Radiant Cooling Makes a Stand in New Mexico

By Michael Weil, Executive Editor, Contracting Business SANTA FE, N.M. With its rugged landscape and colorful mix of inhabitants, the Santa Fe area 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. Mexicans and Spaniards accessed it from the south via El Camino Real (or the Royal Road), and Anglo-Saxon

By Michael Weil, Executive Editor, Contracting Business

SANTA FE, N.M. — With its rugged landscape and colorful mix of inhabitants, the Santa Fe area 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. Mexicans and Spaniards accessed it from the south via El Camino Real (or the Royal Road), and Anglo-Saxon miners and trappers 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 cultures influence everyday life, and to this day many newly built residences are styled after ancient adobe mud dwellings, which offered 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 this southwestern state.

Just ask Ken Hastey, owner of Ken Hastey Plumbing. He says the migration of people to New Mexico has not only been a boon to the new home construction market, but it has led 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.

She says the company grossed almost $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. home just outside 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.

“This home is especially well suited to radiant cooling,” Ken Hastey says. “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 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 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. The tubing is connected to a light commercial chiller through a manifold.

Each loop, both for heating and cooling, consists of 300 ft. of PEX. Fifteen 300-ft. loops are 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,” Hastey explains.

Because marrying a radiant cooling and heating system was new to Hastey, he enlisted engineering help from his local distributor, Dahl Supply of Santa Fe, a division of Hajoca Corp. Dahl heating and cooling specialist Jay Maze notes 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 67° to 72° F 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,” Maze says. “A conservative estimate calls for energy savings of 15% to 20% compared to a traditional forced-air system.”

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 the cooling 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.

To use radiant cooling in a home, Maze says, 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 adds, 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 dew point of the ambient air in the space to be cooled,” he says. “This means that the minimum effective temperature of the radiant cooling surface in most building applications is around 61° to avoid condensation.”

In arid Santa Fe, the ASHRAE design temperature is 64° in summer. If liquid temperatures within the tubing are higher than 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.

In North America, Maze says, 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. That doesn’t have to be the case, however.

“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,” he adds. “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.”

Ken Hastey notes: “The energy efficiency of these systems becomes even better in areas where the night temperatures are regularly below 59° during the summers. 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, millions of square meters of buildings are using radiant cooling. Considering that European building codes are quite stringent in terms of defining indoor air quality and comfort conditions, this speaks highly that this type of system is an acceptable indoor climate control medium.

The system may not be perfect for all climates, Hastey says, 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.

Michael Weil is executive editor of Contracting Business magazine, a sister publication of CONTRACTOR. He can be reached via e-mail at [email protected].