ZERO CARBON HOMES | HEATING SYSTEMS What is the most effective way of heating our future zero carbon homes? Chris Twinn has been researching the issue for five years and has found that standalone autonomous heat recovery systems offer the most benefits when considering high-density homes AUTONOMOUS STATE A A key energy modelling parameter was to be able to adjust heatpump COPs based on outdoor air source temperatures as well as sink temperatures s we ponder the journey to zero carbon for high-density housing, we must consider the most appropriate route for heating our dwellings. A five-year research project has analysed 11 systems for delivering heat and water and concluded that district systems may not be appropriate for the next 50% step-reduction in carbon. The study calculates that alternatives can achieve this with both reduced capital cost and cheaper occupant bills. This research was prompted by concerns from project collaborators: developers questioned the increasing complexity and cost of installation; housing associations were troubled that occupant energy bills were increasing irrespective of targeted energy use reductions; and there was a wide perception of a focus on supply side systems, rather than on cutting energy demand. Context Heating and hot-water systems installation costs have doubled in real terms over the past 10 years (Figure 1). Only a few years ago, the size of a home gas boiler was less than 12kW, Capex comparison for residential in London 2011 2010 measured price converted to 2018 Spons 2011 M&E price book. Includes GFA-NFA/prelims/ OH&P Heat source, space heating and air treatment, ventilation 140 per m2 2019 Budget for 2019 construction start Housing association development of 400 dwellings, 50% affordable in London Budget cost for hot water/ heating 280 per m2 Figure 1: Doubling of residential heating system capital costs for London over fewer than 10 years indicative of increasing system complexity but todays boiler/district heating interface unit (HIU) is typically 35kW or larger. So, counterintuitively, as the thermal performance of buildings has improved over time, the systems capacities and costs have increased. The requirement of adding combined heat and power (CHP) onto the district heating has added a further layer of complexity, and is locking us into high-temperature distribution systems with their fossil-fuels combustion origins and high standing losses. While acknowledging that much of the existing building stock may well continue to have a significant heat demand and so benefit from centralised distribution of heat (particularly as carbon-emitting gas boilers are phased out), there is the opportunity to cut the need for delivered heat in new builds. It has proved difficult to cut the costs and complexity of district systems when serving buildings with reducing demand for delivered heat. Consequently, the service charge portion of the bills which includes central plant and distribution losses, maintenance and management costs are becoming as large as the occupants metered kWh bill (Figure 2). The expected continued reduction in the grid electricity carbon intensity factor and the acknowledgement of increased network losses in SAP 10 means this situation is only going to intensify. So, this research started with the proposition that there may be benefits in both reduced capital and running costs in lowering energy demand en route to zero carbon. Analysis The research programme identified and analysed 11 key system variants (Figure 3), 56 November 2019 www.cibsejournal.com CIBSE Nov19 pp56-59 Chris Twinn.indd 56 25/10/2019 15:53