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    Radiant for All Seasons

    April 1, 2004
    by Michael Weil, executive editor Santa Fe, New Mexico, with its rugged landscape and colorful mix of inhabitants, is a land rich with history, where

    by Michael Weil, executive editor

    Santa Fe, New Mexico, with its rugged landscape and colorful mix of inhabitants, is a land rich with history, where tourism rules, yet people also flock to as a place to live and work.

    Founded in 1607, Santa Fe became a melting pot . It was accessed by the Mexicans and Spanish from the South via El Camino Real (or the Royal Road), and by Anglo-Saxon miners and trappers who trekked in from the North via the Santa Fe Trail. Caught in the middle were the Native Americans who were already living there.

    Elements from each of these three cultures influence everyday life, and to this day many newly built residential homes are styled after ancient adobe mud dwellings, which offer superior insulation against the burning New Mexican sun and the chill of the desert night air.

    These traditional construction philosophies, however, are enhanced with modern conveniences such as hydronic heating. Hydronics, particularly radiant heating, is among the most widely used heating methods throughout the state.

    These days there is a growing twist to this concept — radiant cooling. Though not a new technology, radiant cooling seems to be making a foothold in the HVAC industry in this southwestern state.

    Just ask Ken Hastey, owner of Ken Hastey Plumbing, Inc. He says the migration of people to New Mexico has not only been a boom to the new home construction market, but has lead to a demand for air conditioning in an area where air conditioning hasn't been needed.

    "The adobe style of construction doesn't leave room for ductwork," Hastey explains. "So we didn’t do ductwork. But now out-of-state home purchasers are pushing for their homes to have central cooling, and we had to find a way to accommodate them. Radiant cooling is that accommodation."

    Hastey, who started his company 25 years ago, currently employs nine people, most of whom are installers. In fact, one member of the crew is his daughter Rheanna, who joined the company full time more than two years ago.

    Rheanna says the company grossed near $1 million in sales in 2003 and specializes in new home construction, from which 90% of its revenue is derived. Of that money, Hastey says 98% involves radiant heating.

    On one recent job, a 5,000 sq.ft. residential home just outside of Santa Fe, the customer was having the entire structure zoned with a radiant heating system and wanted cooling as well. Since the house was in the adobe style, ductwork wasn't really an option.

    Says Ken Hastey, "This home is especially well suited to radiant cooling. It has a flat roof with no place for ductwork except in the slab. The house also had high heat loss because of all the glass in it.

    “Our heat load calculations showed that we'd need more heat (BTUs) than we could provide exclusively with the in-floor radiant, so we began planning for supplemental radiant heat in the ceiling. And, with the tubes already going in, why not also use them for cooling in the summer months? So, the systems are married. It’s a great representation of the incredible flexibility of hydronics."

    The radiant cooling system consists of metallic sheets with grooves cut out that hold PEX tubing. These sheets are installed in the ceiling and the PEX is strapped into the grooves, winding across the ceiling and down a wall into a mechanical room located in the garage. It is manifolded into a light commercial chiller.

    Hastey says each loop, both the heating and cooling loops; consist of 300 feet of PEX. In fact, there are 15 300-ft. loops tied into the manifolds. The heating side of the system employs a residential boiler tied into separate loop of PEX that runs under the floor of the house.

    "The floor system provides about 30 BTU/sq.ft. of heating, and the cooling loops provide supplemental heating,” he explains.

    Because marrying a radiant cooling and heating system was new to Hastey, he enlisted engineering help from his local distributor, Dahl of Santa Fe, a division of Hajoca Corp. Dahl Heating and Cooling Specialist Jay Maze explains that in the radiant cooling system, cold water runs through PEX tubing embedded in the ceiling. The cold water acts as a heat sink for the warm air in the room and, as it circulates at temperatures in the 67F to 72F range, it removes heat from indoor space continuously.

    "This means substantial energy savings to cool the home, as water is about three times more efficient than air as a medium for heat transfer. A conservative estimate calls for energy savings of 15% to 20% compared to a traditional forced-air system," Maze says.

    How it Works

    Radiant cooling works like this: heat moves between a space and the radiant-cooled ceiling through a temperature differential. Unlike radiant heating, the colder ceiling absorbs the thermal energy radiating from people and their surroundings. The major difference between cooled ceilings and air-cooling is the heat transport mechanism. Air cooling uses convection only. Hydronic cooling employs a combination of radiation and convection. The amount of radiant heat transfer can be as high as 60%, convection accounts for the remainder.

    A core-cooled ceiling is the cooling equivalent of a floor heating system. In this system, water is circulated through plastic tubes embedded in the core of a concrete ceiling. This layout allows the system to take advantage of the storage capacity of the concrete, and provides the opportunity to shift the building peak load away from the utility grid peak.

    Maze says that to use radiant cooling in a home, it’s important to have detailed knowledge of building envelope and glazing systems, climactic conditions, and an ability to coordinate the design of hydronic piping into large, exposed surfaces of the home — in this case, the ceiling.

    The key, he continues, is to make use of a radiant cooling system in the building envelope. The perimeter solar gains and thermal transmission loads must be reduced to as low as possible to allow the radiant cooling system to operate properly.

    "Radiant cooling capacity is limited by the cooling surface temperature being just above the dewpoint of the ambient air in the space to be cooled. This means that the minimum effective temperature of the radiant cooling surface in most building applications is around 61F to avoid condensation."

    In arid Santa Fe, the ASHRAE design temperature is 64F in summer. If liquid temperatures within the tubing are above that, there are no condensation issues. As stated in a number of occupant comfort studies, the human comfort factors are generally made up of 50% radiation, 30% convection, and 20% evaporation.

    According to Maze, in North America, many mechanical engineers are very concerned with air temperature control and the use of all-air-type air conditioning systems — the radiant comfort part of the equation is more or less ignored.

    However, he adds, "Once the building interior is maintained at a constant temperature, and the transient heat gains around the perimeter have been nearly eliminated, there is very little need for any temperature-compensating controls. The air system just needs to supply outdoor air for ventilation, and does not need to do any space temperature control. This ventilation air can be supplied at near room temperature via a displacement ventilation system. An air-to-air heat exchanger can provide a virtually energy-neutral air system."

    "The energy efficiency of these systems becomes even better in areas where the night temperatures are regularly below 59F during the summers," Ken Hastey says. "This allows a fluid cooler to be used exclusively for heat rejection. Climates where the relative humidity is high throughout the summers may require dehumidification equipment for the ventilation supply air system, which would impact the capital and operating costs."

    In Europe, there are literally millions of square meters of buildings using radiant cooling as the primary climate control system in the occupied areas. Considering that their building codes are quite stringent in terms of defining indoor air quality and indoor comfort conditions, this speaks highly that this type of system is an acceptable indoor climate control medium.

    Hastey concludes that the system may not be perfect for all climates, but in the Southwest, particularly in the drier sections of the country, radiant cooling and heating is a very good option in the residential marketplace.

    “It’s not just for heating,” he concludes. “Radiant is an option for all seasons.”