NABERS UK | DYNAMIC MODELLING TECHNICAL RISKS The authors of a paper on Nabers independent design reviews1 have identified technical risks that could affect target operational ratings and have offered potential solutions Heat-recovery variable refrigerant flow (VRF) system Zonal thermal diversity tends to favour heat-recovery VRF systems. With smaller VRF buildings delivered as a shelland-core base building design, stress test poor design by the tenant by removing the diversified internal zone loads. Water-to-air heat pumps Unless enforced via tenant leases or a CAT-A HVAC design, any condenser water valves for the compressor units are likely to be open for long durations. This will lead to the pumping system acting as a constant flow, instead of variable flow, system. This risk is mitigated if continuously modulating condenser water valves linked to compressor load is specified. Air-to-air heat pumps Where a single heat pump serves interior and perimeter zones with terminal reheats, the high efficiencies of heat pumps in heating mode is not capitalised, and reheat energy will be high. This is because the heat pumps will operate to satisfy the warmest zones, leaving any colder zones to be reheated using terminal reheats. Where this result is not observed in the simulation results, revisit how the air transfer between HVAC thermal zones is modelled by the simulation software engine, or consider adding internal partition walls to ensure this is modelled correctly. From a mechanical design perspective, this system should be designed with separate perimeter and interior zone heat pumps. Centralised cooling and heating plant Standard designs seem wedded to the use of plate heat exchangers at each tenancy for hydraulic separation. This practice is advantageous to enable tenant fit-outs without affecting the broader hydronic network and pseudomonas risk management. However, this is at the expense of pressure losses across the heat exchanger (typically between 10kPa and 40kPa) and, more notably, restricts execution of temperature resets that increases chiller and heating plant efficiency during building partial loading. As a result, the system regresses to operating as a constant temperature system, which can perpetuate the low delta T syndrome. While uncommon, some projects did specify a design without plate heat exchangers, suggesting that the alternative configuration without them, the Australian norm, is possible in the UK. Related to the above issue is the practice of domestic hot water (DHW) calorifiers and space heating sharing the same LTHW plant. Space heating demand is seasonal, while DHW demand is steady-state across the year. The common issue observed was domestic water temperature requirements restricting the ability for the LTHW temperature to be reset downwards when space heating demand is low. Designs should consider the ability to service the DHW load separately from the centralised plant (for example, via a separate hydraulic connection to a dedicated heat pump) or, at minimum, a separate hydraulic riser for DHW, so it is not on a riser shared with space heating. References: 1 Design for Performance: Lessons from the NABERS UK independent design review process, Foo, Bannister, Cohen, CIBSE Technical Symposium, April 22 Download the paper at bit.ly/CJAug22Nab2 This valve comes with dedicated technical support. Just like all our others. Our Technical Team is here to support your project with a collaborative approach and a Fast Track Service. So, when you need the complete valve solution none of the others will do. Find out more at hattersley.com/justlike Alison, Technical Co-ordinator 21400_HATTERSLEY_TECH_SUPPORT_133X186.indd 1 24 August 2022 www.cibsejournal.com 14/02/2022 17:30