Getting fit for retrofit

Much of the UK’s housing stock, particularly homes that are off the gas grid, present a challenge to the drive towards improved energy efficiency

Much of the UK’s housing stock, particularly homes that are off the gas grid, present a challenge to the drive towards improved energy efficiency. However, as oil prices continue to rise and resources become scarcer, practical alternatives need to be found.
The performance of renewable heating and hot water systems in older properties was not always considered adequate. However, more recently it has become apparent that – with a combination of insulation upgrades, precise design and realistic specification – retrofit can work. As a result, more renewable systems are being successfully retrofitted into a variety of older buildings.
Insulation
One of the main reasons that retrofitting renewables presents such a challenge is due to the heat loss associated with older properties. It would be impractical to install a heat pump into a building that has insufficient insulation. In such a case, the cost of running the heat pump might outweigh the benefit of switching from fossil fuels.
However, for properties that can be upgraded so that they require less than 80 watts per square metre (W/m²) of floor space (or for most properties built within the last 10 years that meet the higher insulation requirements of Building Regulations) a heat pump – either on its own or integrated with solar thermal – could present an alternative. The Green Deal specifically targets the retrofitting of older properties and should help with the improvement of insulation levels through the installation of various combinations of loft, door and wall insulation, and double glazing.
Location
Before a heat pump is specified, practical matters must be looked at. A ground source heat pump (GSHP), for example, requires an area of land up to three times the total floor area of the property in which to lay the ground array. Alternatively, boreholes can be drilled – subject to ground conditions. A small plant room is usually required to house the heat pump and the other components.
With regard to air source heat pumps (ASHP), machines should be sited fairly close to the main building because long pipe runs may cause an excessive pressure drop. It is important to avoid any acoustic impact on neighbouring properties, so the unit needs to be located a reasonable distance from occupied rooms. Units are usually placed on a solid base, such as concrete. Suitable indoor space will be required for the rest of the system components, including the DHW cylinder and buffer tank.
Emitters
Key to the effective installation of renewables in retrofit is the design and specification of the heat emitters. Simply oversizing standard panel radiators to compensate for the lower flow rate from the heat pump is unlikely to work. Underfloor heating (UFH) can be a better solution as it can achieve 80W/m² or more in a retrofit situation, depending on the floor construction and covering.
Inrush
A separate consideration in retrofit is the draw on electricity (known as the inrush current) when the machine switches on. This can be very high with some heat pumps – as much as 50 amps or more. In a new-build installation, this is usually less of an issue as it can be factored into the electricity supply arrangement, but it may cause flicker in older properties.
Sizing
One of the concerns with heat pumps, particularly in retrofit, was correct sizing. The MCS standard MIS 3005 was introduced in 2011 to ensure that a heat pump will produce enough power to heat the whole building. A key requirement of the standard is for a detailed heat loss calculation to be performed (as per BS EN 12831). The building’s heat load is worked out room by room – considering the fabric loss of each element (where an element is a wall, window, roof and so on) as well as that room’s ventilation heat loss.
BS EN 12831 specifies the temperature required for each room, for example 18°C for a bedroom and 21°C for a living room. MIS 3005 requires that the heat pump is large enough to be the sole heat source of the building for at least 99% of the heating season to ensure that the need for supplementary heat sources (usually direct electric immersion) is minimised. Location can also play an important part in the sizing requirements of the machine – the heat load in Glasgow will be higher than in Plymouth, for example.
Manufacturers and suppliers are constantly striving to make this process as straightforward as possible. Nu-Heat, for example, has developed its own specialist software, Predictor, to ensure the correct sizing of heat pumps through performance simulation. Predictor takes published data from various manufacturers into a common platform, so performance levels of any make or model can be easily compared.
Solar thermal in retrofit
For older properties that cannot achieve low enough heat loss for renewable space heating, even after insulation upgrades, solar thermal is an excellent alternative because it offers an annual average saving of 50% on hot water bills, depending on location and the amount of sunshine. Across a typical year, around 90% of the hot water demand will be met in July and 10% in December.
Solar thermal is the least intrusive renewable technology for retrofit properties. The roof should ideally be south-facing with a pitch of between 30° to 50°. However, east and west facing roofs can work – even on shallow pitches – it just means that more panels are required. Installation should be relatively straightforward, particularly when on-roof panels are used, as the roof structure is less likely to be disturbed. It is possible to install flush-mounted, so-called “in-roof” systems in retrofit projects as long as appropriate consideration is given to the specific roof structure.
Integrated
Where insulation is sufficient and a heat pump is viable, solar thermal can be integrated with the renewable heating system to achieve even greater energy savings. For example, a combined heat pump and solar system allows each technology to perform in tandem at their most efficient.
Solar thermal panels can only generate hot water during daylight hours, while heat pumps work 24 hours a day. The amount of heat generated by an ASHP is inevitably affected by outside temperatures, but they have been shown to work in temperatures as low as –25˚C. A GSHP utilises latent heat from at least a metre underground where the temperature is generally around 10–12˚C so efficiency is maintained in all weather conditions.
Cylinders
Appropriate hot water cylinders should be specified for any heat pump system, including retrofit. For example, Nu-Heat’s EnergyMaster range includes models that have been specifically designed for such installations and include versions with a solar coil and dedicated solar volume.
MCS and RHI
MCS helps to ensure that renewable heating systems are installed to high standards and that the customer ends up with an efficient system. Both installer and product must be MCS accredited – for the customer’s peace of mind and to enable payments from the domestic Renewable Heat Incentive (RHI).
The opportunities are there for installers to work in the retrofit sector, and it is crucial to make sure that, by using solid design principles, increasing numbers of older buildings are able to benefit from greatly improved energy efficiency.