Consulting-Specifying Engineer - April 2008 - (Page 52) cooling to supplement the radiant cooling system for transient shortacting heat gains (meeting rooms, localized solar gains, etc.). Control your surroundings The fundamental key to enabling radiant heating and cooling systems to work well is to control the building envelope thermal loads first so that the radiant systems can work within their effective surface temperature ranges for the radiant surface areas being used. The goal is to reduce or eliminate high-peak transient thermal loads in a room, so the room cooling is limited to people and equipment heat gains. Heat gains from lights can be eliminated from the room cooling loads through the use of low-level air supply and stratified air extraction systems like UFAD and DV systems. Radiant floor heating systems are limited to a maximum recommended surface temperature of about 85 F for comfort (the average human skin surface temperature). Radiant floor systems also suffer from furniture masking and floor covering variations that can act as insulators, requiring higher radiant heating fluid temperatures to get the right finished floor surface temperatures. Radiant ceiling heating surface temperatures can be much higher because there is no direct contact to them, but the radiant emitter surface temperature versus distance to the human (and other surfaces) becomes a critical design element. Small surface area heaters, such as low- and high-intensity gasfired radiant heaters, are like miniature suns, and the closer things are to them, the hotter the objects get. Ask the how someone felt when paint peeled off the roof of his restored classic car after he installed some “low-intensity” overhead radiant heaters in the garage that operated at a high surface temperature. Consider the limitations Radiant cooling systems likewise are limited in their effective temperature range; floor cooling systems are limited to minimum surface temperatures of about 65 F for direct human contact comfort (and the issues of floor coverings and furniture masking also apply). Radiant ceiling surface temperatures are limited by the average controlled humidity of the indoor air, and generally can be as low as 62 F as a design limitation. The following summary represents the radiant heating and cooling limitations for effective heating and cooling loads: Radiant heating floors limited to 85 to 90 F = 35 Btuh/sq. ft output of radiant surface Hydronic radiant heating ceilings, up to 200 F = 250 Btuh/sq. ft of radiant surface Radiant floor cooling maximum output at 65 F = 12 Btuh/sq. ft of radiant surface area Radiant cooling ceilings output at 63 F = 25 Btuh/sq. ft of radiant surface area So, the object is to “reverse engineer” the building design to make sure that your maximum cooling load would be no more than 25 Btuh/sq. ft where you intend to use a radiant cooling ceiling (chilled ceiling) system, or 12 Btuh/sq. ft for a radiant floor cooling system. For hydronic radiant heating systems, the only real limitation is for radiant floor heating systems to keep the room heating losses below an average of 35 Btuh/sq. ft of the room being served. This can be accomplished easily by an integrated design team, with the mechanical engineer working hand in hand with the architect to get a high-performance envelope designed—toolbox items like exterior sunshades, double-skin facades, high-performance sealed windows with triple-paned or better thermal performance, minimized thermal bridging at the perimeter envelope details, and other similar approaches. Generally, windows are a weak link in terms of allowing high solar heat gains, and excessive heat losses. Policing window thermal performance and exterior solar shading can help reduce the room heating and cooling loads down to within the radiant heating and cooling system design limits. The more windows you want to use, the higher the thermal performance required of those windows, and similarly watch their solar orientation and exterior shading to keep solar heat gains reduced. The inside surface temperature of the glass is a critical comfort factor for the mean radiant temperature of the room as well. While better wall and roof insulation, with reduced building infiltration, is a big help to reduce heat losses and gains in a building, the windows then become the next big element to pay attention to for indoor comfort. If the perimeter transient loads are reduced or eliminated, then the room cooling load is mostly “steady state” from equipment and people density. That’s an ideal fit for radiant cooling and low-level air supply, so that half of those remaining heat gains can be carried off in thermal plumes through the stratified high-level air in the space that is exhausted out of the building. The radiant cooling can deal with the people cooling loads while the equipment heat gains are primarily convective thermal plumes. The small amount of radiant heat being given off by the warm equipment also can be offset from the radiant cooling panels/ceiling. It’s all about mean radiant temperature control for human comfort. Radiant cooling systems can be designed to work effectively in nearly all applications and most climate zones without fears of condensation, provided good design approaches are followed with complimentary fail-safe controls. Very low building energy use can result, primarily from the improved building envelope, as well as from the use of hydronic-based radiant heating and cooling systems. McDonell develops institutional and sustainable projects, including complete mechanical design, design execution through to construction services, and site inspections. He specializes in very low energy semi-passive building systems including radiant cooling, displacement ventilation, and high-performance building envelope design. 52 Consulting-Specifying Engineer • APRIL 2008
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