Module 79: Integrating gas-fired and electrical heat pumps with heating and hot-water systems

This CPD explores how systems using air source heat pumps are evolving

This CPD will consider the integration of (electrically powered) vapour compression and (gas powered) gas absorption air source heat pumps in non-domestic applications.

This CPD will consider the recently updated standards that determine the requirements for ventilation in teaching spaces; the drivers for applying mechanical ventilation; and how equipment certification can provide essential evidence that mechanical ventilation products are fully fit for use.

With the recent moderation1 in projected natural gas price increases over the next 15 years, a strong economic case for using natural gas – in areas where it is available – for heating and hot water services is likely to continue for the foreseeable future. However, even in existing installations the inclusion of a ‘low carbon’ or ‘renewable’ technology – such as heat pumps – can provide an attractive supplemental, or replacement, heat source.

The financial benefit of these alternative methods is magnified by the tax and grant incentives provided by schemes such as the UK’s Renewable Heat Incentive (RHI)2, which makes significant payments for heat sourced from air source heat pumps that meet appropriate quality standards.

The operational characteristics for heat pumps are such that, as the difference in temperature between the source (the outdoor ‘ambient’ air) and the output (the heated water) gets higher, the COPH – the ratio of useful heat output divided by supplied energy – will reduce. This will lower the output of the heat pump, so, at times of a building’s peak heating requirement, when it is coolest outdoors, the heat pump output will be at its lowest, so may require additional – or alternative – heating from another source, such as a condensing gas boiler

The ‘carbon’ benefit for the inclusion of such technology is not always clear-cut, as it is open to the vagaries of the energy supply (electricity and gas) and the carbon ‘embodied’ in the equipment. So to model true ‘lifecycle’ comparative environmental impact requires local, site-specifi c knowledge, as well as assumptions about the production and transportation of the equipment.

Application of modern air source heat pump (ASHP) technology

Until recently, for a typical northern European cold day, the carbon impact – and energy cost – was likely to favour the use of a natural-gasfuelled condensing boiler over the traditional vapour compression cycle heat pump. However, technology that was formerly only applied to larger commercial, multi-stage compressors (known as ‘intercooling’ or ‘economising’) is now available in smaller compressors – in particular, scroll types – that have been developed to allow the injection of refrigerant part way through the compression process.

This is shown in Figure 1, where a small amount of ‘economising’ refrigerant is initially separated off (as a liquid), after it leaves the condenser, passed through an expansion device – thereby reducing its temperature and pressure – and then used to cool the main, high -pressure refrigerant flow, across a heat exchanger . The superheated – but cool, medium pressure – economising refrigerant is then injected into an intermediate point in the compression process. This ‘economised vapour injection’ (EVI) arrangement effectively splits the compression process into two stages within a single compressor. It acts to cool the bulk of the refrigerant in the compression process and so reduces the work of compression.

Figure 1: Simplified functional schematic and P-h process diagram of air source heat pump employing EVI

As illustrated in the pressure-enthalpy diagram of Figure 1, EVI shifts the compression process to the left, and the refrigerant superheat is reduced at the outlet of the compressor. EVI is particularly beneficial at lower evaporating temperatures – that is, at low outdoor air temperatures around 0°C or below . It also increases the refrigeration effect, the useful amount of heat that the refrigerant can gain from the outside air.

In applications that have little or no available natural gas, but do have grid-supplied electricity, there is an advantage in operating a modern ASHP – even in more adverse conditions – because it will out-perform simple electrical resistance heating. This is also very likely to be more carbon effi cient than using other fuels, such as oil and coal. (The excellent research paper3 by Huang and Hewitt reports on the practical implementation of EVI, as well as indicating that such systems can be successfully – and economically – applied at temperatures below -1°C.)

Gas absorption heat pumps (GAHP)

As shown in Figure 2, a GAHP heats an ammonia and water solution with a natural gas burner (in the generator), and the high-pressure vapour is then passed to the condenser, providing heat for the load – for example, hot water. The remaining liquid from the generator (weak ammonia solution) is passed via the heat exchanger into the absorber

Figure 2: The simplified process of a GAHP

The main flow from the condenser is passed through an expansion valve, and the now low-pressure, low-temperature, strong ammonia solution absorbs heat in the evaporator – from the outdoor air – so evaporating the ammonia (the refrigerant), which is then redissolved into the water (the absorbent) in the absorber. The pump then increases its pressure, ready to repeat the cycle in the generator

Effectively, this is the same operating principle as ASHP, but the electrically-driven compressor is replaced by the absorber/generator combination, powered by heat from gas combustion. This can be packaged into a ‘low-noise’ unit that will include appropriate controls and ancillary components – such as that shown in Figure 3.

Figure 3: Commercially available GAHP (Source: Lochinvar)

Compared with a typical vapour compression ASHP, absorption heating COP and output will vary much less with the outdoor temperature.4 As outdoor temperature falls from 5°C to -5°C, the reduction in capacity for an absorption system is less than 10%, compared with more than 30% for a similarly sized electric heat pump. This means that a properly sized GAHP may well be able to provide a consistent source of hot water throughout the seasons.

For a GAHP, the COPH is likely to be 1.3 to 1.54, and a typical seasonal COP will be 1.4.5 This is 40% better than a typical gas-fired condensing boiler. A recent study6 indicated that GAHP were particularly successful when applied as part of a multi-valent system.

Examples of ASHP integration

Figure 4 shows an example of a manufacturer’s application of an electrical air sourced heat pump providing heating for a storage hotwater vessel. ASHP units typically operate with a flow/return temperature difference of approximately 5K, whereas traditional boiler systems worked on 10K, and modern condensing boilers 20K. This means, when sizing an indirect coil (for ASHP), the coil size has to be larger to meet a particular load.

Figure 4: Air source heat pump providing primary heating for domestic hot water (Source: Lochinvar)

Experience shows that it is difficult, practically, to source appropriately sized indirect cylinders, so a heat exchanger – with an effectiveness of around 95% – is used, as shown in Figure 4. The plate heat exchanger thermally connects the ASHP to the buffer vessel – effectively making it part of a primary circuit. The capacity of that buffer should be designed to prevent undue cycling of the ASHP.

This example system comes as a package that can work as a stand-alone water heater, with all controls required built into the ASHP – including the cylinder sensor and a manually reset overheat thermostat

When sensing a demand, the ASHP is switched on while concurrently activating the pump between the heat exchanger and the cylinder. The cylinder immersion heater will be activated if the unit cannot provide domestic hot water (DHW) at suitable temperatures – that is, higher than 60°C. The cylinder temperature sensor provides the signal to activate the shunt pump and immersion heater backup, and even the secondary pump as required. The unit also has a standard anti-legionella programme to ‘pasteurise’ the vessel. Such an application – operating with ASHP flow/return temperatures of 63°C/57°C – will work with a COPH of around 2.4 (dependent on model).

This application of the ASHP has integrated controls to provide weather compensation in heating mode. So, for example, when applied to smaller systems it can provide heat for DHW with a small indirect cylinder (via a three-port valve), and also be used for underfloor heating or oversized standard radiators working on lower temperatures. When supplying DHW, it can operate at 63°C/57°C and then – working solely in heating mode – at 50°C/45°C.

If natural gas is available, it may be better at lower external temperatures to use a condensing gas water heater, with the ASHP pre-heating the incoming water in a similar way to the GAHP example below.

The example illustrated in Figure 6 has a gas absorption heat pump integrated with a gas condensing boiler, providing low-temperature hot water – used, for example, for underfloor heating or low-temperature radiators – and also pre-heated hot water to a gas-fired condensing water heater. The thermal store, as in Figure 5, acts as a large, low-resistance header – which can also integrate heat from other sources, such as solar thermal panels, using the additional coil in the cylinder. This allows the heat pump to continue working in its most efficient mode, while providing useful input to the hot-water system from multi-modal sources. The thermal store also prevents legionella risk, as it is not being used to accumulate domestic hot water

Figure 5: A thermal store (buffer tank) with tappings for multiple direct heat sources and coils for indirect sources and secondary circuits (Source: Lochinvar)

Figure 6: A GAHP combined with a condensing boiler and a thermal store to provide pre-heating for dedicated domestic hot-water heater and heating for low-temperature heating circuit (Source: Lochinvar)

When operating air sourced heat pumps at low external temperatures (approaching 0°C), frost is likely to accumulate on the ambient evaporator coil, as the moisture in the outdoor air freezes, obstructing the coil. For ASHP, a typical solution is to run the heat pump in a reverse cycle – by altering the direction of flow into, and out of, the compressor – thereby using heat from the compressor and the load coil to provide a defrost for the outdoor coil; or by supplying hot gas directly from the outlet of the compressor to the inlet of the evaporator, so bypassing the condenser. During this period, there will be no heating provided to the load. In GAHP, the heat pump has an automatic defrosting system that also enables a continuous, but reduced, supply of heat to the load. With both types of heat pumps, this reduces the system’s overall efficiency and, therefore, will affect the seasonal performance.

Legend for Figures 4 and 6

Thermal stores (buffer vessels) need to be sized to prevent cycling of the air sourced heat pump, but also to provide a heat store for when it is in defrost mode. The size of the store needs to take into account various factors including:

•The output rating of the heat pump

• Whether there are backup gas boilers

• The number of compressors within the heat pump

A typical ‘rule of thumb’ is to size the store at 20 litres/kW output of the heat pump, plus an additional 10 litres/kW output of any modulating boiler. The manufacturer of the equipment should be able to provide explicit guidance pertinent to a particular application.

© Tim Dwyer, 2015.

With thanks to Steve Addis, of Lochinvar, for sharing his practical experience of gas-fired and renewable hot water production.

Further reading:

For a more extensive discussion of the requirements for ventilation in schools, comparison of design solutions and case studies, see the newly-published CIBSE TM57.


  1. 1. DECC Fossil Fuel Price Projections, Department of Energy & Climate Change, September 2014.
  2. 2. Non-Domestic Renewable Heat Incentive (RHI),, accessed 25 May 2015.
  3. 3. Hewitt, N et al, ‘The experimental analysis of the effect of ambient factors on the defrosting of economised vapour injection compressor ASHP in marine climates’, International Journal of Refrigeration, Vol 36, Iss 3, May 2013.
  4. 4. Dieckmann J, Zogg R et al, ‘Heat-Only, HeatActivated Heat Pumps’, ASHRAE Journal, January 2005.
  5. 5. Robur Technical Information, Robur (2008)
  6. 6. Busato, F et al, ‘Two years of recorded data for a multisource heat pump system: a performance analysis’, Applied Thermal Engineering, 2013 57.