CASE STUDY | BRACKEN HOUSE Although limited as-built drawings existed, these were not always a true reflection of site conditions so extensive surveying was required during the strip-out process. A second, more detailed cloud-point survey was undertaken after the building had been handed over to the main contractor and the raised floor and suspended ceiling removed. This survey gave us some of those discovery moments. We found upstand beams where we were not expecting them, which meant we had to rejig the design, moving pipework and ductwork because we actually had less floor void than we initially thought, Berry says. The building imposed a number of constraints when it came to selecting a replacement HVAC system for the refurbishment, including the requirement to maintain the current 2.7m floorto-ceiling height. Other constraints included: a 175mm-deep ceiling void, which is unsuitable for services (except sprinklers and lighting) because it is interrupted by downstand beams; while in the 550mm-deep floor plenum, upstand beams impose a limit on the height of the services that can be distributed within the void and restrict service crossovers to the areas between beams. A number of HVAC systems were reviewed as replacement options including: underfloor FCUs; underfloor VAV units; an enhanced VAV solution, with an increased number of VAV boxes; and a passive chilled beam system with an underfloor fresh air supply. We looked at a number of different ventilation options to evaluate what was feasible, Berry says. Each of the four options was assessed based on its cooling performance, flexibility to accommodate changes to office layout, ease of maintainability and its ability to integrate within the building. The chilled beam solution was rejected because it would compromise the 2.7m floor-to-ceiling height. Underfloor FCUs were rejected on the basis that changing the units filters was considered too disruptive to tenants and because the high density of units required and their associated ductwork would be expensive and require a great deal of coordination. Of the two VAV options underfloor and the enhanced underfloor the more basic solution was least expensive and required the fewest number of services in the floor void, simplifying coordination. This solution, however, lacked the level of zonal control offered by the enhanced VAV option. Although the main drawback of the underfloor VAV solution is the lack of zonal control, underfloor supply systems are, to an extent, self-regulating and the level of cooling provided to There were increased heating loads from IT equipment 30 January 2020 www.cibsejournal.com CIBSE Jan20 pp28-31 Bracken House.indd 30 20/12/2019 15:20