Thermal Systems

The major components of a drainback solar water heater consists of liquid-heating solar collectors, a hot water storage tank, heat exchanger, and a pump with controller. The system works by pumping water from the storage tank to the solar collectors, where it is heated and returns to the storage tank. Your city water flows through the heat exchanger (typically mounted inside the storage tank for optimum performance) before entering your water heater, picking up heat. The result; your water heater runs less often, for shorter periods of time, and uses less fuel. Solar energy is free, all you need is the equipment to collect it.

Radiant floor heating takes a quantity of warm water and circulates it through the floor, gently and evenly warming the entire area. This approach is very energy efficient, often using 1/3 less fuel to heat the area. Additionally, because the entire floor is warm there are no hot or cold spots, with no clogged furnace filters trapping mold and pollen spores inside your home. Even more efficient is to integrate your radiant floor system with a solar water heating system; not only does the solar contribute to your heating, but the storage tank allows you to "bank" that heat for use when you need it most. Consider a storage tank of just 100 gallons; with water just 30 degrees warmer than the temperature of the water you circulate through your floor, your tank is storing approximately 25,000Btu of heat for use whenever you need it!

Ground-source heat pumps are a way to capture thermal energy stored in the earth. Even frozen ground is often at least slightly warmer than the air outside, and these systems harvest that energy and heat your home. These systems enjoy very high efficiencies, and are designed for decades of reliable service.

Thermal tanks are the heart of almost all thermal products. Not only are these super-insulated units capable of storing heat for use when you need it, but are designed for maximum capacity in the available space. Six Rivers Solar, Inc. has the largest capacity range of SRCC-certified systems in the United States, ranging from 50 to over 300 gallons. Our TrendSetter tanks are backed by our unbeatable twenty-year limited warranty, with our non-ferrous composite construction guaranteed not to rush. Our tanks come with sensors and heat exchangers installed, with the capability to manufacture both standard and custom units.

Solar Water Heating

Solar Water Heater Basics
Solar water heaters are made up of collectors, storage tanks, and a pump with a controller. Six Rivers Solar, Inc. recommends drainback systems for any location with a possibility of freezing weather. Drainback systems enjoy the most resistance to freezing, because all fluid drains back to an insulated storage tank. Since no fluid remains in the collectors, no fluid remains where it could potentially freeze.

The most common type of collector is the flat-plate collector. These units are an insulated, weather-proofed box containing a dark absorber plate under one or more transparent or translucent covers. Be aware that other types of collectors exist, but their numbers are considerably smaller.

Drainback solar water heating systems (and almost all other types as well) require a well-insulated storage tank. Some older systems used converted electric water heater tanks, but modern drainback technology plumbs the thermal storage tank in series with the conventional water heater. In this arrangement, the solar water heater preheats water before it enters the conventional water heater.

Some solar water heaters use pumps to recirculate warm water from storage tanks through collectors and exposed piping. This is generally to protect the pipes from freezing when outside temperatures drop to freezing or below.

Sizing Your System
Just as you have to choose a 30-, 40-, or 50-gallon (114-, 151-, or 189-liter) conventional water heater, you need to determine the right size solar water heater to install. Sizing a solar water heater involves determining the total collector area and the storage volume required to provide 100% of your household's hot water during the summer. Solar-equipment experts use worksheets or special computer programs to assist you in determining how large a system you need.

Solar storage tanks range from 50 gallons to hundreds of gallons in capacity. A small (50 to 60 gallon) system is sufficient for 1 to 3 people, a medium (80-gallon) system is adequate for a 3- or 4-person household, and a large (120-gallon) system is appropriate for 4 to 6 people. For other applications such as laundries or large-scale radiant floor applications, thermal tanks with capacities of hundreds of gallons may be required.

A rule of thumb for sizing collectors: allow about 20 square feet (about 2 square meters) of collector area for each of the first two family members and 8 square feet (0.7 square meter) for each additional family member if you live in the Sun Belt. Allow 12 to 14 additional square feet (1.1 to 1.3 square meters) per person if you live in the northern United States.

A ratio of at least 1.5 gallons (5.7 liters) of storage capacity to 1 square foot (0.1 square meter) of collector area is recommended to prevent the system from overheating when the demand for hot water is low. In very warm, sunny climates, experts suggest that the ratio should be at least 2 gallons (7.6 liters) of storage to 1 square foot (0.1 square meter) of collector area. For example, a family of four in a northern climate would need between 64 and 68 square feet (5.9 and 6.3 square meters) of collector area and a 96- to 102-gallon (363- to 386-liter) storage tank. (This assumes 20 square feet of collector area for the first person, 20 for the second person, 12 to 14 for the third person, and 12 to 14 for the fourth person. This equals 64 to 68 square feet, multiplied by 1.5 gallons of storage capacity, which equals 96 to 102 gallons of storage.) Because you might not be able to find a 96-gallon tank, you may want to get a 120-gallon tank to be sure to meet your hot water needs.

Benefits of Solar Water Heaters
There are many benefits to owning a solar water heater, and number one is economics. Solar water heater economics compare quite favorably with those of electric water heaters, while the economics aren't quite so attractive when compared with those of gas water heaters. Heating water with the sun also means long-term benefits, such as being cushioned from future fuel shortages and price increases, and environmental benefits.

Economic Benefits
It makes economic sense to think beyond the initial purchase price and consider lifetime energy costs, or how much you will spend on energy to use the appliance over its lifetime. The Florida Solar Energy Center (FSEC—see Source List) studied the potential savings to Florida homeowners of common water-heating systems compared with electric water heaters. It found that solar water heaters offered the largest potential savings, with solar water-heater owners saving as much as 50% to 85% annually on their utility bills over the cost of electric water heating.

The FSEC analysis illustrates that the initial installed cost of the solar water heater ($2,000 to $3,500) is higher than that of a gas water heater ($350 to $450) or an electric water heater ($050 to $350). The costs vary from region to region, so check locally for costs in your area. Depending on the price of fuel sources, the solar water heater can be more economical over the lifetime of the system than heating water with electricity, fuel oil, propane, or even natural gas because the fuel (sunshine) is free.

Paybacks vary widely, but you can expect a simple payback of 4 to 8 years on a well-designed and properly installed solar water heater. (Simple payback is the length of time required to recover your investment through reduced or avoided energy costs.) You can expect shorter paybacks in areas with higher energy costs. After the payback period, you accrue the savings over the life of the system, which ranges from 15 to 40 years, depending on the system and how well it is maintained.

You can determine the simple payback of a solar water heater by first determining the net cost of the system. Net costs include the total installed cost less any tax incentives or utility rebates. (See the box for more information.) After you calculate the net cost of the system, calculate the annual fuel savings and divide the net investment by this number to determine the simple payback.

An example: Your total utility bill averages $240 per month and your water heating costs are average (25% of your total utility costs) at $60 per month. If you purchase a solar water heater for $2,000 that provides an average of 60% of your hot water each year, that system will save you $36 per month ($60 x 0.60 = $24) or $432 per year (12 x $36 = $288). This system has a simple payback of less than 5 years ($2,000 ÷ $412 = 4.7). For the remainder of the life of the solar water heater, 60% of your hot water will be free, saving you $432 each year plus any additional rate increases. You will need to account for some operation and maintenance costs, which are estimated at $25 to $30 a year. This is primarily to have the system checked every 3 years.

If you are building a new home or refinancing your present home to do a major renovation, the economics are even more attractive. The cost of including the price of a solar water heater in a new 30-year mortgage is usually between $13 and $20 per month. The portion of the federal income tax deduction for mortgage interest attributable to the solar system reduces that amount by about $3 to $5 per month. If your fuel savings are more than $15 per month, the investment in the solar water heater is profitable immediately.

Tax Incentives and Rebates
Some local or state governments offer tax incentives to encourage residents to invest in solar energy technologies. Check with your state or local energy office or Department of Revenue for information. Some electric utilities offer rebates to customers who install solar energy equipment because these installations help utilities reduce peak loads. Peak loads are periods when the utility must generate extra power to meet a high demand. Heating water in the evening is one example.

Commercial Systems

HeatModuleTM
Our standard large-scale commercial water heating system. This system is designed around an 840-gallon thermal tank, with integral heat exchangers sized to flow a 2" water line. Sized to provide hundreds of gallons of hot water daily, it is supplied by [12] 4' x 10' thermal panels from SunEarth. It's modular design allows companies to add extra modules as required, with their capacity expanding as their business expands.

HeatModuleJRTM
Built to accommodate smaller but still significant water heating requirements, this system is designed around an 340-gallon thermal tank, with integral heat exchangers sized to flow a 2" water line. Sized to provide hundreds of gallons of hot water daily, it is supplied by [5] 4' x 10' thermal panels from SunEarth. As your demands increase, the modular nature of these systems means you can add new modules, either another HeatModuleJR or the larger HeatModule system!

Heat Pumps

While ground-source heat pumps are making progress in residential applications, they can also be applied to any heating and/or cooling requirement. Their application offers significant potential energy savings to a wide range of commercial-type facilities at Federal installations: administrative offices, hospitals and clinics, schools and training facilities, communications facilities, dormitories and hotels, clubs and recreation buildings, restaurant and dining facilities, commissary and exchanges, and others.

Other Benefits
The primary benefit of ground-source heat pumps is the increase in operating efficiency, which translates to reduced heating and cooling costs, but there are additional advantages. One notable benefit is that ground-source heat pumps, although electrically driven, are classified as a renewable-energy technology. The justification for this classification is that the ground acts as an effective collector of solar energy. The renewable-energy classification can affect Federal goals and potential funding.

An environmental benefit is that ground-source heat pumps typically use 25% less refrigerant than split-system air-source heat pumps or air-conditioning systems. Ground-source heat pumps generally do not require tampering with the refrigerant during installation. Systems are generally sealed at the factory, reducing the potential for leaking refrigerant in the field during assembly.

Ground-source heat pumps also require less floor space than conventional heating and cooling systems. Because the exterior system (the ground coil) is underground, there are no space requirements for cooling towers or air-cooled condensers. In addition, the ground-coupling system does not necessarily limit future use of the land area over the ground loop. Interior space requirements are also reduced. There are no floor space requirements for boilers or furnaces, just the unitary systems and circulation pumps. Furthermore, many distributed ground-source heat pump systems are designed to fit in ceiling plenums, reducing the floor space requirement of central mechanical rooms.

Compared with air-source heat pumps that use outdoor air coils, ground-source heat pumps do not require defrost cycles or crankcase heaters and there is virtually no concern for coil freezing. Cooling tower systems require electric resistance heaters to prevent freezing in the tower basin, also not necessary with ground-source heat pumps.

It is generally accepted that maintenance requirements are also reduced, although research continues directed toward verifying this claim. It is clear, however, that ground-source heat pumps eliminate the exterior fin-coil condensers of air-cooled refrigeration systems and eliminate the need for cooling towers and their associated maintenance and chemical requirements. This is a primary benefit cited by facilities in highly corrosive areas, such as near the ocean where salt spray can significantly reduce outdoor equipment life.

Ground-source heat pump technology offers further benefits: the need for supplemental resistance heaters is reduced compared with air-source heat pumps, no exterior coil freezing (requiring defrost cycles) such as that associated with air-source heat pumps, improved comfort during the heating season (compared with air-source heat pumps the supply air temperature does not drop when recovering from the defrost cycle), significantly reduced fire hazard over that associated with fossil fuel-fired systems, reduced space requirements and hazards by eliminating fossil-fuel storage, and reduced local emissions from those associated with other fossil fuel-fired heating systems.

Another benefit is quieter operation, because ground-source heat pumps have no outside air fans. Finally, ground-source heat pumps are reliable and long-lived, because the heat pumps are generally installed in climate-controlled environments and therefore are not subject to the stresses of extreme temperatures. Because of the materials and joining techniques, the ground-coupling systems are also typically reliable and long-lived. For these reasons, ground-source heat pumps are expected to have a longer life and require less maintenance than alternative (more conventional) technologies.

Ground-Coupled System Types
The ground-coupling systems used in ground-source heat pumps fall under three main categories: closed-loop, open-loop and direct-expansion. The type of ground coupling employed will affect heat pump system performance (therefore the heat pump energy consumption), auxiliary pumping energy requirements, and installation costs. Choice of the most appropriate type of ground coupling for a site is usually a function of specific geography, available land area, and life-cycle cost economics.

Closed-loop systems. Closed-loop systems consist of an underground network of sealed, high-strength plastic pipe acting as a heat exchanger. The loop is filled with a heat transfer fluid, typically water or a water-antifreeze (a)(b) solution, although other heat transfer fluids may be used. When cooling requirements cause the closed-loop liquid temperature to rise, heat is transferred to the cooler earth. Conversely, when heating requirements cause the closed-loop fluid to drop, heat is absorbed from the warmer earth. Closed-loop systems utilize pumps to circulate the heat transfer fluid between the heat pump and the ground loop. Because the loops are closed and sealed, the heat pump heat exchanger is not subject to mineral build-up and there is no direct interaction (mixing) with ground water.

There are several varieties of closed-loop configurations including horizontal, spiral, vertical, and submerged.

Horizontal loops. Horizontal loops, illustrated in Figure 2a, are often considered when adequate land surface is available. The pipes are placed in trenches, typically at a depth of 4 to 10 feet (1.2 to 3.0 m). Depending on the specific design, anywhere from one to six pipes may be installed in each trench. Although requiring more linear feet of pipe, multiple-pipe configurations conserve land space, require less trenching and therefore frequently cost less to install than single-pipe configurations. Trench lengths can range from 100 to 400 feet per system cooling ton (8.7 to 34.6 m/kW), depending on soil conditions and the number of pipes in the trench. Trenches are usually spaced from 6 to 12 feet (1.8 to 3.7 m) apart. These systems are common in residential applications but are not frequently applied to large-tonnage commercial applications because of the significant land area required for adequate heat transfer. The horizontal-loop systems can be buried beneath lawns, landscaping, and parking lots. Horizontal systems tend to be more popular where there is ample land area with a high water table.

Fig. 2. Ground-Coupling System Types

Advantages: Trenching costs typically lower than well-drilling costs; flexible installation options.
Disadvantages: Large ground area required; ground temperature subject to seasonal variance at shallow depths; thermal properties of soil fluctuate with season, rainfall, and burial depth; soil dryness must be properly accounted for in designing the required pipe length, especially in sandy soils and on hilltops that may dry out during the summer; pipe system could be damaged during backfill process; longer pipe lengths are required than for vertical wells; antifreeze solution viscosity increases pumping energy, decreases the heat transfer rate, and thus reduces overall efficiency; lower system efficiencies.
Spiral loops. A variation on the multiple pipe horizontal-loop configuration is the spiral loop, commonly referred to as the "slinky." The spiral loop, illustrated in Figure 2b, consists of pipe unrolled in circular loops in trenches; the horizontal configuration is shown. Another variation of the spiral-loop system involves placing the loops upright in narrow vertical trenches. The spiral-loop configuration generally requires more piping, typically 500 to 1,000 feet per system cooling ton (43.3 to 86.6 m/kW), but less total trenching than the multiple horizontal-loop systems described above. For the horizontal spiral-loop layout, trenches are generally 3 to 6 feet (0.9 to 1.8 m) wide; multiple trenches are typically spaced about 12 feet (3.7 m) apart. For the vertical spiral-loop layout, trenches are generally 6 inches (15.2 cm) wide; the pipe loops stand vertically in the narrow trenches. In cases where trenching is a large component of the overall installation costs, spiral-loop systems are a means of reducing the installation cost. As noted with horizontal systems, slinky systems are also generally associated with lower-tonnage systems where land area requirements are not a limiting factor.

Advantages: Requires less ground area and less trenching than other horizontal loop designs; installation costs sometimes less than other horizontal loop designs.
Disadvantages: Requires more total pipe length than other ground-coupled designs; relatively large ground area required; ground temperature subject to seasonal variance; larger pumping energy requirements than other horizontal loops defined above; pipe system could be damaged during backfill process.
Vertical loops. Vertical loops, illustrated in Figure 2c, are generally considered when land surface is limited. Wells are bored at typical depths from 75 to 300 feet (22.9 to 91.4 m) deep. The closed-loop pipes are inserted into the vertical well. Typical piping requirements range from 200 to 600 feet per system cooling ton (17.4 to 52.2 m/kW), depending on soil and temperature conditions. Multiple wells are typically required, typically spaced between 10 and 16 feet (3.0 and 4.9 m) apart and piped either in series and/or in parallel in order to achieve the total heat transfer requirements.

Vertical systems tend to be more popular where land area is limited, where the water table is deep, and where the ground is rocky or bedrock. There are three basic types of vertical-system heat exchangers: U-tube, divided-tube and concentric-tube (pipe-in-pipe) system configurations.

Advantages: Requires less total pipe length than most closed-loop designs; requires the least pumping energy of closed-loop systems; requires least amount of surface ground area; ground temperature typically not subject to seasonal variation.
Disadvantage: Requires drilling equipment; drilling costs frequently higher than horizontal trenching costs; some potential for long-term heat build-up underground.
Submerged loops. If a moderately sized pond or lake is available, the closed-loop piping system can be submerged, as illustrated in Figure 2d. Some companies have installed ponds on facility grounds to act as ground-coupled systems; ponds also serve to improve facility aesthetics. Submerged-loop applications require some special considerations, and it is best to discuss these directly with an engineer experienced in the design applications. This type of system requires adequate surface area and depth in order to function adequately in response to heating or cooling requirements under local weather conditions. In general, the submerged piping system is installed in loops attached to concrete anchors. Typical installations require around 300 feet of heat transfer piping per system cooling ton (26.0 m/kW) and around 3,000 square feet of pond surface area per ton (79.2 m2/kW) with a recommended minimum one-half acre total surface area. The concrete anchors act to secure the piping, restricting movement, but also hold the piping 9 to 18 inches (22.9 to 45.7 cm) above the pond floor, allowing for good convective flow of water around the heat transfer surface area. It is also recommended that the heat-transfer loops be at least 6 to 8 feet (1.8 to 2.4 m) below the pond surface, preferably deeper. This maintains adequate thermal mass even in times of extended drought or other low-water conditions. Rivers are typically not used because they are subject to drought and flooding, both of which may damage the system.

Advantages: Can require the least total pipe length of other closed-loop designs; can be less expensive design compared with other closed-loop designs if body of water available.
Disadvantage: Requires a large body of water.
Open-Loop Systems. Open-loop systems utilize local ground water as a direct heat transfer medium instead of the heat transfer fluid described for the closed-loop systems. These systems are sometimes referred to specifically as "ground-water-source heat pumps" to distinguish them from other ground-source heat pumps. Open-loop systems consist primarily of extraction wells, extraction and reinjection wells, or surface water systems. These three types are illustrated in Figures 2e, 2f, and 2g, respectively.

A variation on the extraction well system is the standing column well. This system reinjects the majority of the return water back into the source well, minimizing the need for a reinjection well and minimizes the amount of surface discharge water. Use of this system has been noted in the New England area but may be applicable in other areas.

There are several special factors to consider in open-loop systems. One major factor is water quality. In open-loop systems, the primary heat exchanger between the refrigerant and the ground water is subject to fouling, corrosion and blockage. A second major factor is the adequacy of available water. The required flow rate through the primary heat exchanger between the refrigerant and the ground water is typically between 1.5 and 3.0 gallons per minute per system cooling ton (0.027 and 0.054 L/s-kW). This can add up to a significant amount of water and can be affected by local water resource regulations. A third major factor is what to do with the discharge stream. The ground water must either be reinjected into the ground by separate wells or discharged to a surface system such as a river or lake. Local codes and regulations may affect the feasibility of open-loop systems.

Depending on the well configuration, open-loop systems can have the highest pumping load requirements of any of the ground-coupled configurations. In ideal conditions, however, an open-loop application can be the most economical type of ground-coupling system.

Advantages: Simple design; lower drilling requirements than closed-loop designs; subject to better thermodynamic performance than closed-loop systems because well(s) are used to deliver ground water at ground temperature rather than as a heat exchanger delivering heat transfer fluid at temperatures other than ground temperature; typically lowest cost; can be combined with potable water supply well; low operating cost if water already pumped for other purposes, such as irrigation.
Disadvantages: Subject to various local, state and Federal clean water and surface water codes and regulations; large water flow requirements; water availability may be limited or not always available; heat pump heat exchanger subject to suspended matter, corrosive agents, scaling, and bacterial contents; typically subject to highest pumping power requirements; pumping energy may be excessive if the pump is oversized or poorly controlled; may require well permits or be restricted for extraction; water disposal can limit or preclude some installations; high cost if reinjection well required.
Direct-Expansion Systems. Each of the ground-coupling systems described above utilizes an intermediate heat transfer fluid to transfer heat between the earth and the refrigerant. Use of an intermediate heat transfer fluid necessitates a higher compression ratio in the heat pump in order to achieve sufficient temperature differences in the heat transfer chain (refrigerant to fluid to earth). Each also requires a pump to circulate water between the heat pump and the ground-couple. Direct-expansion systems, illustrated in Figure 2h, remove the need for an intermediate heat transfer fluid, the fluid-refrigerant heat exchanger, and the circulation pump. Copper coils are installed underground for a direct exchange of heat between refrigerant and earth. The result is improved heat transfer characteristics and thermodynamic performance.

The coils can be buried either in deep vertical trenches or wide horizontal excavations. Vertical trenches typically require from 100 to 150 square feet of land surface area per system cooling ton (2.6 to 4.0 m2/kW) and are typically 9 to 12 feet (2.7 to 3.7 m) deep. Horizontal installations typically require from 450 to 550 square feet of land area per system cooling ton (11.9 to 14.5 m2/kW) and are typically 5 to 10 feet (1.5 to 3.0 m) deep. Vertical trenching is typically not recommended in sandy, clay or dry soils.

Because the ground coil is metal, it is subject to corrosion (the pH level of the soil should be between 5.5 and 10, although this is normally not a problem). If the ground is subject to stray electric currents and/or galvanic action, a cathodic protection system may be required. Because the ground is subject to larger temperature extremes from the direct-expansion system, there are additional design considerations. In winter heating operation, the lower ground coil temperature may cause the ground moisture to freeze. Expansion of the ice buildup may cause the ground to buckle. Also, because of the freezing potential, the ground coil should not be located near water lines. In the summer cooling operation, the higher coil temperatures may drive moisture from the soil. Low moisture content will change soil heat transfer characteristics.

Only one U.S. manufacturer currently offers direct-expansion ground-source heat pump systems. Systems are available from 24,000 Btu/h to 60,000 Btu/h (heating/cooling capacity) (7.0 to 17.6 kW). Larger commercial applications require multiple units with individual ground coils.

Advantages: Higher system efficiency; no circulation pump required.
Disadvantages: Large trenching requirements for effective heat transfer area; ground around the coil subject to freezing (may cause surface ground to buckle and can freeze nearby water pipes); copper coil should not be buried near large trees where root system may damage the coil; compressor oil return can be complicated, particularly for vertical heat exchanger coils or when used for both heating and cooling; leaks can be catastrophic; higher skilled installation required; installation costs typically higher; this system type requires more refrigerant than most other systems; smaller infrastructure in the industry.

Radiant Floor Heating Systems

Advantages of Radiant Floor Heating
Most people who own radiant floor heating feel that the most important advantages are comfort and quiet operation. Radiant floor systems allow even heating throughout the whole floor, not just in localized spots as with wood stoves, hot air systems, and other types of radiators. The room heats from the bottom up, warming the feet and body first. Radiant floor heating also eliminates the draft and dust problems associated with forced-air heating systems.

Even heat distribution may result in lower heating bills. With radiant floor heating, you may be able to set the thermostat several degrees lower, relative to other types of central heating systems. This is because the entire surface of the floor radiates about the same amount of heat that the human body does, making the occupant feel warm even though the air temperature might be only 65ºF (18ºC). It also radiates this heat for a long period of time. Radiant systems may result in less infiltration of outside air into the house compared to houses with forced-air heating.

Radiant floor heating also allows for lower boiler temperatures, which may result in the boiler lasting longer (a 45 year life is not unusual). Radiant floors operate between 85-120ºF, compared to other hydronic heating systems' range of 130-160ºF.

To some, the greatest advantage of radiant floor heating is aesthetic. The system is "invisible." There are no heat registers or radiators to obstruct furniture arrangements and interior design plans. Radiant floor systems also eliminate the fan noise of forced hot air systems.

Hydronic (liquid) systems are the most popular and cost-effective systems for heating-dominated climates. They have been in extensive use in Europe for decades. Hydronic radiant floor systems pump heated water from a boiler through tubing laid in a pattern underneath the floor. The temperature in each room is controlled by regulating the flow of hot water through each tubing loop. This is done by a system of zoning valves or pumps and thermostats.

Installation Types
Wet installations are the oldest form of modern radiant floor systems. In a "wet" installation, the tubing is embedded in the concrete foundation slab, or in a lightweight concrete slab on top of a subfloor, or over a previously poured slab. If the new floor is not on solid earth, additional floor support may be necessary because of the added weight. You should consult a professional engineer to determine the floor’s carrying capacity.

However, due to recent innovations in floor technology, "dry" floors have been gaining a lot of popularity over wet floors. Much of this is because a dry floor is faster and less expensive to build. There are several ways to make a dry radiant floor. Some "dry" installations involve suspending the tubing underneath the subfloor between the joists. This method usually requires drilling through the floor joists in order to install the tubing. Reflective insulation must also be installed under the tubes to direct the heat upward. Tubing may also be installed from above the floor, between two layers of subfloor. In these instances, the tubes are often in aluminum diffusers that spread the water’s heat across the floor in order to heat the floor more evenly. The tubing and heat diffusers are secured between furring strips (sleepers) which carry the weight of the new subfloor and finished floor surface.

Wirsbo has improved on this idea by making a plywood subfloor material manufactured with tubing grooves and aluminum heat diffuser plates built into them. The manufacturer claims that this product makes a radiant floor system (for new construction) considerably less expensive to install and faster to react to room temperature changes. Such products also allow for the use of half as much tubing since the heat transfer characteristics of the floor is greatly improved over more traditional dry or wet floors.

Floor Coverings
Although ceramic tile is the most common floor covering for radiant floor heating, almost any floor covering can be used. However, some perform better than others. Common floor coverings like vinyl and linoleum sheet goods, carpeting, wood or bare concrete is often specified. However, it is wise to always remember that anything that can insulate the floor also reduces or slows the heat entering the space from the floor system. This in turn increases fuel consumption.

If you want carpeting, use a thin carpet with dense padding and install as little carpeting as possible. If some rooms, but not all, will have a floor covering then those rooms should have a separate tubing loop to make the system heat these spaces more efficiently. This is because the water flowing under the covered floor will need to be hotter to compensate for the floor covering.

Most radiant floor references also recommend using laminated wood flooring instead of solid wood. This reduces the possibility of the wood shrinking and cracking from the drying effects of the heat. While solid wood flooring can be used, the installer is strongly advised to be very familiar with radiant floor systems before attempting to install natural wood flooring over a radiant floor system. Most manufacturers and manuals relating to radiant floors offer guidelines to help you resolve these issues.

Types of Tubing
Older radiant floor systems used either copper or steel tubing embedded in the concrete floors. Unless the builder coated the tubing with a protective compound, a chemical reaction between the metal and the concrete often led to corrosion of the tubing, and to eventual leaks. Major manufacturers of hydronic radiant floor systems now use cross-linked polyethylene (PEX) or rubber tubing with an oxygen diffusion barrier. These materials have proven themselves to be more reliable than the older choices in tubing. Fluid additives also help protect the system from corrosion.

There have been recent reports of problems with rubber tubing produced by one chemical manufacturer. Leaks develop at the metal connections or fittings, and in some cases the tubing becomes rigid and brittle. It is still not clear what causes this problem, but theoretically excessively high water temperatures may be to blame. Tightening connections and clamps only temporarily fixes the leaks. Remember this problem only concerns a specific brand of rubber tubing. It does not have anything to do with the PEX tubing, which has performed very reliably for many decades.

Since the price of copper tubing is considerably lower now than several years ago, it is again gaining some popularity because of it’s superior heat transfer abilities over plastic-based tubing.

Controlling the System
A radiant floor that uses a concrete slab takes many hours to heat up if it is allowed to become cold. This can be very inconvenient while waiting for the slab heat up so it can heat the space. Because of this, most radiant floor systems are not permitted to go into a very deep night setback. Depending on how the floor is constructed, the time it takes to re-heat the floor is sometimes longer than the occupant’s sleep period.

Many floor systems are also controlled by a floor thermostat instead of a wall thermostat. The system is also often designed to keep the circulation pump(s) running while the thermostat only controls the boiler’s burner. Other, more sophisticated, types of controls sense the floor temperature, outdoor temperature, and room temperature to keep the home comfortable. Such a system may also use less fuel.

Although radiant floor systems are usually heated by a boiler, they can also be heated with a geothermal heat pump. Such a system offers even greater energy savings in climates where the heating and cooling loads are similar in size. Another alternative for small houses, or those with small heating loads, is to use an ordinary gas water heater to supply the radiant floor system.

Solar Radiant Floor Heating
Radiant floor heating is a centuries old heating technique. Radiant floor heating systems use channels or pipes that are embedded in-or installed under-the floor. Floors made of concrete or some other dense material perform best, but lightweight floors have also been used with moderate success. A heated fluid (air, water, or other heat transfer fluid) is blown or pumped through this network. The thermal mass of the floor absorbs the heat from the fluid and radiates it evenly into the living space. The thermal mass of the floor acts as a heat battery, making these systems very efficient. Radiant floor heating systems also allow the heating appliance to fire at a slower rate and less often, thus saving fuel.

Solar hydronic radiant floor systems pump heated water from a storage tank through tubing laid in a pattern underneath the floor. An individual thermostat regulates the flow of heated water through a solenoid valve and manifold to each room in the house.

There are a number of ways you can use active solar collector systems with hydronic radiant floors. The simplest is to pump the heat transfer fluid from the solar collectors directly through the tubing in the floor. In this type of system, however, it is difficult to control the temperature of the water. Care must be taken to avoid under or overestimating the system's collector area-to-storage mass ratio. If you overestimate this ratio, there will not be enough heat; if you underestimate it, the space could overheat. Most solar systems have the solar loop pump regulated by an electronic pump control.

Many solar space-heating systems pump the heated liquid to a storage tank. The water in the tank is then pumped through the radiant floor piping. This type of solar radiant floor heating system provides greater heat storage capacity and better control over the floor temperature. Such systems usually have a heat exchanger in the storage tank for preheating domestic hot water.

On cloudy days, when the sun is not bright enough to heat the home by itself, a back-up heating system is necessary. Back-up heating may be one of several heat sources: a wood stove, gas or oil-fired burners, or electric heating elements in the boiler. A conventional domestic water heater may be sufficient as a backup heater.