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Max Fordham - Cranmer Road Kings College Cambridge WWHR integration.
Image credit Nick Guttridge
This newly built Student Accommodation project at Cranmer Road consists of two new accommodation buildings for Kings College Cambridge, which have been designed to Passivhaus principles. Totalling 59 en-suite student bedrooms, each of differing contemporary character, but both designed to sit comfortably alongside the existing arts and crafts architectural style of the surrounding villas.
As a Passivhaus-designed, ultra-low energy, fully electric development, with a low embodied carbon structure and low operational carbon emissions; the Cranmer Road development should be considered an exemplar student accommodation project, that demonstrates many of the approaches to sustainable and low-energy design that are necessary for projects aspiring to achieve net zero carbon.
Waste Water Heat Recovery for Showers (WWHRS) has been included to reduce both DHW energy demand and the volume of hot water required for showering, by effectively transferring heat energy from shower wastewater (usually destined for the sewer), to preheat the incoming cold water that feeds the shower and water heater.
Recoup supported both Allies and Morrison Architects and Max Fordham, the latter of whom provided the MEP, Passivhaus, and Acoustic services for the project with the WWHRS integration.
"The College has a history of high-quality architecture, and the Cranmer Road student accommodation lives up to these standards. Using timber as the primary structure helped to limit the scheme's embodied carbon, while operational energy is kept low through Passivhaus. Electricity is the sole fuel source, which works well with the ongoing decarbonisation of the UK electricity grid. We've enjoyed working on the project as MEP consultant, Passivhaus designer, and acoustic consultant from early design through site delivery and post occupancy evaluation." - Gwilym Still, Passivhaus Leader at Max Fordham
Electricity is used as the sole energy source for both buildings, chosen to leverage the continued decarbonisation of the electricity grid. Mechanical, plumbing and heating systems are decentralised to limit distribution losses, while Mechanical ventilation with heat recovery (MVHR) is used to ensure good air quality year-round and minimise heat losses further still.
The building services design is lean and intended to minimise internal heat gains and prevent overheating risk. Domestic hot water, the majority of which is used for showering, is produced by instantaneous point-of-use electric water heaters, with WWHRS localised to each student dwelling. A mix of ground-floor and first-floor showers required a combination of Recoup’s Easyfit+ and Pipe HEX products servicing the ground and first floor showers respectively.
The WWHRS units are installed in the most efficient System A method, where pre-heated CWM is sent from the WWHR unit to the point-of-use electric water heater, and to the cold side of the connected showers TMV. This way the Pipe HEX and Easyfit+ units are reducing the amount of energy required for hot water generation and reducing the amount of hot water used for every shower. Low-flow showers are also installed, which results in both, a lower volume of water being used per shower, as well as an increased Heat recovery efficiency of the WWHRS Systems.
The two-storey low-rise blocks utilise the Recoup Pipe HEX vertical WWHR system, sited within service risers on the ground floor, to service the showers on the 1st floor. The shower wastewater is fed to the Pipe HEX under gravity from the floor above.
Recoup had early visibility of the proposed floorplans for the project, and so it was possible to design the layout such that 2 first floor showers are serviced by a single Pipe HEX (2:1), almost halving the upfront cost and ROI versus a 1:1 design, without any compromise on efficiency or effectiveness of the WWHR integration.
For the ground floor showers where a vertical gravity fed WWHR system such as the Pipe HEX cannot be utilised, the showers are instead paired with the Easyfit+ horizontal WWHR unit. The bathroom "pods" used for the bathrooms have been designed to accept the Easyfit+ discreetly below the shower tray (which also bridges across under a false floor in an adjacent service cupboard in each room).
The summer comfort strategy for the accommodation blocks uses a combination of automatic bypass of the heat exchanger on the mechanical ventilation units, and openable windows. The design uses simple, standard components in systems that were easy to commission and will be simple to maintain.
The buildings' achieved airtightness results three to four times better than the Passivhaus limit, measured peak heating demand has been 8 W/m², lower than the Passivhaus target of 10 W/m².
Max Fordham are using Post-Occupancy Evaluation to learn lessons, fine-tune the buildings, and improve future designs. This includes a BUS methodology occupant survey, analysis of meter readings, and continual monitoring against a predicted EUI of 47 kWh/(m².a) baseline energy use, and specific DHW energy use.
Interestingly, initial data from the monitoring demonstrated higher than expected DHW energy use, vs the PHPP model. This has been attributed to the students taking around 7-8 minutes per shower on average, whereas PHPP modelling assumes a 5-minute shower duration, at 0.7 uses/person. per day. This has been borne out in the DHW consumption exceeding the PHPP predictions, which only goes to illustrate the importance of the WWHR integration at Cranmer Road.
"King’s College is delighted with the Cranmer buildings, their quality design, construction, finish and energy effectiveness. From the start the students were involved in the planning for low-carbon yet comfortable buildings, and it is believed this has been achieved. The buildings were constructed to an exacting high standard yet are relatively simple in layout, and ideal for study in quiet and well-insulated rooms. The ‘household’ design of clusters of rooms and associated kitchens proved to be ideal during the current pandemic, where the students had the additional benefit of space to work and live together while not being crowded." - Philip Isaac, Domus Bursar at King's College Cambridge
Cranmer Road was named Project of the Year (non-domestic) at the CIBSE Building Performance Awards 2023. The judges praised the continuity of design, delivery and operation of the new Passivhaus student blocks, and the focus on optimising building performance.
They highlighted the feedback from the surveys used to further improve energy performance; comprehensive commissioning; and detailed analysis of the impact of a range of future climate scenarios, with results of the analysis influencing design and construction.
Cranmer Road was also named People's Choice Winner in the 2021 UK Passivhaus Awards.
Data and quotations have been taken from: Passivhaus Institute, Cranmer Road Awards phase I submission slides and Maxfordham.com https://www.maxfordham.com/projects/cranmer-road-student-accommodation