Heat Pumps and Heat Waves: How overheating complicates ending gas in the UK

by Dr Aaron Gillich | Associate Professor and Director of the BSRIA LSBU Net Zero Building Centre

We have entered what many are calling the decisive decade on climate action. Among the most critical decisions that the UK faces this decade is how it will eliminate carbon emissions from heat. Heat accounts for over a third of our emissions, and over 80% of our buildings are linked to the gas grid. There is no pathway to Net Zero that doesn’t include ending the use of gas as we know it in the UK.

Given the size of the UK gas grid, no single technology or energy vector can replace it. We will need a combination of clean electricity and carbon‐free gas such hydrogen or biogas, delivered by a range of enabling technologies such as heat pumps and heat networks. And of course an extremely ambitious retrofit agenda that reduces the demand for heat in the first place.

The UK is investing widely in low carbon heating innovation. That innovation is essential, but is also unlikely to include any blue‐sky breakthroughs that aren’t currently on the table. In other words, the menu of low carbon heating technology options is set, and this decisive decade will be about deciding what goes best where, and how to ensure a just and equitable heat transition.

Low-carbon heating options

Of all the low‐carbon heating options available, low carbon heat pumps are the most efficient and scalable option that is market ready and can respond to the urgency of climate change this decade. The UK has set a laudable target of installing 600,000 heat pumps per year by 2028. Many have criticized this figure as unrealistic, but I believe that the target is highly achievable, and represents a pace that is in line with past transitions such as ‘the Big Switch’ that put us on the gas grid in the first place.

This race to replace gas in the UK has been widely discussed. As have the many barriers that face heat pump deployment in the UK. What I’ve heard discussed far less are the links between heating in the winter and overheating in the summer. Over the next decade, the end of gas will present both a threat and an opportunity to improve both the winter and summer performance of our building stock.

The threat of climate change is clear. The end of gas increases this threat because gas has allowed the UK to obscure poor building performance, and poor building knowledge for so long. Cheap gas has enabled a ‘set it and forget it’ approach to many building systems, and allowed us to maintain reasonable standards of comfort in most buildings despite very poor fabric performance. The irony is that this poor winter performance actually helps reduce the risk of overheating in the summer, as the leaky and poorly insulated buildings can more easily shed excess heat. It has been widely reported that many newer, better insulated buildings actually face an increased risk of summer overheating.

Replacing gas with heat pumps, or any other low carbon heat source, should be accompanied by ambitious retrofit to improve energy efficiency and reduce heat loss. There are many that argue heat pumps in fact require extensive fabric retrofit in order to function in most UK buildings. This is highly debatable and will be explored in detail in follow-up writings. Regardless, demand reduction and a fabric first approach is a good idea for its own sake.

Replacing gas with heat pumps, or any other low carbon heat source, should be accompanied by ambitious retrofit to improve energy efficiency and reduce heat loss.

But reducing the heat loss in winter will likely trap heat in the summer, presenting a conflict. The UK currently experiences over 20,000 excess winter cold deaths and around 2,000 heat related deaths in summer. It was previously thought that the increased temperatures from climate change would decrease winter cold deaths, but more recent work has shown that due to the increases in extreme weather events at both ends of the spectrum, it is far more likely that winter cold deaths will remain at similar levels, and summer heat deaths will increase dramatically under climate change.

We must use the transition from gas to low carbon heating as an opportunity to better understand our buildings. Many of 600,000 heat pumps we install by 2028 will be in new build, but up to half will need to be from existing homes.

Retrofitting a heat pump is also the time to think about not only how to improve energy efficiency for the winter but how to reduce summer overheating as well. Despite much effort towards a whole‐house approach to retrofit, most work remains quite siloed. Energy efficiency and heating installations are largely in separate supply chains, and the building physics knowledge to carry out an overheating risk assessment is even less likely to sit with the same project team. Overheating is also very poorly captured by the building regulations and planning process.

A holistic approach

The last few years has seen a growing awareness of overheating risk and an emergence of increasingly easy to use assessment tools. A very small fraction of UK homes have comfort cooling. Retrofitting a comfort cooling solution typically requires costly and complex changes to distribution systems. However, there are a range of low cost options, including using local extract fans to create interzonal air movement, or using night purges and thermal mass. Blinds are also incredibly useful, but often misused in summer, and can also help reduce heat loss in winter. There are also ways to use local microclimate features such as shaded areas or the North side of the building to bring in slightly cooler air from outside and reduce peak temperatures.

Improving the air tightness and fabric performance of our buildings to address heating in the winter will change how we implement these solutions for the summer. They require not only careful thought at the design stage, but also strong communication to help end users operate them properly. Simply opening a window is unlikely to help if the outside air is warmer than inside.

A significant problem is that there are insufficient drivers to force this type of holistic approach to design, performance, and communication. It is so often said that we need stronger policies in the area of heat and retrofit, and this is no doubt true. But while we await these policies it is incumbent upon each of us in this sector to share and collaborate as widely as possible, and use whatever influence we have over a given project to encourage a fair and forward looking solution.

In summary, the availability of cheap gas has allowed us to escape having to understand our buildings in much detail. Climate change is the catalyst for an untold level of change in our lives that we are going to start to truly experience in the coming decade. Heating and overheating are coupled issues that must be solved together. We must use the end of gas as an opportunity to understand our buildings better, and implement solutions to climate change that work across seasons, or we risk trading one problem for another.

In summary, the availability of cheap gas has allowed us to escape having to understand our buildings in much detail.

Taking action on Climate Change

by Michelle Agha-Hossein, BSRIA Building Performance Lead

Most nations now recognise climate change as an established, perturbing fact that needs immediate attention. We can see the effects in the worsening and more frequent extremes of weather: flash floods, droughts, strong winds, heavy snow, heat waves, etc.

UK temperatures in 2019 were 1.1°C above the 1961-1990 long-term average and it was a particularly wet year across parts of central and northern England. Still fresh in the memory are storms Ciara and Dennis in February 2020 with strong winds and heavy rain that caused significant damage to homes and commercial buildings. There is growing evidence that periods of intensely strong winds and heavy rain are likely to increase in the future.

The UK is not the only country affected by climate change. Many other countries are (and will be) suffering disproportionately. The world’s leading climate scientists have warned that we might have just 12 years to keep global warming at a maximum of 1.5°C. After this point, the risk of extreme weather conditions will significantly increase. The increased frequency and intensity of extreme weather will affect all but is most likely to bring catastrophic consequences in many less economically developed countries, where food shortages and water scarcity can trigger deep social changes.

Immediate radical action is required to limit carbon emissions, and the built environment industry can play a crucial role by changing the prevailing culture.

Most building-related carbon emissions are generated from energy use in buildings. However, there are choices that building owners/operators can make and initiatives that they can undertake to lessen the related negative impact on the environment:

In brand new buildings, the most effective way for addressing emissions is reducing consumption through energy efficient design. In existing buildings, the issue can be addressed by efficient retrofitting and effective maintenance strategy. Adopting renewable energy technologies in both cases can significantly reduce building emissions.

Steps building owners and operators can take today.

There are several initiatives/activities that can help building owners/operators combat climate change:

  • Consider ‘net-zero carbon’ targets for your building: UKGBC launched its Advancing Net Zero programme in 2018 and published the ‘Net Zero Carbon Buildings: A Framework Definition’ in 2019. The framework provides the construction industry with clarity on the outcomes required for a net zero carbon building.
  • Ensure the required outcomes for a ‘net-zero carbon’ building are achieved: As advised by UKGBC in the framework definition, initiatives like BSRIA Soft Landings should be adopted in new build as well as in refurbishment projects to ensure a net zero carbon building will be achieved. The BSRIA Soft Landings framework provides a platform for project teams to understand the required outcomes for their project and ensure all decisions made during the project are based on meeting those outcomes.
  • Maintain your net zero carbon building effectively: Business-focused maintenance is a methodology developed by BSRIA that can be adopted to help building operators maintain critical assets effectively and efficiently to sustain a net zero carbon building within budget.
  • Investigate failure quickly: Is the energy bill for your building higher than it should be? Investigate the problem as soon as you can. The first and easiest step would be looking at the energy end use breakdown to see which areas are using more energy than expected. If the issue is related to the HVAC system, check the system’s setting points and monitor the indoor air temperature and relative humidity. Thermal imaging of the fabric of the building can also help to identify, thermal bridging, missing/damaged insulation and areas of excessive air leakage.
  • Promote a healthy diet among building occupants: This is a non-technical initiative that building owners/operators can adopt in their buildings. Eating less meat and gradually shifting to more plant-based foods is vital for keeping us and our planet healthy.  It is important to think about initiatives such as using signage or lunchtime talks, to educate building occupants about healthy diets and encourage them to eat more fruit and vegetables. Research has shown that adhering to health guidelines on meat consumption could cut global food-related emissions by nearly a third by 2050. Healthy diet is also supported by Fitwel and the WELL building standard.

Building owners and operators, to play their role in combating climate change, should ensure their decisions and the way they create and run their buildings contribute positively to the wellbeing of our planet and its citizens.

So, make a start today and choose the first thing you are going to assess/change in your building to help combat climate change.

To find out more about how BSRIA can help you improve building performance, visit us here.

Testing of Solid Fuel Stoves

Dr Arnold Teekarem, Head of Combustion at BSRIA

Dr Arnold Teekaram, Head of Combustion at BSRIA

Stoves manufacturers are now CE marking their appliances under The Construction Products Regulation (EU) No 305/2011 (CPR) having successfully completed the CE marking tests at BSRIA on a purpose built facility.  The facility was UKAS assessed this year in accordance with BS EN ISO/IEC 17025 “General requirements for the competence of testing and calibration laboratories”  to become an Accredited and Notified Test laboratory for testing Solid Fuel Stoves  The automated test facility which has a unique data acquisition system for recording real time data such as flue gas emissions, 300 trihedron wall and floor temperatures, flue draught and the fuel load enables both the thermal performance and smoke emissions tests to be carried out on free standing stoves designed to burn wood and mineral fuels.

Thermal Performance and Safety tests

In accordance with BS EN 13240: 2001 +A2:2004 “Room heaters fired by solid fuel-requirements and test methods” BSRIA is able to offer clients the following tests on residentially non mechanical stoves (intermittent and continuous burning) for various types of solid fuels

  •  Thermal performance tests including thermal heat output and efficiency
  • Safety tests- These include operation of appliance to determine the safe combustible distance from the stove to any combustible material
  • Emissions of combustion products
  • Checks to determine whether the materials, design and construction requirements of the appliance comply with the requirements of the standard
  • Checks to determine whether the installation and operating instructions of the appliance comply with the requirements of the standard
  • Checks to determine whether the marking information given on the appliance comply with the requirements of the standard

Thermal performance test facility

Thermal performance test facility

Fundamental to achieving optimum thermal performance of the stove under test are a number of variables.  These include achieving the correct combustion air settings – the appliance air controls must be adjusted accordingly to optimize the flue gas temperature, CO2 and CO levels within the flue gases whist at the same time achieving the optimum burning rate.  Unnecessary variations in the test conditions such as the flue draught can significantly affect the burning rate and thermal performance of the appliance. The test standards require a flue draught of 12 ± 2 Pa for thermal performance test at nominal heat output for appliances up to 25 kW.  For temperature safety tests, the flue draught must be maintained at 15 +2/0 Pa for appliances with a nominal heat output of up to 25 kW.  The test rig is automated to give good controllability of the test conditions within the limits imposed by the test standard.   Minimizing variations in the moisture content of the test fuel is an important parameter that is important both for repeatability of the test results and achieving optimum combustion performance. This variable is also controlled using internal quality control procedures, careful selection of the test fuel and measurement of the moisture content before the tests.

CE marking of stoves became mandatory from the 01 July 2013 and under the CPR, manufacturers products are now required to demonstrate compliance with the above tests by having the appliance tested by a Notified Test Laboratory.  These tests are summarized within annex ZA.1 of the standard.  Manufacturers are also required to implement their own factory production control (FPC) procedures under the current attestation level 3 for room heaters fired by solid fuels.

Smoke Emissions Tests

In addition to the above, BSRIA is also now able to offer DEFRA smoke emissions tests on appliances seeking exemption for

Smoke emission measurement

Smoke emission measurement

burning unauthorized fuel in smoke control areas within the UK (section 21 of the Clean Air Act 1993).  The state of the art test facility uses the dilution tunnel approach with isokinetic sampling of the flue gases, an approach which is also used by some European test houses.  Tests are conducted in accordance with PD 6434, BS 3841 Parts 1 & 2 and the Richardo -AEA Test Protocol issue 3.0.  Because of the variation in the smoke emission between tests, multiple tests are conducted at high heat output as well as reduced heat output.  As the smoke emission rate is dependent on the size of the stove (volume of its firebox), its air controls which affects its combustion performance and the fuel used, the measured heat output at each load must also be reported.  If appliances are fundamentally different in their designs, then individual smoke tests are also required. The exception to this are appliances with cosmetic changes to the exterior.

The smoke emission rate can also be affected by the manner in which the appliance is refuelled.  If the firebed is not established i.e. if there is insufficient burning material to cause the new fuel charge to ignite within a reasonable period or if the appliance is overloaded with a new fuel charge, excessive smoke can occur.  Care has to be therefore taken during the tests to avoid such adverse operating conditions that can cause the stove to unnecessary fail the smoke emission tests.

The iso-kinetic method of sampling the smoke emission within the dilution tunnel used by BSRIA is an accurate and representative method of determining the smoke emission rate from appliances burning wood and mineral fuels. In this technique, the velocity of

Data Acquisition System

Data Acquisition System

the sampled gases within the sampling nozzle is maintained the same as that of the mainstream flue gases within the dilution tunnel.  The specialist instrumentation used for sampling is an automatic gravimetric sampler which continually adjusts the sample volume flow rate and hence the flue gas velocity in the smoke sampling nozzle.

In tests conducted at BSRIA, iso-kinetic sampling has been consistently maintained during the tests.  The maximum isokinetic deviation on the velocity is around 0.8% compared to 2.5% which is required in the test standard.

BSRIA’s smoke emission test results are expressed in grams/hour to enable comparison with the permitted smoke emission rate given in BS PD 6463 and represent the average smoke emission rate taken over the test cycle which is typically 45 minutes to an hour. Alternatively the smoke emission rate can be expressed in mg/m3 at a reference oxygen content of 13% in the undiluted flue.

Opacity measurements are also taken during the tests to detect the peaks in the smoke emissions during refuelling and de-ashing.

Further information on testing of stoves and other types of solid fuel appliances can be obtained by contacting Dr Arnold Teekaram, Head of Combustion BSRIA Tel 01344-465538 or by e-mail at arnold.teekaram@bsria.co.uk or by visiting BSRIA web site http://www.bsria.co.uk/services/testing/standard-testing/solid-fuel-stoves/

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