CASE STUDY | HOULTON SCHOOL Most of the leaking, single-glazed windows were replaced with bespoke lookalike high performance units, and in some cases, with the addition of secondary glazing control of the assumptions, he explains. On the existing buildings, as the insulation increases he says PHPP gets more and more accurate because your temperature doesnt fluctuate as much. Alongside fabric thermal improvements, the team also set out to minimise the existing buildings fabric air permeability. For the Grade II listed buildings an aspirational air permeability target of 3m3h-1m-2 was set with a backstop minimum of 5 m3h-1m-2, while for the new build an aspirational target of 1m3h-1m-2 at 50Pa was set with a backstop minimum of 3 m3h-1m-2. Work to the existing fabric included removal of the historic plaster to expose the wall so that problem areas could be repaired. In addition, most of the leaking, single-glazed windows were replaced with bespoke lookalike high performance units, and in some cases, with the addition of secondary glazing. The fabric modifications were successful; tests showed the actual air permeability of the Power Hall was less than 3m3h-1m-2, while in the Transmission Hall it was still less than the 5m3h-1m-2. backstop. The success in reducing the air permeability in each building to this extent shows what can be achieved in historic buildings like this, says Amy Punter, an associate director and historic building specialist at Hoare Lea, the projects MEP engineers. The reason minimising air permeability is so important, is that it brings the heating loads down to a manageable level; without that we would have had to install considerably more plant, which would have been a real problem given its listed status, she says. Etude and Hoare Lea worked in close collaboration. Punter says that while Etude was developing the energy targets, Hoare Lea was carrying out studies to assess the targets impact on loads and implication for plant sizes. The more you can bring the loads down, the smaller the plant, which in historic buildings like these is really important as it reduces the extent of intervention required, she says. HANDLING AIR The use of AHUs was contentious because the ESFA is not generally in favour of mechanical ventilation, says Punter. Because we were targeting an air permeability akin to Passivhaus standards in the new build blocks and were targeting significant reductions in the air permeability of the existing buildings, a need for mechanical ventilation was established, particularly when the benefit of heat recovery was taken into account in the energy consumption calculations, she explains. Air handling plant is located internally on the historic buildings to avoid external additions; air intakes and exhaust louvres are located in dummy windows to minimise visual impact. The air handling units each have an integral reverse cycle heat pump, which has the benefit of providing an element of cooling to temper the supply air in summer. Two air handling units also serve each of the new build teaching blocks; these are located in enclosed rooftop plantrooms to provide protection, reduce visual impact, and reduce long term maintenance. The only space without mechanical ventilation is the sixth form space on the top of the Transmission Hall because it has opening windows on two elevations to make use of the opportunity to cross ventilate. All classrooms and areas with occasional high occupancy levels, such as the main assembly space, have option of opening windows should temperature and/or CO2 levels get too high. Historical equipment from the radio station has been left in situ A computer generated image of the site All teaching spaces in the historic buildings have mechanical ventilation, although the ESFA is not generally in favour of it. See panel below, Handling air. The new blocks are orientated north-south to reduce overheating risk and maximise the opportunities for winter solar gain. On the north elevation, windows are in line with the facades to maximise daylight in the classrooms, whereas on the southern elevation brick fins shade the triple glazed windows and their hinged side ventilation panels that allow additional natural ventilation. The teaching blocks low energy design meant that their heat loads were low enough to enable heating using air source heat pumps (ASHPs). Wall mounted radiators sized for a 45oC flow temp heat the classrooms while ceiling mounted radiant panels, also sized for a 45oC flow temperature, are used to provide heat to laboratories and larger spaces. Heat pumps were a relatively unusual solution at the time and we were keen to show what a new build school could look like with this technology, but now most new buildings are being heated with these, says Etudes South. Heating design guidance He says that this project highlighted the need for design guidance to keep pace with the evolution of heating systems. In particular, he says there is an issue with heat emitter sizes designed for buildings with a very low heat load, which he says could lead to emitters being oversized. South also says that design standards often assume new 34 November 2022 www.cibsejournal.com CIBSE Nov 22 pp32-34, 36 Houghton school.indd 34 21/10/2022 17:19