ZERO CARBON HOMES | HEATING SYSTEMS Results This demonstrated that comparisons based on systems efficiencies can be misleading. It shows the shortcoming of SAP in its inability to accommodate varying COPs across the year. To take account of the outdoor environment, a temperature banding annual model was set up. This was run for a representative outdoor temperature in each band to establish a heat balance for a representative dwelling, and permitted an appropriate heat pump COP to be selected for each outdoor temperature band. A zero carbon future-proofing assessment relates to the ability of a system to switch to 100% zero carbon energy sources within the life of the system. Onsite PV renewable energy generation was assumed to be generally insufficient because the study was focused on high-density development. Biomass and similar renewables were disregarded because of urban air quality concerns. Hence, all renewable energy for heating was assumed to be delivered via the electrical grid. For assessment of maintenance and service charges, experience has been drawn from the FM side of the construction industry. The level of maintenance was assessed as being broadly in proportion to the number of active components in the system. Network energy standing losses were also considered as part of the standing charge using SAP10 defaults, as these better reflected monitored data from recent completed projects. included the building fabric enhancements. This unit was originally developed and has gained a proven track record as a Passivhaus-certified product. Many thousands of the units have been installed, largely across southern Scandinavia and Germany. A key aspect of its proven track record is the integration of MVHR, heat pump, hot-water storage and controls in a single unitary box that is factory assembled and tested before delivery to the site. This is unlike the UK convention of multiple components from various suppliers such as MVHR, HIU, metering, heatingemitter system, and controls being assembled and configured under less-thanoptimum site conditions. Its unitary configuration and lack of distribution heat pipework makes it ideal for modular offsite building fabrication. Its electrical power demand is small enough to operate from a standard 13-amp outlet without any enhancement to dwelling electrics. Each system was ranked under each characteristic and then these were combined to give an overall rating (Figure 4). This preserved a level of transparency given that each of the different contributing stakeholders had differing opinions on relative importance of each criterion. The highest-rated shortlisted systems were then tested for market acceptability, proven kit availability and indirect cost implications. This meant systems such as the individual air source heat pumps, although dominant in various markets abroad, were discarded as unsuitable for UK high-density housing. The system that emerged as most favoured was the one that harnessed most of its heat from that already available inside each dwelling and that avoided needing any siteimported heat deliver. This heat autonomy system uses an in-dwelling unitary two-stage ventilation heat recovery and heat pump (two-stage MVHR+EAHP) (Figure 5). It exploits the waste heat from occupants, appliances, cooking and showers, which is captured via extract ventilation and upgraded using the small exhaust air heat pump. This has a heating capacity of around 1.5kW for delivering both space heating and hot water. To achieve this requires high dwelling envelope thermal performance, so the assessed cost for this system also Enhanced fabric To allow the use of this two-stage MVHR+EAHP unit, building fabric enhancement was investigated in some depth, and a stripped-down version of Passivhaus was found to deliver a capital cost sweet point. The modelled housing in London achieved a peak heating capacity closely related to Passivhaus recommendations. This was Zero carbon future Energy saving Service charge Bills Capital cost Sum 1. Individual gas boilers 1 1 3 3 5 13 2. Gas boiler district heating 1 1 2 2 2 8 3. Gas boiler + CHP district heating 1 2 1 1 1 6 4. Gas boiler + ASHP district heating 3 3 1 1 1 9 5. Gas boiler + GSHP district heating 3 3 1 1 1 9 6. Central ASHP + heat network + direct-electric DHW 4 3 2 2 3 14 7. Central ASHP + heat network + individual WSHP 5 4 2 4 2 17 8. Individual ASHP with direct-electric DHW top-up 4 3 5 3 5 20 9. One-stage EAHP + direct-electric heating 4 2 4 2 4 16 10. One-stage EAHP + gas boiler district heating 1 3 2 3 2 11 11. Two-stage MVHR+EAHP 5 5 4 5 4 23 Ranking: 1 Poor 2 3 Mid 4 5 Best Grid electricity accessibility; peak demand management; smoothing of demand peaks Code compliance; standing losses; COPs; source temperature; network temperature System extent; network losses; component count; interface units; billing system; gas servicing Energy billed; service charges; standing losses; outsourcing overhead System extent; component count; construction interfaces; modularisation potential Figure 4: Ranking of systems Key considerations: System 58 November 2019 www.cibsejournal.com CIBSE Nov19 pp56-59 Chris Twinn.indd 58 25/10/2019 18:06