air heat pumps, electricity use and comfort. If used properly, heat pumps can provide high efficient heating of houses. However, a Danish spot test indicates that airto-air heat pumps not always result in energy savings. The use of heat pumps might involve changes in the residents’ thermal comfort practices like higher indoor temperatures in the winter or air-conditioning (cooling) in the summer. The reasons for this might be both technical and behavioural. The paper examines the comfort practices that influence the electricity consumption related to air-to-air heat pumps: How do residents use heat pumps? And what are the consequences for the comfort practices and the electricity use? The analysis is based on results from a survey and qualitative interviews among Danish owners of dwellings and summerhouses with focus on their comfort practices. The study also includes results from metering data on the households’ actual electricity consumption and technical inspections of heat pumps. The paper draws on a practice theoretical approach, which understands energy consumption as an integral part of everyday practices that integrate different elements, including habits and technologies. Introduction In Denmark, air-to-air heat pumps are promoted by energy authorities and in energy saving campaigns as an energy-efficient alternative to direct electric heating (usually convection heaters) in dwellings and summerhouses. Thus, visitors to the website of the Danish Energy Saving Trust (an independent, public sector authority that promotes energy savings) can read that “air-to-air heat pumps are a good and cost-efficient alternative to direct electric heating, especially if you also use your summerhouse outside the summer season” (Danish Energy Saving Trust 2011). There are about 215,000 summerhouses in Denmark (Statistics Denmark 2010), and the majority of these (app. 84 %) have direct electric heating installed while only about one out of ten has an air-to-air heat pump (Kofoed et al. 2010). Furthermore, app. 119,000 dwellings, or 8 % of all single-detached, semi-detached, terraced and farm houses, are heated by direct electric heating, while only 7,700 have a heat pump as their primary heating form (Dansk Energi 2010). Thus, the total potential for substituting electric heating with air-to-air heat pumps is considerable. The Danish Energy Agency estimates that the number of installed air-to-air heat pumps is about 75,000 (Wittrup 2010). Many of these probably supplement other forms of heat supply (e.g. direct electric heating). Typical air-source heat pumps deliver an amount of energy for space heating that is 3 to 4 times the electricity consumed. Thus, replacing direct electric heating with an air-to-air heat pump should ideally reduce the electricity consumption for heating by about two-third. However, as documented by studies of the so-called rebound effect (see review in Sorrell et al. 2009), theoretical energy savings from energy efficiency improvements are in general only partly realized due to increased quantities of consumption or a general increase in consump