In recent years, there has been a resurgence of interest in hydronic heating, especially in the form of radiant floors. While interest has been high, hydronic heating is gaining only a tiny share of the market. Ironically, it's the lack of a cooling method that has limited hydronic heating in the marketplace. Even in heating-dominated climates, cooling has become a standard feature of new homes. Many home buyers can't justify the cost of adding a complete duct system to provide cooling. Recent developments in hydronic cooling promise to remove this limitation. If that happens, forced-air comfort systems will have stiff competition.
Hydronic comfort systems offer several advantages over forced air. Circulating water requires less energy than blowing air through ducts. Ducted systems are notoriously leaky, which reduces system efficiency and indoor air quality. Forced air systems can harbor dust and organisms that reduce indoor air quality and might affect occupant health. Blowers, cabinets and ducts consume considerable space inside a building. Hydronic systems are quiet, easy to zone, and often allow unrestricted furniture placement. They are well suited to high-efficiency heat pumps.
These advantages support the idea that thermal comfort could be provided by a hydronic system, while indoor air quality and moisture control could be provided by a much smaller fresh air ventilation system.
Types of Hydronic Cooling
Residential and commercial buildings can incorporate hydronic cooling in several ways. Cool water is circulated through tubes to each room or zone. Tubes can be embedded in concrete floors (or ceilings in commercial buildings), attached to aluminum panels or attached to fin-tube convectors.
Baseboard convectors are the most common distribution method for hydronic heating. However, they have two fatal flaws for cooling. First, there is no way to capture condensation. Second, cool air pooling near the floor would be uncomfortable. A valance convector, made by Edwards Engineering for more than 20 years, solves both problems. Now used primarily in hospitals and nursing homes, the system is poised to enter the residential market.
Locating the convector near the ceiling promotes air circulation as the cooler air falls down the wall surface. In cooling mode, water temperature ranges from 45 degrees F to 65 degrees F. Condensation is not only accommodated but encouraged as a way to dehumidify the air. A condensation tray--formed by the valance's casing--catches condensation.
Like radiant floors, valances allow unrestricted furniture placement. Unlike tubes embedded in the floor, they can be added to existing buildings without much trouble.
Greg Baer, P.E., owner of SunQest, Inc., has been installing the Edwards valances in conjunction with radiant floor heating systems. "The floors do a better job of heating," he says, "the valances provide better cooling."
The growing popularity of radiant floors offers another opportunity for radiant cooling. With the addition of a control device and one or two valves, the same hydronic tubing can cool a home as well as heat it. In an American Society of Heating, Cooling and Air-conditioning Engineers (ASHRAE) paper presented in early 1997, Bjarne Olesen explained design issues related to comfort and performance.
Condensation is the main issue. If the relative humidity of the air is between 60 and 70 percent, condensation can be prevented by keeping the floor temperature between 63 degreees F and 68 degrees F. Comfort is another issue. Even with shoes, people probably will feel uncomfortable if the floor temperature falls below 66 degrees F. As a general rule, Olesen recommends that the temperature of a radiant floor not drop below 68 degrees F in order to prevent condensation and maintain comfort.
The temperature limitation affects cooling capacity. In a typical residential setting a hydronic floor system can deliver only 13 Btu/ft2-hr-degrees F. This may not be sufficient for cooling-dominated southern climates. However, in energy-efficient homes, this limited cooling capacity may be adequate.
Olesen notes that spaces with large expanses of glazing, such as solariums, could be good applications. Where the sun strikes the floor directly, the cooling capacity is about three-and-a-half times greater (47 Btu/ft2-hr-degrees F ).
Without the comfort and condensation problems, it would be possible to increase cooling capacity by spacing the floor tubes closer together (about 6 in.), reducing the slab thickness above the tubes and possibly increasing tube diameter. A dehumidifier can help boost cooling capacity by reducing relative humidity. That allows a slightly lower floor temperature.
One design issue deserves careful planning. For heating purposes, floor slabs generally need about an 18 degree F difference in temperature between the supply and return water temperatures. This is regulated by adjusting the rate of water flow. However, cooling will require a temperature difference of only 5 to 9 degrees F. This requires a higher flow rate that increases pressure drop in the tubing.
Because of the limited cooling capacity, the system probably will not be able to maintain a set temperature. Some fluctuation should be expected unless an auxiliary cooling device is available. Olesen recommends that the tube spacing, water flow and other design variables be sized for heating. Then, you try to squeeze as much cooling as possible out of the system.
Olesen's report titled "Possibilities and Limitations of Radiant Floor Cooling" can be purchased for $7 from ASHRAE.
© 1995 - 2000 by American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Reprinted by permission from ASHRAE Transactions, Vol. 103, Part 1 pp. 42-48.
This diagram shows one way to integrate cooling with a hydronic radiant floor heating system.
Chilled Water Sources
To add cooling to a hydronic distribution system requires a source of cool water. In some cases, water can be cooled directly by a loop of tubing buried in the ground or placed in a well. Average ground temperature in cool climates may be sufficient for the modest cooling capacity.
A heat pump would be a good choice because it can make both hot and cold water. Many major manufacturers now make air-to-water heat pumps and most ground-source heat pumps have the option of heating water instead of air.
Greg Baer of SunQest has used both sources. "In mild conditions, we have used well water to cool homes," he says. "Water circulates from the bottom of the well, through valance convectors and then back to the top of the well." For greater cooling power, valances can be fed water chilled to 45 degrees F by a ground-source heat pump."
A new device called Smart Cool provides another option. Smart Cool is an evaporative cooler with a Seasonal Energy Efficiency Ratio (SEER) of 36. (An air conditioner is considered efficient when its SEER is 12 or higher.) Its primary application is to cool air and deliver it directly to the house or through a single, short duct. The developers have cleverly offered an option that allows Smart Cool to be attached to a hydronic distribution system. Air cooled by direct and indirect evaporation generates about the same cooling power as a three-ton air conditioner. The hydronic loop adds about one ton more cooling capacity. (See August 1997 for more information on Smart Cool.)
"Floor condensation won't occur because the cooling capacity of the evaporative cooler declines as the outside relative humidity rises. Smart Cool is essentially self-limiting and won't be able to reduce the floor temperature below the dew point," says Morgan Muir with Smart Cool distributor, Hydronic Specialties Co.
A highly efficient indirect/direct evaporative cooler also offers a hydronic option.
Cooling for All Climates
Builders who install radiant floor systems now have a viable method to offer cooling at a reasonable cost. Because of their limited cooling capacity, hydronic floors may find a market only in northern areas with smaller cooling loads. Valances can handle the higher cooling loads and dehumidification needs of hot, humid areas.
These two distribution options, plus the availability of highly efficient equipment to drive them add up to a bright future for hydronic cooling.
This article appeared in Energy Source Builder #53 October 1997,