John O’Sullivan writes that carbon-free heat pumps, combined with a renewable-powered grid, could one day render a 100% CO2 emissions-free system for Ireland


For today’s modern building design, heat pumps provide a competitive energy cost solution for space and water heating. While this is true for standard designs or retrofits with strong insulation specifications, the advantage in cost savings is even more distinct when heat pumps are integrated with the passive building concepts of air tightness and low-temperature heating design.

Heat-pump action is 100% carbon free, and as a result, the building’s CO2 footprint is contingent on the electrical supply from the grid. Therefore, in the context of national CO2 reduction targets, heat-pump penetration into the market offers the best technology for completing the renewable-energy power generation and user consumption cycle. Heat pumps combined with a 100% renewable-powered grid may one day render a 100% free CO2 emission system. This would be in line with international long-term goals for 2050 and beyond.

In the near future, heat-pump market penetration can contribute towards medium-term reduction goals. This is evident in some EU countries, where governments incentivise installations through grants and reduced electrical utility tariffs. Today’s challenge is for heat pumps to outrank the efficiency of gas- or oil-condensing boilers as a converter of primary fossil fuel energy into building heat – both in terms of economic cost and CO2 emissions.

Culture and behavioural patterns are also a factor where homeowners need assurance that a comfortable domestic environment can be met without the necessity for traditional fire burners.

Air source or ground source?

Ground-source heat pumps (GSHP) have had the advantage of operating at a better design efficiency due to the relatively stable source of its geothermal subsoil heat source. Air-source heat pumps (ASHP) have closed this advantage gap in recent years with better control technology that adapts its performance characteristics to the outside air source temperature and humidity.

Where a design has a mechanical-ventilation heat recovery (MVHR) system, ASHPs may capture the extra heat energy in the building’s exhaust air for improved efficiency. This system is also likely to incorporate an auxiliary back-up from either an outside air or geothermal source (see Case Study A, below). Heat distribution networks may be a combination of air-air and air-water in ASHP installations.

In efficient operations, typical output temperatures are ~50 ﹾC for DHW and < 35ﹾC for under-floor heating or low-temperature radiator space heating. Piped warm-air heating systems are typically set for ~20 ﹾC room temperature. Air humidity can be controlled within a set range for comfortable occupancy. In continental climates, air-air ASHPs are also used in reverse-cycle for cooling during hot summer periods. These features make ASHP solutions a viable alternative for the large proportion of building stock in urban environments where land space is unavailable for GSHP.

The capital cost of an ASHP installation is significantly cheaper. Because the vast majority of existing Irish homes have a hydronic-based plumbing system, air-air ASHPs are only likely to be suitable for new building designs.

Heat-pump installations represent only a marginal proportion of Irish building stock. Increasing the market share requires an adaptive approach. The overall proportion of buildings that may be rendered suitable would be dependent on successful co-operation between different sectors for an integrated approach. This includes:

  • Lowering the carbon intensity of the power grid;
  • Reducing the average heat load of buildings;
  • Smart-grid technology; and
  • Government policy and public awareness.

Lowering the carbon intensity of the power grid

Heat pump

Diagram of a heat pump’s vapour-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor

Ireland’s grid delivers power to the user with a primary energy-conversion factor of approximately 40%. The product of this value and the performance factor of a heat-pump installation give a total primary efficiency value for a building’s electrical heat system. The performance factor of the heat-pump installation is known as the Seasonal Performance Factor (SPF) [1] – an EU standard for the ratio of heat units produced for consumption (e.g. water tap temperature) versus total primary electrical energy consumed, i.e. fluid pumps, fans and auxiliary immersion system.

As an example: a heat-pump installation having a SPF factor of 3.0 (x 40% grid factor) will give an overall primary energy-conversion factor of 120%. The carbon efficiency of the installation may then be calculated based on the product of this figure and the averaged carbon intensity value (kg/kWh) powering Ireland’s generation fuel mix.

Under the EU Renewable Energy Directive (2009), building heat-pump installations giving a SPF of 2.5 or greater may be classified as renewable. At SPF 2.5, an installation is considered to outrank the most efficient of gas condensing boilers in terms of carbon efficiency, i.e. <277g CO2/kWh [2]. Based on this rationale, there are many grounds for optimism that this qualifying mark will be gradually lowered due to forecast improvements in the Irish grid’s primary energy-conversion factor, along with a greener primary fuel mix. Such a SPF renewable-mark reduction means that more of existing housing stock may gradually be gathered into the renewable net.

It is important to note that converting older buildings into renewable stock through improvements in grid-based carbon efficiency figures would not alone render a satisfactory CO2 reduction target. It would also be expensive and impractical to reinforce the grid network for the power levels necessary to supply the total overall heat load of present Irish building stock (urban and rural), were it to significantly convert to heat-pump technology.

However, a policy advisory by the Irish Academy of Engineering (2013 p10) [3] recommends that heat-pump installation should be incentivised in rural dwellings remote from the gas network (400’000 homes) to take advantage of recent, significant, upgrade of the rural-electricity-network. Electrification of rural heating would make good use of approximately 2GW of excess capacity currently available on the grid. Generally, for colder winter periods, most of the heat-pump installations would need to work in conjunction with existing condensing boiler installations.

Consequently, with a 75% rural conversion to heat-pump installations, oil consumption would be reduced by approximately 90%. Energy import savings would be €230 million per annum and carbon savings amount to 1.3Mt.

Reducing the average heat load of buildings

The operating efficiency of a heat pump depends primarily on the temperature difference between the heat source (air or geothermal) and the heat distribution system. Thus, low energy homes with trickle, 24-hour, low-temperature heating systems achieve the best SPF results.

Generally, the economic benefits of heat pumps reduce for older building stock with greater heat loads. Government sponsored, wide-scale retrofitting of insulation and the Building Energy Rating (BER) rating programme continues to improve building suitability. Current opinion within the industry indicates that buildings with a BER of ‘B’ or better become viable candidates for heat-pump installations.

A strong note of caution is that the Dwelling Energy Assessment Procedure (DEAP) of BER rating may not always correlate with lower heat-load measurements (kW/m2). For example, a BER ‘C’ rating may be converted into a higher ‘B’ rating through the installation of PV solar panels. Energy calculations through the Passive House Planning Package (PHPP) [4] concentrates on reduction of a property’s actual heat load as opposed to the carbon intensity of the energy it consumes. PHPP ranking is therefore more reliable for estimating heat-pump sizing.

Passive building designs achieve the highest installation efficiencies. Well designed and insulated buildings are also likely to benefit economically from heat-pump technology. Along with efficient low-energy consumption, the economic benefit also includes a longer average lifespan for a heat pump (25 years versus 15 years for an oil or gas boiler). There may also be the possibility of incentivised ‘green’ electricity tariffs in the future (vs carbon taxes on fossil fuels).

Smartgrid technology

The penetration of intermittent renewable supplies into the grid means that power generation does not follow demand as closely as the traditional centralised power-network model. Increasing the existing variety of power-generation dispatch options along with modifying demand patterns is key to increasing the penetration of renewable generation.

Smartgrid information technology manages a greater share of distributed and intermittent generation points (e.g. wind) in a manner that tailors complex power flows in response to demand. In addition, smartgrids avail of storage, either on a large scale (e.g. Turlough Hill reservoir) or distributed on a local scale in the form of thermal storage within buildings.

Energy storage allows power to be banked during periods of excess renewable supply, e.g. wind during off-peak early morning periods, and released during peak demand periods.

In the case of low-energy buildings with low-energy heat loss, excess electrical energy can also be thermally stored within the building. This thermal energy store can be a combination of the building’s fabric (concrete walls and floors) and the hydronic heating system, e.g. large warm water tank. Such designs may withstand periods of electrical interruption for two-to-three hours, without any appreciable degree of discomfort or the employment of auxiliary heat sources.

Adopted on a wide scale, this model of heat-pump installation would help the grid to operate a more efficient generation cycle that balances energy supplies between peak demand and trough periods.

Digital information connectivity between operator and consumer also helps smartgrid management. Here, building heat management systems contribute information to the aggregated demand profile of a distribution network. The system operator in turn signals availability of supply and might encourage modification of that demand through ‘smart’ tariffs.

Government policy and public awareness

Heat pumps are an integral component of building energy CO2 reduction programmes in leading EU economies such as Germany and Scandinavian countries. The UK has recently developed a policy and grant-aid programme. The goal is to increase heat-pump penetration amongst the bulk of buildings presently connected to its extensive urban gas network. The policy is evidence based on the results of comprehensive field trials carried out in two phases through 2008-2013, by the Energy Saving Trust [5].

For Phase 1, a sample of 83 heat-pump-installed dwellings was measured for performance. User behaviour and perception was also assessed. A Phase 2 sample of field trials carried out on 44 sites included mostly original Phase 1 sites that were adjusted with customised interventions to improve performance. This resulted in significant improvements in performance and proved to be a useful technical background for formulating policy.

Amongst the finding were that heat-pump installations were carried out piecemeal between different trades, with no single professional holding responsibility for the overall project. As a result, the national quality standards for technical installation were revised and improved. This is particularly pertinent in an Irish context, because stakeholders within the heat-pump industry do express concern that poorly planned installations damage the overall reputation of the technology.

In order for UK domestic homeowners to qualify for reduced electrical utility tariffs (Renewable Heat Incentives) [6]; candidates need to achieve an installation SPF of >2.5. The tariff rate improves for efficient sites with improved SPF ratings. So along with consuming less electricity, the consumer also pays a reduced unit rate according to: Electrical Supply (kWh units) x (1 – 1/SPF rating) x unit price. For example, for a modest installation (SPF of 2.7), the homeowner gains a 25% rebate.

Heat pumps: conclusion

Heat pumps offer the homeowner much greater comfort at appreciably less cost. A greater market share will assist in meeting international CO2 reduction targets. The infrastructure required for a notable increase in market penetration is already in place.

For most of Ireland’s geographic area, there is not the legacy constraint of an existing gas network to compete against and replace. The present programme for development of a national electric vehicle charging infrastructure complements that necessary for heat-pump penetration, especially in the area of smartgrid integration.

A dedicated government policy for the promotion of heat pumps would be helped by field trials on a representative sample of present installations in building stock. Calculation of overall building efficiency factors, along with the present available metrics of primary grid efficiency factor and carbon intensity would make it possible to estimate a carbon-reduction target for a nominal percentage of building conversion to heat pumps. Such field trials would also contribute to the technical knowledge base necessary for competent installations.

In conclusion, the traditional view of home comfort is centred on the warm stove or open fire. A challenging, yet important, element of policy would be an information campaign to demonstrate to the general public the improved comforts of well designed, electrically based and low-temperature-heated homes – without the fire.

Acknowledgements: SEAI Sligo, and

Case Study A: certified passive house example

 Domestic heat-pump solution for detached 167m2, three-bedroom bungalow on exposed rural site, south-western Ireland. Passive timber frame construction; no stove; air-tight test: 0.56 N50

Passive House Space Heating Certificate 15kWh/m2 pa (10 W/m2) Space heating ambient temperature: 20°C constant year round. Constant DHW availability (52°C Top 180l DHW tank).

Heat-pump design: Integrated ASHP and MVHR unit. Space heating provided by air-air unit. Air to DHW unit with 180Ltr tank. 2kW geothermal auxiliary backup (winter-time operation) with integrated space heating through under-floor heating installed in bathroom and hall areas. Summer bypass valve for ventilation-only operation. 4.12m2 flat-plane solar thermal panels and 300l tank integrated with heat-pump system.

 Results for 2012: ASHP Air to Air COP: 3.4 and Air to Water COP 2.3

Annual Operation Periods:

Summer ventilation-only: 46%, Passive Recovery Unit: 54%, Compressor-on: 7.6%.

Energy consumption of ASHP, Ventilation fan and Solar circulation pump:

1200kWh/year or € 220.71 for constant DHW and 20°C space heat.

 Capital cost of heat-pump installation: €8,730 (+€2,760 on conventional oil-boiler installation)

Estimated payback at running cost difference (€1,100 per annum): 2.5 years.

Case Study B: conventional modern home air source HP retrofit

Retrofit domestic heat-pump solution for detached, four-bedroom, two-storey conventional build design (2005-2006) construction in south-western Ireland. Conversion from OFCH to ASHP.

Insulated, cavity-block wall construction. 200 m2 floor space. Under-floor heating on the ground floor and standard radiator installation upstairs. 200l DHW tank. One heating stove installed. No solar panels.

U-Values: Walls 0.27: Floor 0.24: Window 2.6: Roof 0.16

9 kW, air-water heat-pump installation with auxiliary electrical immersion system. 95% annual energy supply is covered by heat-pump system.

 Annual gross energy demand: Space heating, 22 MWh/annum; DHW 4.9 MWh/annum.

Energy supplied by heat pump: 23.60 MWh

Energy consumed by heat pump: 5.60 MWh

Energy used by auxiliary heater: 0.64 MWh

Overall seasonal performance factor: 3.88

Total annual energy costs (heat-pump and immersion): €936/year

Previous annual oil-boiler installation cost1:€2,286/year

Annual savings: €1,350/year

Payback period for net installation cost of heat pump: 3.7 years

1Cost calculation based on current oil prices: €850/m3

Electricity price for the heat pump: €0.15/kWh.

New PictureJohn O’Sullivan, BEng Elec MA, currently works as a watch officer with the Irish Coastguard Service. He is also currently pursuing an MSc in Renewable Energy Technology Systems with Loughborough University through distance learning. O’Sullivan began his career as a merchant marine radio officer, mainly with the Dutch deep-sea merchant navy. He has overwintered on the British Scientific Base of Halley, Antarctica. In more recent years, O’Sullivan has worked as a project engineer in windfarm developments and has also worked as a consultant engineer in the area of ocean-energy development.




[1] (p22-23)

[2] EU figure based on 87% efficient boiler and gas CO2 Life Cycle equivalent of 237g/kWh




[6] O'RiordanEleccarbon,energy,heat
  For today’s modern building design, heat pumps provide a competitive energy cost solution for space and water heating. While this is true for standard designs or retrofits with strong insulation specifications, the advantage in cost savings is even more distinct when heat pumps are integrated with the passive building...