Consulting-Specifying Engineer - April 2008 - (Page 48) cooling systems, primarily at “point source” or small area radiant cooling surfaces like passive chilled beams and suspended radiant cooling panels, which generally are operated at temperatures of 57 to 64 F, and dissolved oxygen generate convective cool air movement as part of their function and overall cooling capacity rating. If you maintain the indoor temperature comfort with radiant heat system, the air side of the indoor comfort system must supply fresh air (filtered, treated/tempered, humidified/dehumidified) or outdoor air at a rate to ensure healthy conditions, and at an air movement rate that promotes a healthy human comfort, as well as enough make-up air for the building exhaust systems. The fresh air can be supplied at or near room temperature because it does not need to provide heating or cooling, assuming the radiant system is able to deal with the room heat gains and heat losses. Increase energy efficiency This presents a significant leap toward lower HVAC system energy use and requirements, compared to using all-air systems to provide room air temperature control combined with the ventilation function. Moving a lot less air saves energy while the hydronic radiant system efficiently transfers energy and transports it around the building to the central plant. Water can hold more than 3,400 times as much energy per unit volume compared to air, so moving energy around a building with a hydronic system is much more energy-efficient compared to moving large volumes of warm and cool air around the building. While some may assume that central hydronic heating/cooling plants are more costly, the total cooling and heating plant size required for a radiant cooling and heating system is much smaller, mainly due to the higher performance building envelope that also is required to go hand in hand with a radiant slab/radiant cooling system. The proper application of radiant cooling that ensures that you will not get condensation requires the of use large surface areas, like ceilings, walls, floors, or combinations of all three surfaces. This keeps the mean radiant cooling surface temperatures as high as possible above dewpoints, yet still provides an effective amount of cooling capacity. Sprinkler piping (exposed) Thermo - active slab Radiant cooling/heating Suspended lighting: diffused indirect light achieved 10.5 ft. to 11 ft. floor to floor 100% outdoor air from adjustable floor air supply outlets Displacement ventilation system Raised - access floor 100% outdoor air from adjustable floor air supply outlets Electrical 10 in. to 14 in. data trays Supply air : ventilation only (20% of an all-air HVAC system ) This shows an ideal room environment with a radiant cooling/heating ceiling, indirect lighting for glare elimination, and a low-level ventilation only-air supply to provide displacement air movement system in the room. The use of a radiant slab system to perform all the sensible cooling in the space at an average ceiling temperature of 63 to 65 F allows the ventilation air system to be reduced to just the 100% fresh, tempered outdoor air needed for IAQ and make-up air requirements for building exhaust systems. This allows a substantial amount of formerly required ceiling plenum space to be eliminated, potentially saving up to 2.5 ft of floor-to-floor height, allowing additional stories to be added in a fixed building height, or simply envelope cost savings, to allow project capital to be spent on the high-performance envelope that allows the radiant systems to work within their capacity and dewpoint tolerance limits. Source: Geoff McDonell 48 Consulting-Specifying Engineer • APRIL 2008
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