This article was originally published in the November/December 1993 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.



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Home Energy Magazine Online November/December 1993




Don't Force Air, Go with the Flow


by David Springer

David Springer is vice president of Davis Energy Group in Davis, California.

Hydronic heating systems can offer a comfortable and energy-efficient alternative to forced air distribution systems.

In our efforts to improve forced-air distribution systems, alternatives to forced air are often overlooked, but hydronic heating systems are an attractive option that can deliver superior efficiency and comfort.

Hydronic systems distribute heat by circulating hot water through baseboard convectors, radiators, or low-temperature radiant panel surfaces (most commonly concrete floors). Census figures show baseboard and other hydronic system types were installed in 39% of new homes in the northeast states in 1991. Though hydronic heating had only a 6% national market share, its popularity is evidenced by a 30% increase in system shipments between 1991 and 1992.1 The growth can be attributed to aggressive marketing by manufacturers, technical refinements, and a general perception that this is new technology. Radiant floors are most appropriate in heating climates, where a second distribution system isn't needed.

Radiant floor heating was actually popularized by Frank Lloyd Wright in the late 1940s, and was widely used in residential developments in the 1950s. Despite comfort advantages, the industry experienced market setbacks resulting from piping failures, high material costs, and growing demand for air conditioning. A resurgence occurred in the 1970s with the introduction of inexpensive non-metallic tubing (polybutylene and cross-linked polyethylene). Non-metallic tubing is resilient, reducing the likelihood of rupture due to floor structural failure, and immune to the corrosion which has plagued copper and steel tubing, previously used in these systems.2

Unfortunately, empirical data comparing performance of typical residential forced-air systems to radiant heating systems is lacking. However, with the introduction of methods for evaluating distribution efficiency, a reasonable comparison can be made using quantifiable thermal energy distribution system characteristics (see One Size Fits All: A Thermal Distribution Efficiency Standard, HE Sep/Oct '93, p.62). For forced-air systems, distribution efficiency is reduced by duct leakage, duct conduction, induced infiltration, and electricity used for the air handler fan. The distribution system can also affect building loads.3

Tests we conducted on a variable speed heat pump system with ceiling supply registers demonstrate the effect of air distribution on building loads. This study showed high correlations between fan operation, thermal stratification, and building heat loss rates. Since the upper half of exterior wall surfaces usually have a higher percentage of window areas than the lower half, it is evident that thermal stratification will increase envelope heat loss. Further, increased thermal stratification means that higher average air temperatures are needed to maintain comfort at the occupant level. With this background, let's look at hydronic thermal distribution efficiency.

For hydronic baseboard systems, efficiency is reduced by pipe heat conduction and pumping energy. Since pipes have much less surface area than ducts, are easier to install within insulated spaces, and usually do not leak, conduction and leakage losses can be almost negligible. Induced infiltration is non-existent. Also, pumps typically require less than one-fourth the electrical power of fans (but may run longer).

These same considerations also apply to hydronic radiant floor systems, though heat loss from the warmer slab degrades efficiency, and is the most significant loss factor. On the positive side, thermal stratification is almost non--existent with radiant floor systems, so envelope loads are probably lower than for ducted distribution.

How significant is floor loss? For heated slab floors, the heat flow is more complicated. The uncertainty of soil thermal properties (mass and conductivity) and three-dimensional heat transfer makes calculation of heat loss very difficult. However, radiant heating design equations can be used to estimate the extent to which heating a floor increases its losses.

Hot Air versus Radiant Heating

Figure 1 was developed using ASHRAE design data (1992 HVAC Systems and Equipment, Chapter 6) and provides a method for estimating distribution efficiency for slab-on-grade or raised-floor radiant heating systems. The multiple curves marked R0-R4 represent floor covering thermal resistances (insulating the top of the floor with carpet requires warmer water and thus, increases downward heat flow). For raised floors, the U-value is the downward heat loss coefficient of the floor assembly. For slab floors, the U-value can be estimated by multiplying the perimeter heat loss coefficient by the building perimeter and dividing by the floor area. For example, a 1,500 ft2 house with a perimeter of 160 ft and a perimeter loss coefficient of 0.40 Btu/ft-deg.F-hour would have a below-floor U-value of 0.04. If the entire house were carpeted with an R-2 carpet and pad, the thermal efficiency value would be 90%. If the floor were not carpeted (R-0), the efficiency value would be 98%.

Heat retention in slab floors causes continuous heat loss. However, low-mass raised floors can cool down between operating cycles, thus reducing building heat loss during off periods.

With forced-air heating, distribution efficiency may be lower than 50% while the blower is operating, especially if the blower creates pressure imbalances between rooms. (Five-fold increases in infiltration resulting from blower operation have been measured.) Seasonal distribution efficiency depends largely on blower operating hours. More research is needed to develop reliable comparisons of forced-air and hydronic heating distribution systems over complete heating seasons.

Radiant floor heating blends nicely into energy-efficient home designs. Exposing slab floors is an inexpensive way to provide thermal mass to reduce heating and cooling energy use. Radiant heating keeps the exposed floors warm and comfortable in winter. Leaving the slab exposed (or covered with vinyl or tile) also results in high distribution efficiency.

Radiant floor heating offers other energy advantages. Exposing slab floors is an inexpensive way to provide thermal mass to reduce heating and cooling energy use. Radiant heating keeps the exposed floors warm and comfortable in winter. Higher mean radiant temperatures (average wall, floor, and ceiling temperatures) produced by radiant systems allow lower indoor temperatures to be maintained without sacrificing comfort. Hydronic zoning is more simple and less costly than for forced air systems.

Since conventional air conditioning requires ducts, cooling is the greatest obstacle to hydronic heating. However, mechanical air conditioning is unnecessary in some climates with careful envelope design. Non-ducted cooling alternatives which also eliminate summer distribution losses include radiant floor or ceiling cooling, mini-split air conditioners, and one or two-stage evaporative coolers.


End Notes

1. Hydronic heating market data were from the June 1, 1992 issue of Air Conditioning, Heating & Refrigeration News. Additional market information is available from the Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ. The Hydronics Institute has been tracking sales of tubing for hydronic radiant heating for the past three years.

2. ASHRAE design procedures, simplified and modified for non-metallic tubing, are included in the Hydronic Radiant Heating Handbook (1989), Davis Energy Group Inc., 123 C St., Davis, CA 95616, (916) 753-1100.

3.Studies of existing houses completed by the Florida Solar Energy Center in 1989 indicated that blower operation increased infiltration 1.2 to 6.0 times (ASHRAE Transactions, 1989, Vol. 95, Part 2). If infiltration with the blower off is 50% of conduction loss, a four-fold increase in infiltration (induced by the blower) would double the required output of the heat source, resulting in a distribution efficiency below 50%.


Hydronics Fly At United

United Airlines is installing one of the world's largest hydronic systems at a new maintenance complex near the Indianapolis Airport. Thirteen aircraft hangars, an outdoor bridgeway, and two other buildings will be constructed with under-floor radiant heating, allowing crews to fix jets faster. Radiant heating will be used with forced air to keep the indoor temperatures of a hanger at 65deg.F. United needed a hydronic system because too much heat would be lost if only forced air was used to warm the large hangars. With a combination of hydronics and forced air, the outer skin of a freezing cold aircraft can be heated to room temperature within an hour. Some of the tubing will be laid in concrete slabs outside the entrance of each of the 33,000 ft2 hangars to melt the ice and snow off an aircraft before it is towed inside for repairs. Indoors, tubing will be installed in strips one foot apart.

Radiant heating will also melt snow on an outdoor walkway. The temperature will be controlled by outdoor sensors that detect snowfall as it lands, so the sidewalk will only heat up when necessary. United also plans to use hydronics to heat the ground equipment building.

After testing various types of tubing, the airline chose electronic-cross linked polyethelene (pex) tubing provided by Stadler Corporation of Bedford, Massachusetts. Radiant heat was specified because it works, said Jerry Rothfeld, technical director of Efficient Energy, a San Rafael, California-based manufacturing representative for Stadler. Stadler offered United a 30-year warranty on the tubing and provided ten-year product liability insurance if there is a system failure. United broke ground on the project in August 1992 and plans to complete it in 1995.

--Linda Berlin

Linda Berlin is a freelance writer based in Stinson Beach, California.


Figure 1. Distribution efficiency versus floor U-value and floor covering resistance (R-value).



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