Finned heat exchanger
Low-pressure vapour
Cooled air outlet
Hot high-pressure refrigerant vapour
Condenser
Ambient air inlet
Hot water out (flow)
Refrigerant boils at low temperature
Cold low-pressure liquid/vapour
Fan
Refrigerant flow valve causing pressure drop
Figure 1: Internal components of an air source heat pump
Hot water return
Liquid refrigerant Compressor (electric)
ASHPs in particular, had turned out to be. Given the high cost of installing any heat pump system, and the relatively high cash and carbon costs of the electricity they use, doubts began to arise as to whether heat pumps really did offer benefits, either to the consumer, or to the wider environment. To cap it all, there was anecdotal evidence of dissatisfied people experiencing inadequate levels of comfort, and those in the countryside with electricity bills higher than with their previous oil condensing boiler.
What can we realistically achieve? The first thing to note is that, for reasons as discussed below, a heat pump can’t work at ‘optimum’ efficiency when delivering hot (tap) water – so, in systems producing both space heating and tap water, like most of those in the table below (in all three countries), the overall efficiency is bound to be lower than the impressive-sounding COPs cited in some of the adverts for heat pumps. In addition, the EST (UK) results were
not a direct measure of the seasonal performance factor because they added losses from the hot water system to the load, so the actual SPFs (the standard comparison measure) might, in practice, have been a little more favourable. However, perhaps most significantly, the
way many heat pumps have been installed and set up in the UK falls short of the ideal, so it is little wonder that some performances
Heat pump performance Tim Dwyer reports on a government guide that helps
In an attempt to overcome one of the factors contributing to the poor performance of many installed heat pump systems identified in the Energy Saving Trust’s report Getting warmer: a field trial of heat pumps,the Department of Energy and Climate Change (DECC) has published a new document, Heat Emitter Guide for Domestic Heat Pumps. The new guide provides a
relatively simple, paper-based tool to establish – prior to installation – the capability of both existing and new heating systems to use heat pumps that will reduce overall carbon emissions when used for space heating (not including
52 CIBSE Journal October 2011
domestic water heating). To apply the technique to
radiator systems in existing properties (so replacing gas or oil boilers) the actual radiator size and heat output potential is compared with a calculated space heating load to determine an ‘oversize factor’ for the radiators. This oversize factor can then be used to provide an indication of the how successful it would be to use lower temperature heating water supplied by a heat pump.
Oversize factors The oversize factor is given a ‘temperature star rating’. Traditionally, radiators may have been oversized to allow for extra
heat output when the heating systems are switched on and to provide a ‘safety margin’ – or they simply may have been oversized for aesthetic reasons, or even due to poor design practice. Whatever the reason for the
original oversizing, it will now be a positive benefit – as the greater the radiator oversize, the greater the opportunity to increase the operating performance of a heat pump, since larger heat emitter areas will allow the use of lower water flow temperatures. Heat pumps will perform at higher efficiencies at lower heating system water flow temperatures. A one-star rating is given for
systems with an oversize factor of
between 1.6 and 1.9: that is, the radiators are between 30% and 90% larger than they need to be to meet the room heating load, rising to six stars for an overload factor of 6.8 or above.
Temperature rating The temperature star rating is then used directly to indicate the potential for UK installations of ground and air sourced heat pumps. So, for example a one-star heat emitter system is shown to need a flow water temperature of 60C and a likely seasonal performance factor (SPF) of 2.8 for ground-source heat pumps and 2.1 for air-source. SPF is a annual ratio of [Useful
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