Mixed and match

Throughout the last two decades the UK has seen a growth in the construction of naturally ventilated and mixed-mode buildings. Often backed by enlightened clients and companies looking to make an ‘environmental statement’ prominent schemes such as De Montfort University, Elizabeth Fry and Jubilee Campus have given greater credibility to the technologies developed. But despite this, naturally ventilated and mixed-mode buildings still only constitute a small proportion of all new buildings constructed.

So what has prevented the trickle down of this technology to smaller everyday commercial developments? Richard Quincey, a former partner at Max Fordham LLP puts it down to a number of factors not least that many engineers view it as a highly specialised field, requiring expensive analysis tools and as such do not readily offer it as a potential solution. He sees the use of bespoke solutions and the lack of commercially available systems as further drawbacks, as well as the perception that designs are complicated and difficult to commission. Manageability problems have also been a deterrent, with complex controls and strategies often being employed.

“Rather than trying to push the performance envelope of this type of building on each new project,” says Quincey, “we should be looking at consolidating and refining design experience and making these ideas into a more accessible mainstream commercial technology.” A recently completed project by Fordhams seeks to do just this.

Granta Park Amenities building

Granta Park is a hi-tech research and development park about 12 km south east of Cambridge. The two-storey mixed-use amenities building is a speculative development consisting of lettable office space, a cafe and a fitness suite. “The client wanted an affordable low energy building but following a reasonably familiar construction route,” says Quincey. “For that reason they weren’t going to build a very high mass building.”

The building is divided into north and south blocks separated by a triple height atrium. The structure is relatively conventional, clad in titanium-zinc alloy panels, with exposed dense blockwork/concrete where possible and dry-lined with thick heavyweight plasterboard elsewhere. It has typically low U-values and low leakage manual windows. Solar control is simple, limited to a number of fixed overhangs, internal roller blinds and Pilkington’s new generation solar control glass Titan. “Early on it looked like it was a typical fan coil project,” Quincey explains. “But when we did the calculations it seemed a shame to install radiators and fan coils to cater for such a small demand.” Instead Fordhams adopted a simple low intensity mixed-mode approach.

Ventilation strategy

The low intensity mixed-mode solution is characterised by very low-pressure mechanical ventilation and simple opening windows. The low-pressure air paths are created by installing small speed controlled dc fan/damper units at ground and first floor level around the building’s perimeter. These sit in a 200 mm raised access floor void drawing air in and jetting it across the concealed concrete floor slab and into the room via floor grilles. Extract air vents into the atrium and exits via automatically controlled high-level windows.

The raised access floor incorporates an underfloor heating/cooling system developed for the project in conjunction with Osma Underfloor Heating and Hewetson Floors. The system, which is now commercially available, allows the pipes circulating the hot or chilled water to conduct heat to the underside of the galvanised steel floor deck when the floor tiles are dropped into place.

The mechanical supply air jet is tempered by the thermal mass of the floor slab and the underside of the floor preheats/cools the air as it mixes in the void. It then enters the rooms at low velocity through floor grilles, giving a gentle displacement system. “This solution removes the problem of getting fresh air into the space without cold draughts or areas of reduced comfort that is an issue with a perimeter natural ventilation system,” says Quincey. It reacts to occupants opening windows by automatically backing off or assisting as required.

“Having the heating/cooling dispersed across the floor area means that the mechanical heating/cooling is mostly de-coupled from the ventilation system,” adds Quincey, “thereby allowing full modulation of the air volume when occupancy is low with obvious savings in fan energy.”

Total heating output is 120 W/m² at floor temperatures of about 27°C. By using an underfloor heating system and draught free ventilation Quincey suggests it is possible to lower the winter room air temperature to around 18-19°C and thereby reduce heat losses by around 12%.

At Granta the underfloor heating system is also connected to a heat exchanger on the chilled water system serving the fitness suite. This provides a limited amount of changeover summertime cooling, approximately 20-30 W/m² (or higher if used to mechanical pre-cool the building fabric), for peak internal conditions. However this cooling source could just as easily by provided by groundwater or a ground coupled heat pump.

It is interesting to note that lower chilled water flow temperatures than chilled ceilings (around 1·25°C lower) can be used as the dewpoint of the cooled surface is dependent only on outside air conditions and is not affected by the occupant latent load.

But what about expectations on the internal comfort of the offices? Project engineer Mark Maidment explains that this was discussed with the client early on. “Clients can be unintentionally misled about the performance of buildings. On this job we worked with the GIR 30 document, rather than saying it will be 21°C or 23°C, we said it would only get above 25°C or 27°C for so much of the year and they were reasonably happy with this approach.”

The prefabricated fan/damper units are powered off local 13 A power track sockets that also feed traditional floor boxes. The system operates at up to 3 ac/h during the day and 6 ac/h during night cooling mode – the specific fan power is dramatically lower than currently recommended by CIBSE best practice (up to 0·1 W/l/s and 0·38 W/l/s respectively).

The arrangement allows for secure night-time ventilation for fabric cooling and some external noise attenuation.

Controls

“The controls for naturally ventilated buildings are too expensive and defects are an issue,” comments Quincey, “with the design of the controls often starting from scratch for each new project and defects being resolved over too long a period.” Standardised natural ventilation controls would obviously be a key technology to making natural and mixed-mode ventilation designs more widely used.

Fordhams worked in conjunction with Smartkontrols to develop an off-the-shelf, low-cost controller that would be configurable to a large number of natural and mixed-mode applications. The NatVent controller and associated window controller that evolved now sit in the company’s SeaChange range of flexible controllers, allowing it to be augmented by other standard modules to add further functionality as required. In addition the use of standard ‘distribution board’ cabinets significantly reduces costs, making a bems a real solution even on very small or inexpensive projects.

The building is divided into flexible zones with users able to set up and adjust temperatures and occupancy times for their area through wall-mounted zone controllers. Control modules are decentralised and fully networked doing away with the need for most control panels. Those for the ventilation are located in the floor void alongside the damper/fan assemblies.

The next generation

Although the Granta Park scheme initiated the move towards a simple low intensity mixed-mode approach the designers have not been slow to develop the idea. Other schemes under development propose increasing the mass of the standard Kal-dek type roof construction to improve control of solar loads. This involves increasing the density of the mineral wool (with rock wool the U-value roughly remains unchanged at the densities used) and incorporating additional layers of cement bonded chipboard immediately above the perforated liner tray to give a roof with a six to seven hour time lag and an admittance of 3-4 W/m². This arrangement only adds about 10% to the weight of the structural steel work for the roof.

Although the underfloor heating system is particularly suited to condensing boiler technology Quincey highlights the possibilities when connected to heat pump installations, for either heating or cooling, because of the low operating temperatures. “If you’re using a ground coupled heat pump you’ll probably get a coefficient of performance of around 4:1 and in lower cost buildings you get a heating system with beneficial cooling.”

The future

Cost and time issues were obviously important for the Granta Park scheme, which was undertaken as a construction management project. The integration of the underfloor heating/cooling system with the standard raised access floor has produced a fast-track solution. “The m&e installation can quite literally follow in the footsteps of raised floor; it involves only clip or push fixings and is not heavily reliant on site workmanship,” says Quincey.

The approach doesn’t compete with fan coils or chilled beams in the cooling sense because of its low output, but often it is all that is needed. “In cost terms this approach comes into its own when you go for open-plan buildings because of its simplicity and speed of installation” says Maidment. The installed cost of the heating/cooling pipework, manifolds and insulation is in the region of £30/m², putting it closer to the cost of radiators rather than installations using fan coils or chilled ceilings/beams.

Furthermore it allows some flexibility to move away from very high mass buildings, typical of earlier naturally ventilated buildings, which often equate to a high embodied energy and high construction cost.

Time will tell just how successful this solution is in reducing energy use and gaining wider acceptance of the mixed-mode approach – particularly for speculative office developments. However those involved already report keen interest from a number of sources. Developers aren’t renowned for their altruism but as Quincey says: “If we manage to produce solutions that are attractive to developers we have a real chance at making a large scale impact on carbon emissions.”