Outside her house, all that can be seen of the pump is an innocuous metal box. Inside the machine is a closed circuit of liquid that cycles in a pipe running from the box outdoors to a water tank indoors. This liquid is a refrigerant with a very low boiling temperature, so even winter air has enough energy to vaporise it. At the outdoor box, the heat pump fans air over the refrigerant, helping it to absorb heat, which it then transfers to the water tank inside the house.

Craig has one of these pumps to generate hot water for an underfloor heating system and another that generates hot water that runs from the taps. On her countertop, an owl-shaped monitor on the tells her how much she is using. It’s usually good news. Heat pumps use only around a quarter of the energy needed for a traditional gas boiler and Craig is able to get the electricity needed to keep the system running from solar panels she has had installed on her roof.

What makes heat pumps so efficient is the refrigerant gas inside the network of pipes. It flows through a cycle of evaporation and condensation as it absorbs heat from the air, turns to vapour and is then compressed with an electric-powered pump. This compression step helps to concentrate the energy stored in the refrigerant. When it gets inside, the refrigerant cools and condenses as it transfers its heat to the water in the inside tank.

Even in low outside temperatures, it can generate enough heat to keep Craig’s home warm and produce enough hot water for her needs. It is a highly efficient process, and is leading many to see heat pumps as the future of home heating.

The energy used to heat the spaces we live and work in is one of the highest contributors to our individual carbon footprints. Globally, heat accounts for nearly half of all energy consumption and 40% of energy-related carbon dioxide emissions. Of course, exactly how much of your own emissions come from heating your home will depend on where you live. In cooler and temperate climates, that tends to be a far bigger proportion.

In the US, 38% of greenhouse gas emissions from residential housing are produced from heating and cooling rooms, while a further 15% are produced heating water. It is estimated that 19% of the UK’s greenhouse gas emissions come from warming up the places we live and work, with more than three-quarters of this coming from domestic buildings. The vast majority of houses in the UK rely on gas boilers, but the British Government has moved to phase these out in the next five years.

There are plenty of less carbon intensive alternatives for householders to switch to, such as using electric heating, heat pumps, or even district heat networks, where a central source is used to heat water, which is then shared among nearby houses through networks of pipes.

The carbon footprint of heating, however, is only one half of the story. In order to cut down on emissions quickly, as well as prepare for low energy alternatives, “our houses need to be warmer”, says Hannah Jones, a mechanical engineer at a UK-based green design consultancy called Greengauge. “If we all just add heat pumps to most houses as they are we’ll need more energy than they can provide.”

This means we need to be able to trap what heat we get into our homes inside rather than allowing it to leak out.

This challenge motivated Craig to participate in Muswell Hill Sustainability group, a network of carbon-friendly homeowners in north London who are trying to demonstrate how carbon savings can be made. Through the network’s thermal imaging group – “the TIGgers” – homeowners can have their property photographed using heat-sensitive cameras to reveal where they are wasting the most energy.

“People are often surprised to see their roofs are bright red with heat loss,” says Craig. “It’s one way to motivate communities to decarbonise the country’s housing stock en-masse.“

Once you have found where the heat is leaking out, it is then possible to do something about it.

Where to start

New builds are easier to tackle. Installing cavity insulation and new double-glazing during the construction stage saves replacement costs in the future.

For the majority of the people, however, the challenge is how to adapting their existing properties. Much of the housing in Europe and North America is more than 40 years old. In the US 13.5% of homes were built before 1940 while a third of the housing in the UK, Denmark and Belgium were constructed prior to 1946. In Germany, Hungary and Sweden 45-50% of the houses were built between 1946 and 1980 while in Italy, Slovakia and Romania the share of housing of that age rises to 50-60%.

Despite this, huge carbon savings can still be made by retrofitting the existing housing stock, according to Jones. Making sure doors shut properly is a good first step, and that they are draught proofed, with no gaps. “Door brushes, beading and rubber seals alldo their bit,” she says. “Not everyone can afford new cavity wall insulation, but everyone can drought-proof their letterbox.”

According to a report from the University of Sussex, new double-glazing (manufactured after 2002) provides one of the most significant carbon savings. If all houses got new double-glazing it could save the UK 20.3 TWh of fuel per year. Assuming the heat saved is generated from natural gas, the savings would amount to 3.76 million tonnes of CO2 per year – around 1.2% of the nation’s total annual carbon emissions.

For households that can’t afford to refit windows, there are also magnetised versions available, where a second pane can be stuck over the top superficially. They fit different size and types of window and come at about one-fifth of the cost.

Making the best of a boiler

While the energy you use to heat your home makes a big difference to the carbon footprint, there are other ways of reducing emissions.

Terence Jeffrey is a green energy consultant, based in Islington, London, where he advises residents how to get more heat out of their existing boiler systems. He recommends people pay attention to their radiators.

“We always recommend radiator reflectors,” he says. “Much of radiator heat is lost through the mounting wall, a reflector it bounces back 95% of it.” In a house with five radiators, reflectors could save £20 ($26) a year from energy bills, he estimates.

More efficient still is to turn off radiators that aren’t being used. Thermostatic radiator valves can be added and used to control the heat in individual rooms. “Letting the other rooms be a little cooler is going to save carbon,” Jeffrey says. Internet-connected “smart” valves allow even greater control.

Turning down room temperature down by 1C can have a big cumulative effect. According to Element Energy, the UK would save 1.18 million tonnes of CO2 if everyone carried out such a decrease. Jeffrey is quick to add, however, that many of his clients are in fuel poverty, so dropping the temperature further is not always an option for those already scrimping by on the barely any heat anyway. 

He does recommend lowering the running boiler temperature to 60C. “A lower temperature over a longer period is better than firing it up to higher temperatures in short bursts” he says.

Insulation and passive designs

After draught-proofing, insulation is the most important way to cut back on carbon.

At its most extreme is a rigorous sustainability energy certification called “passive house”. The movement has its roots in 1970s American architecture when the energy crisis meant homes were designed to maximise as much as possible on insulation and “solar gains” – the free heat that comes from the sun.

“It was a case of necessity becoming the mother of invention,” says Julie Torres Moskovitz, who completed the first certified passive house in New York. Growing up in the 70s, she remembers colouring-in exercises about how to save energy during the crisis at school. “Energy has always been a theme for me,” she says.

To achieve passive house level of energy efficiency, a home must use below 15 kilowatt hours per square meter for heat for the whole year annually – about a 10% of that used by the average home.

“Virtually airtight” is how Moskovitz describes the requirements of a passive house. “If the standard for insulation are met, a passive house can reduce heating energy consumption of its occupants by up to 90%,” she says.

A passive house needs a high level of insulation, and to eke as much use as possible from the heat available inside. External insulation on the outside of the building is one of the most effective at creating a sealed envelope and avoid “thermal bridges”, which wick heat from warm indoors to cold outdoors. But it can also be the most expensive and, thanks to its change of appearance, runs up against planning permission problems.

There are easier and cheaper options – 10mm-thick insulation can be rolled on like wallpaper to internal walls. “Adding a layer roof insulation is also very easy to do,” says Jones. “A roll of mineral wool to the floor of cold loft space could make a noticeable difference.”

Insulation doesn’t just trap heat from radiators – it also keeps in warmth produced by other sources too. “Each person in the house is going to be giving off 100 watts,” says Justin Bere, a sustainable architect in London. In a 100sq m (1,076 sq ft)  three bedroom passive house, it should only require 1kW to heat the building even in the coldest months, he estimates. “So in a family of five you’ve already got half the warmth you need for the house.”

A dog might give off another 50 watts while appliances such as an oven, a refrigerator and electric lights also add to the warmth inside the building.

Bere’s point is that if the house can be fitted to hold onto the warmth inside, there is less need for more from carbon-emitting sources.

While many new builds are being fitted to passive house standards, existing buildings can also be retrofitted with insulation. Bere often uses insulation panels that are as thick as a mattress and are attached to wooden board. These can be drilled on the interior of existing walls. “It means the rooms come in by 10cm (4in) on every side,” he says. “However, most people barely notice the difference in the end, and the added comfort is far more valuable than a few centimetres of wall.”

Jones cautions that making a building too airtight can cause problems when it comes to ventilation. “You really don’t want to encourage mould growth, or create poor air quality,” she says. Installing a mechanical vent that circulates fresh air, while recovering the heat can help here.

Accidental emissions

While adding insulation and other measures to make your home more energy efficient might seem to make sense, there is a risk that the carbon footprint of the construction materials themselves could cancel out any savings made.

“It can sometimes be a zero sum game,” says Ahmed Khan, a professor of sustainable architecture in Belgium.

Through research at the Université Libre de Bruxelles, he found some of the most energy-efficient products, such as brand new insulation materials and triple glazing, can be more carbon heavy over their lifetime due to the energy needed to manufacture and transport the materials. This is called embodied energy.

There is a sweet spot though. “It’s possible to achieve a very high energy efficiency and low embodied energy, so long as you pay attention to the sources of materials,” says André Stephan, expert in energy efficiency at the Univerité Catholique de Louvain.

Favouring materials that come from natural sources, repurposed waste or reused materials can help. “It truly depends on the location of your home, but as a general rule materials which are bio-based and not fired at very high temperatures are typically less energy intensive,” says Stephan. Less energy intensive means less greenhouse gas emissions.

Rather than fibreglass insulation, cellulose fibres, mineral wool from mineral waste, cork and even wasted blue jean fabric can be used. “Reaching passive house standards or equivalent with these materials is ideal for a low carbon footprint across the life cycle of the property,” says Stephan.

Smart heating

In the remains of a coal mine in east Belgium is a laboratory which is experimenting on another potential solution for keeping carbon emissions down. This is the EnergyVille complex where Chris Caerts researches “smart” fixes to improve energy efficiency. He’s often asked, is intelligent technology the answer to emitting less carbon? 

“Compared to having a boiler switched on all the time, a remote app which turns it off when it is not needed will appear revolutionary,” he says. “But a well-programmed ‘dumb’ thermostat could achieve near the same savings.”

“We need systems that work automatically and learn how to make the most of changeable energy such as wind and solar.” That might mean a hot water system that activates when it receives a strong signal from the solar panels, or even knows when to expect one.

In theory, an intelligent heating system should be able to make similar decisions on a daily basis, with information from regional wind and solar farms. Consumption could be timed when there is the greenest energy mix forecast.

For those with gas boilers, a smart thermostat which learns which rooms you use and when could also help to reduce your carbon footprint some, says Caerts.

While truly intelligent, flexible systems that adapt to the energy mix might still be some years away, a change in mentality can start now. The National Grid, for example, launched an app earlier this year which allows users to work out when the greenest time to use electricity is. One website even offered those taking up baking during the pandemic lockdown to decide the best time to crank up their ovens in order to have the least environmental impact.

“We need to be thinking about how we can steer consumption to maximally coincide with times in the day when the carbon intensity is the lowest,” adds Caerts.

With many of us likely to be spending more time in our homes this winter, it could make a big difference.

BBC

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