Sustainable energy without the hot air: Ten years later

Updated: May 16, 2019

Dan Wright | Loughborough, UK

The 1st of May marks the ten-year anniversary of the publication of Sustainable Energy - Without the hot air, the seminal book by Sir David J. C. MacKay who sadly passed away in early 2016. In this article, we revisit MacKay’s thoughts on Smarter Heating and consider how technology and policy has changed in the decade since the book was released.

Article summary

In his chapter on Smarter Heating, MacKay summarises that a holistic approach to tackling the power used for heating requires that we reduce the leakiness of buildings, increase the efficiency of heating technologies, and reduce the average home indoor temperature.

  • MacKay identified the need to retrofit households with better insulation. The UK government's Green Deal policy sought to support the retrofit of millions of homes each year, but only achieved a couple of thousand before being stopped. Policy instruments and incentives have a valuable role to play, however, it is unclear how to catch the attention of the millions of households whom could benefit from such retrofits.

  • MacKay saw the potential of heat pumps. While we now know that MacKay may have been optimistic with the levels of efficiency stated (likely from lab studies rather than field studies) they are still a means of home heating that would reduce domestic reliance on natural gas, and could be matched with renewable electricity generation for eco-friendly home heating.

  • MacKay highlighted the technology needed for reducing average temperature in homes, with thermostats being central. Advances in technology now mean that occupants have more control than ever before to decide how their home is heated... when, to what temperature and which room. Could optimised self-learning heating systems take the hassle out of home heating while delivering savings to no detriment to the occupants?

Approaches to reducing the leakiness of buildings

The draught you might feel under an old front door and the cold you feel when placing your hand on the inside face of a solid wall during winter are both examples of issues that can be remedied to improve thermal comfort and energy efficiency. MacKay proposed that improving airtightness and insulation was essential:

This can be done by improving the building’s insulation – think triple glazing, draught-proofing, and fluffy blankets in the loft.

In 2013, four years after the launch of MacKay’s book, the Department of Energy and Climate Change launched the Green Deal. This was a pioneering policy initiative to bring the UK’s old and leaky housing stock [1] into the era of energy efficiency through financing for the installation of insulation and renewable technologies. Approximately 2.5 years post-launch, the Green Deal was effectively terminated (Rosenow & Eyre, 2016).

While the policy had aimed to reduce the leakiness and overall energy efficiency of two million homes per year, the actual annual number of households benefiting from the policy was around 6,000 (DECC, 2016). Now, retrofitting homes with energy efficiency measures continues to be expensive, but there are options for loans through private companies [2]. It is likely that a revised and renewed Green Deal could be necessary to make gains in reducing the leakiness of the privately-owned UK housing stock. Could it be possible that the UK public might be more likely to engage with new technologies rather than structural modifications?

Increasing the efficiency of heating technologies

The power requirements and financial cost of light has dropped dramatically since 2010 as ultra-high-efficiency light-emitting diode (LED) bulbs have replaced traditional incandescent bulbs. What they may lack in the warmth factor of tungsten bulbs they make up for in lifespan and cost-effectiveness. In the same way, new boilers with a rated efficiency of 94% are considerably more efficient than their predecessors [3], but MacKay had the following advice regarding heating technologies:

You might think that 90% sounds hard to beat, but actually we can do much better.

MacKay believed that rather than look to combined heat and power generation which utilises the by-product of fossil fuel-fired power stations to heat homes [4], we should leapfrog directly to heat pumps, a technology that can be much more than 90% efficient!

The rise of the heat pump

Heat pumps work like reverse refrigerators, using electricity to turn cold air into warm air which can then be used in home central heating systems (Figure 1). Heat pumps are already widely used across many European nations (EHPA, 2015), but still only make up a small fraction of the UK home heating market (Hannon, 2015).

Figure 1. Air-source and ground-source heat pumps (adapted from MacKay, 2009)

I spoke with Stephen Watson, a doctoral research with the LoLo Centre for Doctoral Training in Energy Demand based at Loughborough University, about what how the UK context might need to change to usher in the age of the heat pump.

Q: Stephen, on paper, it feels like heat pumps are a no brainer for meeting the government’s carbon emission reduction goals, as long as installation is matched with clean generation of electricity. What do you think is preventing wider uptake across the UK?

Indeed, I think that heat pumps do have a significant role to play in the decarbonisation of domestic space and water heating in the UK. Even with the current electricity generation mix and levels of heat pump performance, they result in lower emissions than gas boilers. It is worth noting that David MacKay was quite optimistic about the performance that can be achieved by heat pumps, since he was basing this on lab-based tests. Since Sustainable Energy - Without the hot air was published there have been two large field trials of heat pumps in the UK, so we now know more about how they perform in real homes. There could be a number of barriers to widespread heat pump use: cost, unfamiliarity with the technology, lack of installers, noise, space, requiring changes to emitters, etc.

Q: In your opinion, do you think that the government should step in to further subsidise the installation of renewable heating sources (e.g. through the Renewable Heating Incentive [5]) or should it be down to industry to crack the marketing message that might make heat pumps a more attractive purchase to homeowners?

I know that EON have been producing some rather mysterious adverts to promote heat pumps - I'm not sure how well these are understood! The Renewable Heat Incentive is a payment per unit of heat used. This is an unusual way of providing a subsidy, since we are paying people to consume heat, which may not be something we want to do. This type of subsidy may also be more likely to go awry, as has happened in Northern Ireland. Since the capital costs of heat pumps are high, but the running costs are not, it is my view that it would be better to focus on grants to reduce the cost of installation, rather than ongoing subsidies.

Reducing the average home indoor temperature

The warmer we want our homes, the harder our heating systems have to work. The second law of thermodynamics states that warm goes to cold. The larger the difference between the indoor temperature and the outdoor temperature, known in the business as the ΔT (Delta-T), the quicker that the warmth will rush out of the room. For this reason, minimising the difference between indoor and outdoor temperature is important for reducing energy use in homes. MacKay made suggestions for how we can achieve this:

Increase the efficiency of the heating system. This can be achieved by turning thermostats down (or, if you have friends in high places, by changing the weather).

Thermostats are the main method of controlling household temperatures, however, 23% of UK homes do not have a thermostat (Park et al., 2017). A thermostat is a basic form of heating control that enables occupants to set a temperature to which they would like to heat their home.

Heating control: Thermostat

By having a central thermostat, occupants can have thermal control of their home. Occupants can turn down their set temperature (for example, from 22°C to 18°C) to reduce the total average temperature.

Heating control: Thermostat and programmer

With a thermostat and a programmer, occupants can add temporal control of their home heating system. After setting a central temperature for the home, occupants can also and can also decide when the heating comes and goes off, either by using a digital and analogue controller. By turning off the heating when the house is empty, this can reduce average indoor temperature.

As US President Jimmy Carter found during the 1977 natural gas crisis when imploring the public to turn their thermostats down to 65°F / 18.3°C [6], giving advice to occupants is unlikely to be enough to reduce usage (DECC, 2014) so this is where technology that engages with occupants could provide the answer…

Advanced heating controls

Advanced heating controls allow us to go beyond the temporal and thermal control and add spatial control. Zonal space heating controls work by enabling occupants to set a preferred temperature and time of operation for each individual space in their home. The occupant is then able to control the temperature of each zone individually and set automated routines for how each room is to be heated to fit with their lifestyle.

Heating control: Zonal space heating controls

These add spatial control to average home indoor temperature. Each zone can be set to a temperature, and each zone can come on and off dependent on the occupant programme. This reduces the average home indoor temperature by keeping unused rooms cool.


Technology has progressed remarkably in ten years. Faster and cheaper domestic internet speeds and growing mobile network coverage has ushered in the age of interconnectivity. In home energy research, the Internet of Things presents new ways to investigate interaction with energy routines and gather data on demand. This means that it is becoming easier to take actions based on proof, whether that is monitored increase in room temperature following insulation or the reduction of energy bills following the installation of imprpved heating controls. Technology can make the impact of MacKay's advice tangible.

Ten years after its publication, Sustainable Energy - Without the hot air continues to be a fascinating book that has inspired at least one person (me!) to get involved with the energy space. Revisiting the book reminded me of MacKay’s engaging writing style, the clarity with which he presents complex concepts and his insightful comments, many of which are very closely aligned to what we are seeing (or hope to be seeing) today. If you are interested in tooling up to be ready for conversations about renewable energy and what the future may hold within this space, MacKay’s free e-book is a very good place to start.

You can download the Sustainable Energy – Without the hot air eBook here and purchase the paper copy here.

For Stephen’s latest papers on heat pumps with the UK context, please see:

Dan Wright is a doctoral researcher with the School of Architecture, Building and Civil Engineering at Loughborough University funded by the EPSRC London-Loughborough (LoLo) Centre for Doctoral Training in Energy Demand (Grant No. EP/L01517X/1) and supported by Simble Solutions Limited, an innovative, Australia-based SaaS specialist.


[1] In the foreword preceding the summary of the Green Deal, then-Secretary of State of Energy and Climate Change, Chris Huhne, stated that: ‘Britain as some of the oldest building stock in Europe’ (DECC, 2010).

[2] The 2019 Green Deal Guide

[3] A Worcester/Greenstar 25i 25kW combi boiler has rated efficiency of 94%, while a boiler that is more than 20 years old would be closer to 60% efficient (Boiler Efficiency Calculator)

[4] For further information on how combined heat and power generation works, please see this great article on combined heat and power (CHP) cogeneration by Chris Woodford.

[5] The Renewable Heating Incentive is a tariff-based approach to encourage installation of renewable heating technologies introduced by the UK government in 2014 (Snape, Boait, & Rylatt, 2015).

[6] The presidential plea to the US public is thought to have had a relatively minimal effect, with 27% of households being monitored to have their thermostat set to 65°F / 18.3°C or below following the speech from President Carter, but little causation detected (Lubyen, 1982).


DECC. (2010). The Green Deal: A summary of the Government’s proposals. Retrieved from

DECC. (2014). Advice on how to use heating controls: Evaluation of a trial in Newcastle. Retrieved from

DECC. (2016). Household Energy Efficiency National Statistics Headline Release. Retrieved from

EHPA. (2015). European Heat Pump Market and Statistics Report 2015, 4. Retrieved from

Hannon, M. J. (2015). Raising the temperature of the UK heat pump market: Learning lessons from Finland. Energy Policy, 85, 369–375.

Lubyen, P. D. (1982). Prompting Thermostat Setting Behavior: Public Response to a Presidential Appeal for Conservation. Environment and Behavior, 14(1), 113–128.

Park, T., Solomon, P., Sanders, M., Dietsch, M., Faller, J., Gibbons, D., … Tupper, A. (2017). Evaluating the Nest Learning Thermostat. Retrieved from

Rosenow, J., & Eyre, N. (2016). A post mortem of the Green Deal: Austerity, energy efficiency, and failure in British energy policy. Energy Research & Social Science, 21, 141–144.

Snape, J. R., Boait, P. J., & Rylatt, R. M. (2015). Will domestic consumers take up the renewable heat incentive? An analysis of the barriers to heat pump adoption using agent-based modelling. Energy Policy, 85, 32–38.


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