C36 -- Net Zero Performance Assessment in the United Kingdom: Predicting and Optimising Electric Hot Water Energy Usage

In the United Kingdom, decarbonization assessments for new and existing buildings are now being implemented to achieve Net Zero operation as part of whole life carbon assessment. This is achieved with low fabric U values, low air permeability, low window g-values and use of low and zero carbon heating and cooling technology, such as high co-efficient of performance air source heat pumps. Another high energy usage system is domestic hot water, as electric type systems have a detrimental effect on energy consumption during the year. Hot water usage can have a significant impact on predicted energy usage intensity (EUI) for a building by up to approximately 30 percent, particularly when all electric strategies are adopted. As buildings move towards Net Zero hot water heating strategies, local point of use water heaters and minimizing large calorifier storage vessels are utilized, predicting accurate energy use is particularly difficult due to lack of available tools and approximating usage guidance by the end users. In the UK, simplified building energy models (SBEM) approximate EUI using generic formula in the form of pro-rata steady state correlations linked to generic national calculation methodology (NCM) parameters. Another method of assessing operational hot water demand is via CIBSE TM54, which provides more detailed control parameters but lacks dynamic and response factors. At present, there are no digital tools available that thermodynamically assess dynamic hourly operation of the electric hot water systems. To provide more accurate EUI associated with commercial hot water strategies, this paper provides fundamental analytical assessment of modern electric hot water strategies generating EUI values (kwh/m².yr) that can be used within ASHRAE 90.1-2022, 189.1-2020 and CIBSE TM54 assessments. All values are intercompared with existing standards including empirical point of time verification.