Getting the most out of heat pumps and heating

1 A quick introduction to heat pumps
2 How heat pumps work, in brief
3 Why are some heat pumps more efficient?
4 Why do some heating systems work better?
5 Measures of efficiency
6 New breeds of heat pumps
7 Alternative refrigerants
8 Related articles on Designing Buildings

[edit] A quick introduction to heat pumps

Heat pumps are increasingly seen as the most likely alternative to gas heating systems because, depending on their performance rating and conditions, they can produce between 3-5 units of heat for each unit of electricity. Unlike wood stoves and gas boilers, no combustion is involved, and the only emissions associated with the production of heat relate to the manufacture, disposal of the unit at the end of its useful life, and the source of electricity used to run it. However, unlike producing heat through combustion, which by its nature occurs at high temperatures, heat pumps often run at lower temperatures.

[edit] How heat pumps work, in brief

Heat pumps extract heat from a source, such as the surrounding air, waste heat, geothermal energy in the ground, or sources of water, and amplify that heat. This amplification is based on three interrelated gas laws that relate the pressure, volume, and temperature of a gas, which in the case of a heat pump is called a gas refrigerant (though there are also liquid refrigerants). The effective result of these laws is that when the pressure of a gas increases, its temperature also increases. So a heat pump extracts what heat it can from its heat source and exchanges this heat with a refrigerant, which is then partially warmed and compressed to increase the temperature, the heat of which is then extracted for use. The refrigerant then cools and expands through an expansion valve. The cooled refrigerant then performs the same cycle as, and when required, to meet the demand temperature.

[edit] Why are some heat pumps more efficient?

The electrical energy of a heat pump supports the extraction of heat from the source, often with the use of fans or pumps, and the application of pressure to the gas refrigerant to increase its temperature and pump the heat via a medium through the system to where it is needed internally. The efficiencies of the system lie in the number of cycles, which in turn relates to the heat source temperatures, the expected resultant temperatures, and the efficiency of the fans, pumps, and refrigerant.

While the source temperature will vary, the expected output temperature can be controlled by system design, such as larger surface areas of heat emitters and increased efficiency in terms of fabric loss, insulation, airtightness, etc. Likewise, the efficiency of the fans and pumps can be maximised through different controls and designs.

[edit] Why do some heating systems work better?

Heat pump flow temperature is typically set by the Microgeneration Certification Scheme (MCS) at 55 degrees C, and the minimum seasonal coefficient of production (SCOP) is around 2.9. However, running below 50C can be more efficient, and a 45C flow temp can be ideal, which can help achieve a SCOP of up to 4. In order to achieve the right comfort levels when running heating systems at these lower temperatures larger surface areas of the heating emitter is needed along with better fabric performance to keep that heat in the building. For older buildings, this might be around 65-85 W/m2, which in comparison to newer buildings (post 2006) might be around 20-40 W/m2, with Passivhaus much lower at nearer 2-10 W/m2.

A heating system in the floor creates a large emitting surface area, and also places the heat being emitted at the most efficient place at the lowest point in the room (as heat will rise). If the fabric of the building is very good, ie airtight and with high levels of insulation it will have two effects: Firstly the low temperature heat being emitted will stay in the room for longer, secondly the improved airtightness will both reduce the warm air being lost but also reduce any cold flows of air into the space, or cold chills. Furthermore high levels of insulation on areas such as the windows will increase their surface temperature in general. So levels of comfort might be improved because their are effectively fewer radiant cold spots impacting the occupants.

[edit] Measures of efficiency

The efficiency of the refrigerant, being its ability to carry temperature and increase its temperature under pressure and then cool, can vary and can be denoted by its volumetric cooling capacity (VCC). Many different refrigerants have been used over time, and the efficiency of these needs to also be balanced with their environmental impact, which is why the refrigerants used have continually changed.

The terms coefficient of performance (COP) and seasonal coefficient of performance (SCOP) are standardised measurements of the efficiency of the system in heating overall, or in the case of seasonal, in relation to seasonal temperature changes. These are used to describe how well a heat pump has the potential to work. COP and SCOP are denoted by a factor 1:3 for example expressing how many units in heat output can be achieved from the electrical units of input, in this case one unit of electricity produces three units of heat.

The terms energy efficiency ratio (EER) and seasonal energy efficiency ratio (SEER) work on the same principle but are most commonly used to describe efficiencies of systems in cooling, such as air conditioning systems or heat pumps used for cooling in warmer climates.

[edit] New breeds of heat pumps

Today, in general there are two main types of heat pump used; monobloc systems and split systems.

A monobloc heat pump system is made up of a single unit heat pump that is located outside of the home, on a wall or area that has free air and maintenance access. This outdoor unit is draws heat or coolth from the outside air to heat or cool a home. It consists of a compressor, a heat exchanger and the water side of the refrigeration cycle. In this system the entire process happens in the same unit with pre-heating occurring outdoors before moving inside to directly heat the house or with top up electrical heating.

In the newer split systems, a fan and condenser draws heat or coolth from the outside air, collecting and transferring the now pre-heated (or cooled) gas to another unit which is located inside the home. This secondary unit then transfers the heat from the compressed gas refrigerant to the water systems in the house where it is required, such as in a hot water cylinder. The newer split unit systems in general have the advantage of smaller outside units, that run more quietly and are often more efficient, however they are more complex and often more expensive to install than monoblock systems.

In September 2023 the UK government Department for Energy Security and Net Zero, Boiler Upgrade Scheme increased the grants available for UK households to install new systems. Acting on behalf of property owners, installers can apply for between £5,000 and £7,500 of the cost and installation of an air source heat pump, ground source heat pump (including water source heat pumps) or a biomass boiler (the lower amount).

In response to feedback and in relation to the efficiency points that are described above, eligibility for the grant is dependent of the receipt of an Energy Performance Certificate (EPC) and were improved fabric performance is a recommendation, it must be carried out to a degree before the property is eligible to receive a BUS grant. Elements such as heat emitters with larger surface areas to counter balance the often lower temperature outputs of heat pumps are not currently a stipulation in the government scheme but a consideration that lies with the system designer.

[edit] Alternative refrigerants

Natural refrigerants such as propane and ammonia have been used for many years, but as the need to keep food safe and cool became more difficult with ice over longer transport distances, a variety of refrigerants were used. Prior to the 1900s many refrigerants were unsafe and dangerous. By the late 1920s Thomas Midgley Junior developed the first synthetic refrigerant chlorofluorocarbon (CFC) R-12, or Freon, then a safer, non-flammable, and non-toxic refrigerant. It was stable, and could be used under different operating conditions, and could be combined with oil for use in compressors/ Its effect on the ozone layer was not at that time known, and its use was widespread. Iterations such as R-22 (a HCFC) and R-134a, are still found today in applications like refrigerators and vehicle air conditioners.

The range of potential refrigerants is broad such as chlorofluorocarbons, ammonia, hydrocarbons, carbon dioxide, chlorodifluoromethane, tetrafluoroethane, freon, CO2, natural refrigerants, dichlorodifluoromethane and water. As such many are now referred to by a type and with a prefixed R, which was introduced by DuPont which eventually owned the brand Freon. There other classifications that may be used such as by class 1, 2 and 3 denoting the way the refrigerant works by storing latent heat, sensible heat or the ability to carry liquefiable vapours.