CPD PROGRAMME
Figure 3: MIS 3005 Heat Emitter Guide for Domestic Heat Pumps, showing part of the matrix to establish appropriate heat pump/heat emitter design criteria, based on room heat loss and heating flow water temperature
Guide for domestic heat pumps To size heat pumps for heating systems requires some fairly standard calculations that should be within the capabilities of design engineers. However, to assist in the appropriate application of heat pumps to both new and retrofit applications, a supplementary guide to the MIS 3005 has recently been published. The Heat Emitter Guide for Domestic Heat Pumps attempts to provide a relatively simple (paper- based) tool to establish, prior to installation, the capability of both existing and new heating systems (excluding domestic hot water) to employ heat pumps. The document is particularly useful
when assessing the potential for existing heating systems to be reused when installing heat pumps. It provides a simplified method to assess the suitability of different heating emitter systems, including radiators and fan convectors, in existing properties when replacing gas or oil boilers. Since ‘traditional’ systems were typically designed with mean heating water temperatures of around 76C (far higher than is currently available from heat pumps), the guide determines the opportunities for using lower heating flow temperatures by examining the degree of oversizing in the existing heating distribution system. The heat emitters may have been originally oversized to allow for cold start up, to provide a ‘safety margin’ or simply for aesthetic reasons. And subsequently, as a result of energy saving measures (for example, draught proofing, improved thermal insulation and double glazing) the heating loss in the building may have reduced substantially below the available output of the heating distribution system. The heat loss to potential heat output ratio is termed the oversize factor. Whatever the reason for the system’s
oversizing, it provides an opportunity to increase the operating performance of a heat pump, since larger heat emitter
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areas allow lower water flow temperatures to meet the required heat output. The combination of oversize factor and heating system flow water temperature is used to give a predicted SPF for the retrofitted heat pump installation. For new buildings, the guide provides a
matrix to assist in the selection of heating systems that are likely to operate at an appropriate SPF. This uses a combination of room heat loss, heating emitter types and heating flow temperatures to assess the likely SPF for air (and ground) source heat pumps. It identifies the most appropriate solutions, using a ‘traffic light’ colour coding system, in terms of the required ‘oversize factor’ for radiators and convectors (i.e., the multiplier of their catalogue output at an emitter-room temperature difference of 50K), and ‘pipe spacing’ (PS) for underfloor systems, related to various floor finishes. The matrix is shown in Figure 3 and
can be freely downloaded from www.
microgenerationcertification.org The guide may well assist in improving the application of heat pumps, and provides a quick first feasibility check.
Carbon dioxide heat pumps The commercialisation of small scale heat pumps employing CO2
as a refrigerant
potentially offers some key benefits to the domestic and small commercial marketplace. To act as a refrigerant, CO2 uses far higher pressures than HFC refrigerant, making more demands in the engineering of the underlying system. However, the major benefit is that the systems can be designed so that they can efficiently produce heat at 65C – a temperature that can satisfy the requirements of hot water systems, without needing top up supplementary heating. However, due to the way that CO2 pumps work (known as supercritical
operation), they require a large temperature drop across the load to achieve consistently good COPs. This means that they are particularly effective when heating up low temperature loads (such as incoming cold water supplying a hot water service) but not so effective when heating the returning water from a heating system. Hence, they are likely to show the greatest benefit where a building has a relatively large domestic hot water load compared with the heating load – a trend that is becoming more likely with increasing building fabric performance. © Tim Dwyer 2012
For further reading in this area see: CIBSE Journal, October 2011 pp50-56. The article ‘Hot Prospect’ covers the realities of heat pump application. See: http://
content.yudu.com/A1u3w5/CIBSEoct11/ resources/
50.htm The BSRIA guide Heat Pumps – A guidance document for designers (BG 7/2009) provides an excellent overview of heat pump technology and application. HVCA’s Guide to Good Practice – Heat Pumps (TR/30) covers the practical aspects of installation.
References 1.
www.energysavingtrust.org.uk/Generate-your- own-energy/Financial-incentives/Renewable-Heat- Premium-Payment Accessed 7 December 2011.
2.
www.energysavingtrust.org.uk/Generate-your-own- energy/Air-source-heat-pumps
3. Legionnaires’ disease. The control of legionella bacteria in water systems. Approved Code of Practice and guidance L8, HSE 2007
www.hse.gov.uk/pubns/books/l8.htm
4.
www.planningportal.gov.uk/buildingregulations/ approveddocuments/partl/approved Accessed 3 December 2011.
5.
Getting warmer: a field trial of heat pumps, Energy Saving Trust, 2010.
6. Microgeneration Installation Standard, MIS 3005, Issue 3 2011,
www.microgenerationcertification.org
7.
www.energysavingtrust.org.uk/Generate-your- own-energy/Financial-incentives/Renewable-Heat- Premium-Payment Accessed 2 December 2011.
heat
8. BS EN 12831:2003 Heating systems in buildings. Method for calculation of the design heat load.
January 2012 CIBSE Journal
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