Riverside Primary School in Perth, Scotland’s first Passivhaus-accredited primary school, has released its first-year report on actual operational energy performance – and the results are outstanding.
With an energy assessment of just 43kWh·m–² per year, the school, designed by Architype, is significantly outperforming the classic Passivhaus target of 60kWh·m–² per year.
Passivhaus standards and certification requirements prioritise energy efficiency and minimal heat loss. But while the focus is rightly placed on the primary building geometry and fabric performance, careful consideration must also be given to engineering solutions, plant selection and building-user operations.
‘When designing the system, the hot-water strategy was one of the main challenges, as we needed to avoid large-scale energy use and heat losses,’ says David Coulter, associate engineer and certified Passivhaus designer at BakerHicks (Motherwell), which provided mechanical and electrical design services on the project.
Energy use of Riverside Primary School
43kWh·m–² per year
Traditional buildings are often designed with a centralised hot-water system, which can result in significant heat losses during distribution and long wait times for water. Both factors contribute to energy waste and increase a building’s overall energy demand. The challenge of a low-energy design solution is overcoming the key sources of energy waste associated with a centralised water-heating system. These include heat loss in long pipe runs, standing energy losses and lag time in hot-water delivery.
‘We need a solution that heats water only where and when it is needed,’ says Coulter. ‘Point-of-use [POU] water heating is an effective approach. By providing immediate hot water at the source, it ensures availability when required while significantly reducing distribution losses and reheat times.’
Selecting a standard POU water heater with 10-15 litres of storage volume to serve a single appliance is common practice. While the capacity typically provides sufficient hot water, and aligns with the principles of POU water heating, it can often be oversized compared with the actual hot-water demand of the appliance. Oversizing can lead to unnecessary energy consumption, reducing the overall efficiency of the system.
Working with Baxi’s sales and specification team Coulter identified the multiple POU water heaters required to serve wash hand basins (WHBs) located near classrooms at the school.
‘As these basins are primarily used for handwashing, it was possible to reduce the storage volume and associated electrical energy use [electric kW duty],’ says Coulter.
‘The general guidance for handwashing is 20 seconds per person. By comparing this timeframe with the available storage volume in the POU water heater and the maximum flowrate of the wash hand basin, we identified an opportunity to further optimise the design, enhancing efficiency while maintaining functionality.’
A significant reduction is possible when downsizing from a standard 15-litre (3kW@240V) storage unit to a more efficient 7-litre (2.2kW@240V) storage unit. Even greater savings can be achieved by considering the temperature range against flowrates, as the water is regulated to a lower flow temperature via a mixing valve.
Making the calculation
The energy use of the 7-litre option was calculated as follows [where the flowrate l/min is WHB flowrate l/min x (mix temp-cold temp)/(hot temp-cold temp].
For one specific WHB in this project (used in general teaching spaces), the calculated flowrate is 2.44 l/min, allowing for up to eight handwashes from the 7-litre storage capacity. This calculation does not account for the unit’s recovery rate, with 2.2kW equating to a 10-12 minute recovery time, which further informed the design decision.
The overall reduction in total volume and electrical energy resulted in significant energy savings compared to the standard selection process featuring a 15-litre vessel. When scaled across multiple units, this significantly reduced the building’s overall energy demand.
Taking into account flow rates, recovery times and usage, we were able to provide a diversified approach based on operational usage rather than a worst-case scenario of the sink always being on at 100% duty. The overall water storage for the project was reduced by 25%, with hot water W/K heat loss decreased by 30%.
A centralised approach
The concept design for the nursery followed the POU approach that had been applied successfully in the teaching spaces. However, during energy calculations, BakerHicks noted that the electrical energy use associated with the nursery was significantly higher because of the number of POU water heaters. This was having a negative impact on the building’s overall energy efficiency.
‘We investigated various solutions, including multipoint POU water heaters and a centralised storage electrical calorifier,’ says Coulter. ‘When we analysed the area in isolation, and assessed the impact of electrical demand in relation to standing energy losses and the delivery of hot water, we found that transitioning to a centralised approach could offer huge benefits.’
The electrical demand of the proposed centralised calorifier was 6kW, but, in practice, only 3kW was used, with 3kW provided for resilience. In comparison, multiple POU water heaters had a maximum demand of 15kW. The result was a significant reduction in the building’s overall energy demand by treating these isolated areas with a centralised system rather than a POU.
‘As the area contained appliances within a short distance, the centralised approach provided a saving against standing losses, reheat times and other factors,’ explained Coulter.
‘While low-energy principles were applied in selecting POUs, engineers should still explore alternative solutions [such as the centralised approach], as these may prove more beneficial when considering factors such as location, usage patterns and overall system energy demand.
