Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 45 often necessary to seek specialist guidance and take precautions to avoid affecting significant archaeological finds, which can include whole Saxon villages. This can make it problematic to excavate large areas of a site to develop schemes such as ground source heat pumps. Choosing the most appropriate type of energy for some National Trust properties can be a difficult challenge. For example, the trust’s historic art and house collections at Beningborough Hall in North Yorkshire require lower heat levels and would only need a 90kW biomass boiler, but this option may not be financially viable, especially following substantial reductions in payments made under the Renewable Heat Incentive in 2015/16 for biomass heating installations of less than 200kW. On the other hand, a heat pump might not provide the right solution either, because the heat emitters are too small. Ground-source heat pumps work most efficiently with underfloor heating systems due to the lower temperature requirements of a large emitter, but installation is rarely possible due to both conservation issues and financial constraints. WOOD FUEL Log heating systems such as stoves and boilers are ideal for houses but in larger properties they require more frequent refilling so in these environments other types of wood fuel such as wood chips and pellets are mostly used in automated systems. Wood chips can be made from virtually any kind of woody biomass, including whole trees, by a chipping machine. This makes it possible for the trust to supply fuel from its own estates. Wood chips are typically used in automated systems making them a clean and convenient heating option for the trust. Pellets are relatively new in the UK but they have been used in central Europe for some time. They are produced from wood by-products such as sawdust and have a better calorific value which means the energy to weight ratio is very favourable, so they are more appropriate for smaller spaces. The visitor building at Penrhyn Castle in Wales has a new wood pellet space heater with a hot-air convector built in, which heats the whole building. In simple carbon dioxide emission terms the log stove at another trust property, Llanerchaeron tea room, emits considerably less carbon dioxide per kWh than the new high-tech pellet stove at Penrhyn. Research has shown that log stoves emit around 4g of carbon dioxide per kWh compared to 34g per kWh for a wood pellet system (and around 500g per kWh for an electric heater using power from the grid). Deciding whether a pellet stove or a log stove is more suitable for a particular site can come down to the ability to manage the stoves. Cutting, hauling, drying and splitting logs, or just supplying them, as well as loading and cleaning the stove are all time-consuming and members of staff have other tasks to carry out. It can come down to the simple question: ‘Do you have the space to store the fuel and the time to manage the fire?’ Expense can also be a consideration. The Tigchelaar wood-fired storage heater or ‘masonry stove’ at Llanerchaeron is over 90 per cent efficient and a very good space heater but it is also twice the price of some stoves. On the other hand, there is a simple Clearview stove space heater in Colby Woodland Garden which is significantly cheaper than the masonry stove, and far cheaper than any pellet system. Fuel is important and the trust ensures that its wood fuels are produced in the UK from FSC timber and from as local a supplier as possible, if not from its own estates. Wood chip and pellets must also conform to the relevant standards (including DIN 66 165). In some cases, using the natural resources that properties and estates have access to creates additional conservation wins. The biomass system at Croft Castle in Herefordshire uses wood from conifer trees on the estate to heat the property. Removing the conifers has exposed ancient wood pasture and led to an increase in biodiversity. CASE STUDY 1: A wood pellet range cooker in a farmhouse in Snowdonia At Hafod y Llan in Snowdonia, the trust experimented with a Klover 120 wood pellet range cooker. The requirement was for a viable, economic, manageable biomass cooker and central heating appliance which could simply replace the host of oil-fired range cookers (Aga, Stanley, Rayburn, Esse, etc) used in many similar farmhouses and cottages. The building is a fairly typical three- bedroom farm-house with moderate levels of insulation, draught-proofing and retrofitted windows. The appliance had no problem at all heating it. When used all day, two 20kg bags of pellets were consumed and this fell to one bag a day if the property was heated only in the morning and evening. In the summer months just half a bag a day was used for hot water. A fossil-fuel boiler was retained as a backup, but it has never been needed. In winter-heating mode the fuel was burning much more cleanly and only leaving a very fine ash. In summer there was some partially burnt pellet but this was not an issue. The daily and weekly controls were not very intuitive at first but are adequate once members of staff get used to them. As a cooker it performed well overall, if a little less refined than an Aga. The oven could be a tad hot (200°C+ top and 180°C bottom of the oven), and it was a matter of trial and error at the start. Using the hob plate also took some practice, with a range of temperatures across the surface. A simple slot-in electric plate cooker was also provided for minor cooking requirements (boiling an egg for Wood-fired storage heater or ‘masonry stove’ at Llanerchaeron, Ceredigion