25 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 PROFESSIONAL SERVICES 1 and reliable, but generally they deliver water at a lower temperature than a gas boiler and are better suited to underfloor heating than conventional radiators. The size/capacity of the heat emitters (radiators or underfloor heating) must be carefully matched to the heat demand of the building to ensure that the heat pump can deliver sufficient energy. Threading new pipes through historic fabric can be a challenge but there are often opportunities to think creatively and work with the existing fabric that allows work to be carried out without any damage to the building. Heat pumps typically produce two to three times as much heat energy than the electrical energy they require to work. The integration of the heat pump with a hot water tank to heat water is highly recommended as a heat pump cannot provide the instantaneous supply of a combi boiler. Also, the requirement for additional space can be particularly difficult in smaller dwellings where the introduction of a dedicated energy centre or the use of the basement to locate the equipment may be needed. Heat pumps can be coupled with photovoltaic systems to increase the efficiency of a property, but consent for the panels will be required where the building is listed. Given the phasing out of conventional gas boilers and where future heat demand of a building during colder periods cannot be met by a heat pump alone, a hybrid boiler/heat pump could be considered, but only if the fabric is to be improved to a point where the boiler is no longer required before gas supplies end. Biomass is another option and wood burning stoves and boilers may seem to offer attractive low-carbon alternatives. However, sustainable timber takes time to grow and needs to be transported and there is, at best, a short-term carbon penalty for using biomass. Proper evaluation is rarely given to the embodied carbon in harvesting and processing the crop and in transporting it. Biomass use also impacts on air quality and its production competes with our farmland resource for food production. Hydrogen could also help us to move away from gas without the requirement to improve the fabric performance but currently it has many unknowns. At present, most hydrogen is produced from fossil fuels, and with carbon capture it is neither a cheap nor an easy option for domestic heating and would be discounted for any retrofit taking place in the short to medium term. Direct electric heating systems use electrical energy without any supporting mechanisms such as heat pumps. Electric heating and hot water systems can be attractive due to their simplicity and typically, installation comes at a lower capital cost when compared to a ‘wet system’. However, the building will be significantly more expensive to run when compared to either a boiler or heat pump. A direct electric system also results in higher peak loads and so is not desirable at scale for the grid system. The use of storage heaters can help to mitigate this, but generally direct electric should only be considered where heat pumps are not feasible and where heat demand is very low. District heating systems for heating and hot water are available in certain locations, but many networks are powered by gas. Combined heat and power (CHP) systems, too, require a transition plan away from fossil fuels. If heat pumps are used as an alternative generation plant, the lower temperature of the hot water generated can present issues such as heat loss so such systems will need to be carefully modelled. Heating networks can be expensive and/ or unreliable but could be considered where large-scale retrofit is taking place, for example, as part of a neighbourhood regeneration scheme. For larger historic properties, an ‘energy centre’ can often be created on site using redundant buildings while retaining their historic fabric. The centre can provide heat, electricity and cooling to a group of buildings through a decentralised network with no need for an individual plant room in the main building itself. Heat losses for such local distribution networks can be mitigated by reducing the distance between the energy centre and the main buildings. Table 2 summarises the active strategies that can be deployed. BALANCING PASSIVE AND ACTIVE STRATEGIES In reality, the solution may lie in not one or the other but a combination of both passive and active strategies and, as said, every project comes with its own challenges and opportunities. While broadly some of the solutions and optimal ways forward may be similar, the case for every property should be addressed on its own particular circumstance and an appropriate strategy for raising its energy performance level should be developed accordingly. When it comes to historic and traditional property there is no one size fits all. A too rigorous fabric-first approach, following Passivhaus standard for example, could be unrealistic, especially when applied to listed properties. Systems could be incompatible with the significance of the building or the components affected, and in some cases, the embodied carbon of insulation systems can be too high to be offset by the operational savings, particularly where a component has a relatively short service-life. Systems could be incompatible with the significance of the building or the components affected and embodied carbon of insulations too high to be offset by the operational savings achieved by their application. The right selection of passive and active actions is project dependent, working with a team that has researched, understood, and aligned the proposed strategies with the principles of construction of historic buildings. As part of this design process, the use of building performance evaluation (BPE) can help to inform decision-making and evaluate the individual and collective impacts of the solutions chosen. BPE can be undertaken in the form of dynamic energy modelling studies, steady state calculations, life-cycle carbon assessments (LCA) and post-occupancy evaluation (POE) with the scope to assess energy performance, carbon footprint and occupant comfort. They allow a building’s future performance to be predicted and also make comparisons with the established design targets. There is much to consider but what is certain is that accounting of embodied carbon and assessing the different strategies, whether active or passive, in approaching retrofit of our historic and listed buildings are absolutely key in adopting the right approach to the preservation and safeguarding of our heritage and built environment. DR MARTINA PACIFICI is Senior Associate Sustainability Lead at ADAM Architecture (see page 13). Natural sheep insulation being installed between the existing rafters
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