The next big step--slab radiant heating and cooling.

The advantages of radiant floor heating systems, especially for high-end private residences, are fairly well known. Infloor radiant heating operates quietly while delivering excellent comfort levels. Radiant energy heats people directly so room temperatures can usually be kept lower and the power consumed by a pump to move an equivalent amount of energy is a fraction of that of a fan. Both factors result in substantial reductions in energy use.

The traditional drawback to the use of radiant floor heating has been its cost since a separate air conditioning system is needed.

However, for commercial applications radiant systems for both heating and cooling have been used. Systems that use hot water for heating and chilled water for cooling have been used in office buildings and hospitals since the 1960s. These systems use either insulated metal ceiling panels with water tubing bonded to them or exposed metal panels called valances. A separated ducted air conditioning system is used to provide dehumidified and cooled ventilation air to the space.

While these systems are effective in terms of comfort and energy efficiency they too have drawbacks. The cooling capacity of the panels is fairly low since very cold water can't be used because condensate can occur on the panels. Therefore, many panels are needed. In fact, in some installations where the cooling loads are high, the entire ceiling needs to be constructed of panels. This results in an extraordinary amount of piping and tubing connections in the ceiling. The up front cost associated with installing many panels is also very high.

Work is now being done on finding ways to use tubing embedded in an exposed concrete ceiling slab to provide heating and cooling. The current popularity of exposed concrete ceilings and indirect lighting is helping to make these systems more acceptable. The biggest advancement has been the realization that the use of slab radiant heating and cooling is not a substitute for an all air system in a conventional building, rather it is only one important element in a "whole building approach" to sustainable design and energy conservation.

For this to work, buildings must be designed so that the low density but steady heating and cooling ability of slab systems becomes a major advantage. The thermal mass of the building is used as a control element to moderate fluctuations in the indoor environment. High performance glazing is necessary to limit heat gain. External and tracking shading is important to limit dramatic fluctuations in solar load. Efficient lighting and daylighting strategies must be incorporated in the building design. Ventilation and dehumidification systems that introduce 100% outside air can also be used effectively to ensure excellent indoor air quality.

By combining these design strategies it is now possible to design a building where the slab radiant heating and cooling system and a dedicated outside air system actually cost less than conventional all-air counterparts. Air handlers and ductwork for ventilation become a fraction of the size and less space is needed for mechanical equipment and shafts. Thermal mass is used, rather than controls, to maintain a constant indoor environment. The reductions in mechanical system costs can then be used to offset the higher cost of the improved building envelope.

However, there is a learning curve associated with slab heating and cooling. Some aspects of this approach run counter to what we have come to expect in a high quality, modern building. For instance, most building control systems lower the temperature of the building at night to reduce energy usage. Slab heating and cooling systems act counter to this approach. An 8-inch slab may take hours to change temperature.

Therefore, the best approach is to use the moderating nature of thermal mass to maintain fairly constant temperatures throughout the building at all times by adjusting the temperature of the water in the slab based on the trends-in outdoor temperature.

DOMINICK DEPINTO P.E.

PARTNER, AMBROSINO, DEPINTO &

SCHMIEDER CONSULTING ENGINEERS