In 1987, Bethel Baptist Church in Prospect, New York, began planning for a 6000 square foot addition to its existing building. The majority of the space would serve as a new sanctuary for the growing membership. The building proposal included hydronic radiant floor heating in its concrete slab-on-grade floor––a system few of the members had any familiarity with at the time. However, most members had experienced the discomfort of uncovered, unheated concrete floors in similar buildings. The decision was made to give the new approach a try.
In the years that followed, the comfort, efficiency and reliability the heated floor provided confirmed they made a wise decision. So much so that floor heating was again specified in 1992 when the church added a 4200 square foot Sunday school addition.
Perhaps it’s not too surprising that church members stuck with the proven performance of hydronic radiant floor heating when planning a new multi-purpose room last spring. The new 5300 square foot slab-on-grade structure serves as an activity center for church-sponsored youth programs, athletic activities, dinners and other special events. It houses a 40 by 80-foot court area designed to withstand the daily pounding of basketballs, volleyballs and the like. The walls are clad with 3/4-inch plywood on the interior. All windows are equipped with removable interior chain-link panels. Even the ceilings are finished with 5/8-inch plywood with hardwood battens over the seams. This was definitely not a place for “wimpy” heat emitters.
A Match Made in Heaven
Several factors made this building an ideal candidate for hydronic radiant floor heating:
- An in-slab system is virtually indestructible. It’s use precluded any concern about people or equipment damaging the heat emitters or vice versa.
- The ceilings in the court area are 18 feet above the floor. Floor heating has a proven history of NOT creating air temperature stratification in such areas. Eliminating stratification also eliminated any need for ceiling fans. The latter being a virtually assured target for high-flying balls.
- Any water or snow tracked in on the floor would quickly dry up reducing the chance of slips or falls.
- A large ventilation fan would be temporarily operated to provide a high rate of air exchange during “heated” games. Under these conditions the thermal mass of the slab could provide a surge of heat output to help maintain comfort at lowered air temperatures, as well as quickly reestablish the normal 65°F air temperature after the fan was turned off.
- The same boiler that supplies the floor heating system could also provide ample amounts of domestic hot water to the kitchen during periods of heavy usage.
CADing Before Cutting
Prior to installation, all floor heating circuits were drawn and accurately measured using CAD software. The resulting layout drawing made for a fast and accurate installation, as well as an accurate record of circuit placement for future reference. In total the building contains just over 4300 feet of 5/8-inch PEX-AL-PEX tubing divided into 11 circuits.Most of the tubing was installed at 12-inch spacing, which was conservative given the load and the fact that the only floor covering is two coats of epoxy paint. However, a cushioned athletic tile floor may be installed in the future. The spacing used could still deliver the necessary heat output without resorting to excessive water temperatures.
After the stem wall foundation was constructed, backfilled and compacted, the entire floor area was insulated. The edge and outer 4 feet of the slab rest against 2 inches of extruded polystyrene insulation. All interior areas are insulated with 1 inch of the same material. The manifold stations were accurately located based on floor plan measurements. Welded wire reinforcing was laid over the insulation, and the tubing fastened to it using wire twist ties. When all circuits were in place both manifold stations were air pressurized to approximately 75 psi and checked for leaks. A few days later the 80-cubic-yard concrete slab was placed in just over two hours with the aid of a boom pumper.
Watch Out Below!
The north-facing slope of the metal roof measures 48 feet from peak to eave and has a 6/12 slope. The snow pack that periodically slides off this roof could appropriately be described as a mini-avalanche. The roof overhangs the entrance sidewalk to prevent any person who might be on it during a snow slide from being buried. Instead they can step back and watch a pile of snow 50 feet long and several feet deep form in a matter of seconds.To keep this sidewalk clear without having to shovel what could be tons of snow each time the roof sheds, the same type of tubing was installed in both the sidewalk and a dedicated “melting pad” adjacent to it. Figure 1 shows a cross section of the sidewalk and melt pad area.
The concrete portion of the melt pad area is covered with 3 to 4 inches of #2 crushed limestone. The stone was chosen over an exposed concrete surface for two reasons. First, it keeps people from thinking of the melt pad area as an extension of the sidewalk. This pad is NOT the place to be standing or walking when the roof decides to dump its snow pack. Second, the stone makes a good landing pad for water cascading off the long roof slope during a heavy rain. The stone breaks up the water without excessive spatter or eventual erosion as would be the case with a concrete pavement. Melt water runs down the center channel between the concrete pads and into a stone-filled leaching trench. The soil beneath the drainage area is almost pure sand with excellent percolation. A layer of 1-inch extruded polystyrene beneath the sidewalk and melt pad minimizes downward loss from the slab during warm-up as well as helps retain ground warmth to prevent soil freezing.
The snowmelting manifold is piped to a small brazed plate stainless steel heat exchanger. This assembly forms an isolated closed-loop hydronic circuit and therefore requires its own expansion tank, air separator and relief valve. The temperature of glycol solution is regulated by a variable-speed injection pump between the primary loop and the high temperature side of the heat exchanger. A strap-on aquastat on the primary loop temporarily “sheds” the snowmelting load whenever the primary loop temperature drops below 140°F. This gives priority to space heating and domestic water heating should all loads call for heat simultaneously.
Separate tubing circuits were used for the sidewalk areas and the melt pad. Each circuit is capable of being individually controlled using valve actuators on the manifold. This allows the possibility of operating the melt pad by itself without wasting energy on what could be bare sidewalk areas.
For the present, snowmelting is initiated by a manual switch. The plan is to gain operating experience with the system, especially how the melt pad handles deep snow piles, and then develop a custom automatic control strategy.
Blown Away
Heat is supplied to the system from a 98,000 Btu/hr. oil-fired boiler using a sealed combustion system and sidewall venting. The latter was chosen to eliminate the need for a chimney through the metal roof near the bottom of the long roof slope. In this part of the country chimneys penetrating through the lower portions of long metal roofs have literally been ripped off during snow slides. Although reinforced metal “crickets” have been used to split the sliding snow pack apart up-slope of such chimneys, they require substantial structural reinforcement and several additional penetrations of the roof.
With the sealed combustion system, flue gases are pushed out of the boiler by the static pressure of the oil burner through a short (2.5-foot) length of 4-inch, sealed stainless pipe leading to the outside vent terminal. Combustion air comes in through a coaxial galvanized sleeve, which surrounds the stainless vent pipe. In this application sidewall venting provided a simple, less expensive and certainly less vulnerable alternative to a chimney.
System Piping
Space heating and snow melting are configured as reduced-temperature secondary circuits supplied from a common primary loop. The indirect water heater is piped as a parallel circuit to the primary (rather than as another secondary circuit). This allows domestic water heating to take place without heating up the primary loop, and hence reduces extraneous heat loss during warm weather. A piping schematic of the system is shown inFigure 2.
Water temperature to the floor circuits is regulated by variable-speed injection pumping between the primary loop and the distribution system. Water temperature is reset based on outdoor temperature. At an outdoor design temperature of -15°F, water is supplied to the floor at approximately 96°F. The injection control also monitors the water temperature returning to the boiler to prevent sustained flue gas condensation during recovery from setbacks or other transient loading situations.
A Radiant Future
Hydronic floor heating provided an excellent solution to the heating needs and usage of the new multi-purpose room. The space heating and domestic water heating portions of this system were professionally installed for approximately $3.80 per square foot of heated space (including underslab insulation and moisture barrier). Operating costs are not yet available, but are expected to be very modest in comparison to other projects of similar size and occupancy.It may be some time before the Bethel Baptist needs to build another addition. But when and if that time comes it’s pretty easy to guess how it will be heated.
