REFURBISHMENT CASE STUDY OFFICE BLOCK
the year, the system is able to cope with additional air ‘better than most other systems’. The windows do not form part of the night time ventilation strategy; instead the air handling units run at night, with the cooling system off, to purge the structure of the day’s residual heat. The lighting scheme, too, was influenced
We inherited a building with fairly generous floor slab to ceiling slab heights, which opened up a lot of options for the type of cooling and heating systems we could install
by the cooling system. NDY restricted the heat output from the lighting to a maximum of 12W/sq m. ‘It was quite a challenge to maintain the design at that level but if we’d gone over, the lighting would have impacted on the cooling available for the space,’ explains Hall. The solution developed uses suspended high efficiency fluorescent fittings with daylight and PIR controls as part of a DALI system. The lighting solution has been enhanced
around the atrium, where the new infill has created deeper floor plates. The number of fittings has been increased in this area to give the impression of higher levels of light entering from the atrium than is actually the case. ‘The light in this area has been enhanced to make the offices feel light and open,’ says Hall.
Heating In winter the return air from the offices, which can be at temperatures as high as 27C, is mixed with the outside air to achieve the 19C supply air temperature
without having to use additional heat. The solution can recover up to 65% of the heat that would otherwise be thrown away. ‘It’s a really efficient system,’ says Hall. When heating is needed, it is provided
by a combination of three gas and two biomass boilers. The biomass was installed because, at the time the scheme achieved planning approval, the rules required a 10% reduction in the building’s carbon emissions from renewable technologies. ‘We tried to minimise energy use by making the building as lean as possible through the improved façade performance, our system selection and the plant efficiencies,’ says Hall. As a result, the biomass boilers are the scheme’s only renewables. The two biomass boilers, located in the
main ground floor plant room, provide 15% of the overall heat demand. In summer, a 550 kW boiler is sufficient to meet the hot water demand. Hot water is delivered to each toilet block via a hydraulic interface unit complete with a heat exchanger. In winter, a larger 900 kW boiler will supply additional heat for the ventilation system heater batteries, office perimeter trench heating and the below-floor fan coil unit for the main entrance door. The gas boilers provide supplementary heating when necessary. Variable speed pumps on both the heating and chilled water systems help match the circuit flow rate to demand. Unlike gas boilers, which can be turned
Windows are fitted with internal blinds for glare control
Basic building loads Heating (gas and biomass)
Electrical building services Electrical cooling Subtotal
kWhr/sq m/yr kg/CO/sq m/yr 6.43
Hot water (gas and biomass) 5.32 Electricity – lighting
0.25 0.15
26.40 8.92 4.44 51.5
11.14 3.76 1.87 17.7
Projected, estimated energy loads of the services, based on Energy Performance Certificate data. (Source: Norman Disney & Young)
on and off instantly, biomass boilers are less responsive. ‘If the biomass boilers are servicing a big demand and that demand suddenly drops you cannot turn them off instantly,’ says Hall. To allow the boilers to consume any remaining fuel, Hall has included a giant 20,000 litre hot water storage vessel in the heating/hot water primary circuit to act as a thermal buffer, ensuring biomass boilers can run efficiently. The hot water store also helps prevent corrosion in the boilers by keeping the water circuit warm. ‘Biomass boilers don’t like being fed cold water, so the hot water store helps provide back-end protection by maintaining a minimum return temperature,’ explains Hall. Proof of the effectiveness of the building services solution is in the scheme being awarded a BREEAM Excellent rating. It has also achieved a B rating Energy Performance Certificate. ‘It’s an impressive performance from a 1980s building,’ says Hall. ‘But then it was a pretty heavy refurbishment,’ he adds.CJ
30 CIBSE Journal May 2012
www.cibsejournal.com
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