11 minute read
Low-carbon heating
Could hybrid heating be the ideal stepping stone to help the UK start a large-scale shift to low-carbon heating systems? Registered Gas Engineer finds out.
The march to net-zero by 2050 is continuing, with the government pushing hard to reduce carbon emissions generated from the heating and hot water in our homes. Current plans to ban gas boilers in favour of heat pumps take a long-term view, with the Heat and Buildings Strategy signalling the government’s aim to “phase out the installation of new natural gas boilers beyond 2035”.1
But there are concerns that the UK may not be able to reach its goal of 600,000 heat pump installations a year by 2028.2 Hybrid heating systems could offer a more immediate solution, encouraging people to take that first step towards decarbonisation while having the reassurance of a fossil-fuel boiler to provide their heat and hot water if needed. They can used to renewable technologies without having to switch completely now.
What is hybrid heating?
Hybrid systems combine traditional fossil-fuel heating, such as a gas boiler, with a renewable technology, usually a heat pump, to work together to provide heat and hot water. The heat pump acts as the primary energy source and the boiler provides back-up heat and hot water for times when the heat pump is insufficient, such as in cold weather or when a boost of hot water is required.
Not all hybrid systems are the same: some are readily available while others are being trialled in pilot projects across the country. Systems that link a boiler inside the property with a separate heat pump outside are particularly suitable for retrofit because the existing gas boiler can often be used. In other systems, the boiler and heat pump sit together inside a single unit.
Mark Wilkins, technologies and training director at Vaillant, says: “Hybrid systems work with all types of boiler in on and off-gas areas. Where space is limited, a heat pump can also be installed with a combi boiler, removing the need for a hot water cylinder in the property. The heat pump would provide up to 80 per cent of the heat load, with the boiler providing the hot water and back-up heating for cold winter days.”
Terri Hickman, marketing communications manager at Intergas, says: “When it comes to heat pumps, we’re all for them as long as they’re installed in the right homes. If they’re not, you’ll spend more money to be less comfortable.
“But decarbonising our homes is vital, so right now Intergas supports the hybrid system. This system has proven to deliver up to a 50 per cent reduction in energy bills, which is really encouraging.”
How does it work?
Hybrid heating systems are particularly suitable for harder-to-heat properties, such as older, less well-insulated buildings, where a heat pump alone may struggle to meet the full heat demand.
As well as the heat pump and boiler, systems include a hydraulic module and a control system that monitors the property and automatically switches between the two heat sources to ensure the best efficiency.
The heat pump will generally be the most efficient heat source during the warmer months because the outside temperature is higher, while the boiler will perform better during times of peak demand and in the winter. Once the weather warms up again, the heat pump will take over to minimise the use of fossil fuels, helping users save money and reduce their carbon emissions.
Jonathan Grist, renewable sales director at Viessmann UK, says: “Although heat pumps require electricity to run, they produce around three to four times as much energy as they use, which is an energy efficiency level of 300-400 per cent. By contrast, even the most efficient fossil-fuel boiler rarely achieves an efficiency of more than 94 per cent.
“Many heat pumps can also act in reverse and offer cooling in the summer. By investing in a hybrid system, consumers are getting heating, hot water and cooling from one automated, energy-efficient solution.”
As the UK continues to invest in its energy infrastructure, electricity is expected to become even more low carbon, further improving the energy efficiency of systems that use it – including heat pumps and hybrid systems.
Where can they be installed?
One of the problems with switching to a fully renewable system is that many homes are older and not well enough
insulated to enable them to run efficiently. Because hybrid systems have the addition of a fossil-fuel boiler as well as the renewable technology, they can be installed in more properties, including those with higher heat losses.
Martyn Bridges, Worcester Bosch’s director of technical communications and product management, says: “Almost half the buildings in the UK are currently unsuitable for all-electric heat pump systems, and would need to undergo a deep retrofit, such as solid or cavity wall insulation and floor insulation. Installing hybrid heating in the same properties would likely require a more moderate retrofit, such as high-performance glazing and loft insulation.”
Space can be a factor, because smaller properties may not have enough space for the additional equipment needed for a hybrid system.
Fitting a hybrid system now can be seen as the first step towards making the property’s heating and hot water zero carbon at a later date, spreading out the installation costs. Mark says: “If a hydrogen-ready boiler was going to be installed later, fitting a hybrid system now would ensure that the property was fully future-proofed and, provided that hydrogen gas was made available to the property and the electricity supply is from a renewable source, the home could be subsequently upgraded to a zero-carbon solution by converting the boiler or upgrading to a full heat pump system.”
How much carbon could be saved?
It’s difficult to estimate the exact carbon and cost savings that could be made by switching from a gas boiler to a hybrid system. Running costs depend on the technologies chosen, how the system is used, and the property’s heat loss. But manufacturer estimates put the possible household CO2 savings at 50-55 per cent compared with a natural gas boiler, with some giving higher estimates, especially if the boiler is then converted to run on low-carbon fuels.
Neil Sawers, Grant UK’s commercial technical manager, says: “There are two energy bills (electric for the heat pump and gas/oil for the boiler), but with careful design the boiler should only become active when either the heat pump cannot deliver the power required, or the heat emitters cannot satisfy the heat load at the design flow temperature.
“This ‘bivalent’ point will become the pivot around which the heat pump will decide whether to go alone or request the support of the boiler. From our perspective, the maximum heat loss (at design conditions) should not exceed twice the maximum output of the heat pump. This will help achieve a minimum heat pump contribution of at least 50 per cent of the annual heat loss.
“If the fossil fuel is replaced with biofuel, for example, the carbon footprint could be reduced even further (HVO could reduce oil boiler emissions by up to a further 88 per cent). The more improvements are made, the greater the emission savings.”
How are they installed?
Before installing a hybrid system, engineers must carry out a full room-by-room heat-loss calculation. Mark says: “Generally, hybrid systems are easier to install than a heat pump because there is no requirement to resize the radiators, because the boiler is operating at a higher flow temperature in the coldest part of the year. Wherever possible, we encourage reducing the maximum flow rate to 55°C to make the system more efficient and reduce carbon emissions from the boiler.
“If adding a heat pump to an existing fossil fuel boiler system, this should be sited outside the property. A hydraulic module is then needed to integrate the pipework of the heat pump and the boiler.
“When installing a ground source or water source heat pump, these units should be installed inside the property and a hydraulic module is still required.”
Jonathan adds: “Hybrid systems are generally a retrofit solution: installation is fairly simple, with only a few minor alterations to the system hydraulics required in most cases. The existing gas or oil boiler will remain if it is compatible with the new system, but should a new boiler be needed, it is usually fitted in the same location.
“The new heat pump will just require hydraulic and electrical connections. That said, internal space requirements will need to be considered if an internal hydroblock needs to be installed, as well as wall-mounted or floor-standing buffer vessels.”
What qualifications do engineers need?
The qualifications needed to install a hybrid system do vary. Manufacturers recommend that engineers first attend the relevant product training before starting to install these systems. You will also
1 https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/ file/1032119/heat-buildings-strategy.pdf 2 www.gov.uk/government/publications/the-ten-point-plan-for-a-green-industrial-revolution/ title#point-7-greener-buildings “Fitting a hybrid system now can be seen as the first step towards making the property’s heating and hot water zero carbon at a later date, spreading out the installation costs.”
Why hybrid heating could be the best
of both worlds continued
need to carry out room-by-room heat loss calculations and should seek advice before attempting one if you are unsure.
To wire the heat pump correctly you will need to be Part P qualified, or use a qualified electrician to carry out that work. You also need to be Gas Safe registered to install a new gas boiler, or work on the existing one.
If you’re installing a monobloctype heat pump, then you do not need to be F-Gas certified because you won’t be working with refrigerant. Working on split heat pump systems, however, where the outdoor and indoor units must be connected using refrigerant pipework, may require F-Gas certification.
CIPHE has developed a low-temperature heating course where you can find out more about low-temperature systems, and the Heat Pump Association (HPA) also has a five-day heat pump foundation course.
Iain Bevan, commercial manager, heating and renewables at Daikin UK, says: “The HPA course covers the fundamentals of how a heat pump works, as well as the finer technical details of the installation, and how to select the right heat pump for a particular home. The government is looking at whether to make courses like this one mandatory for heat pump installers.”
Do they qualify for government incentives?
Hybrid systems currently qualify for payments under the domestic RHI, but this scheme comes to an end on 31 March 2022. They can also be installed under the Sustainable Warmth Fund’s Local Authority Delivery Scheme provided that the boiler used is A rated, and under the ECO3 Energy Company Obligation scheme.
However, they will not qualify for the forthcoming Boiler Upgrade Scheme grant. The government says it wants to focus on “the technologies that offer the greatest carbon savings, rather than those which would continue the burning of fossil fuels for heating”.
Andrew Cowdrill from BEIS told the audience at a Hybrid Heating Great Britain coalition webinar late in 2021: “It’s too early to rule hybrid heating systems in or out of the 2050 energy mix.
“They offer a good level of thermal comfort with potentially less need for thermal efficiency improvements [but] we wouldn’t want hybrids to be seen as a route around making costeffective building upgrades.”
Viessmann’s Jonathan Grist says: “The government admits their decision not to include hybrid systems in the Boiler Upgrade Scheme is based upon limited available evidence on how hybrid systems perform in off-gas grid installations. We could argue the fact that further testing is needed in this area, as well as investigating the use of biofuels with hybrid systems.”
And Daikin’s Iain Bevan adds: “We believe that sustainable heating should be accessible for all. We feel strongly that hybrids should be included.” ■
Hybrid heating in action
Heating company GreenACT installed a hybrid heating system created with a ground-source heat pump, gas condensing boiler, solar thermal and solar PV technologies, in a self-build property in the West Midlands.
The room heating was split between the heat pump and the condensing gas boiler. This allowed most of the heating to come from renewable sources, with the boiler topping up the system when the temperature dropped below -3°C.
Domestic hot water was provided by a roof-integrated, flat panel solar thermal system backed up by the boiler, with water stored in a twin-coil solar cylinder. Communication between the solar thermal array and back-up system means that, where feasible, solar energy will always be the priority technology for heating water. A single control system optimises which of the three energy sources is used.
Overall, the system has a high seasonal performance factor of 4.3, delivering a total of 25,956 kWh per year, while only needing 6,036kWh of electrical energy to run. That equates to 77 per cent of the energy for the home coming from renewable sources, with the system expected to save 6.35 tonnes of CO2 per year.
Weather compensation on the heat pump and gas boiler also contributes to a 20 per cent reduction in energy requirements.
