RSES Heated air rises over cooler air in a room with a con vective heating system. This creates a circulating pattern. As the air passes over you, heat is transferred by conduction to your skin or clothing. Your body s heat radiates heat to the surfaces of the room and allows these surfaces to re-radiate heat to your body. Another example is when air passes over baseboard convector fins to carry the heat from the fins to a room. A reverse transfer may occur if the fins are cooler than the air. Hydronic radiant heating uses the surfaces of the room itself as the source of heat. Tubing is imbedded in or attac hed to the floor underlayment (although tubing may also be installed in w alls or ceilings). Heated water is then pumped through the tubing, which in turn transfers heat (by conduction) to the slab or flooring. Heat is radiated by the floor to all other surf aces in the room including furniture. These surfaces re-radiate this energy in an even, uniform distribution. The walls and ceiling of the room are at a rela tively uniform temperature, so there is very little stratification of air temperature. As a result, the heat loss in the room may be signif icantly lower than other types of heating systems. Supply and return manifolds Brackets Clamps Cable ties Pressure test kits Cross-sectional view of in-slab application FIGURE 10-1. Tubing imbedded within a concrete slab (w ater temperatures from 80 to 135 F) 4
LESSON 10 It is not unusual to see air temperatures for r adiant systems set at 65 to 68 F in rooms that would have been heated to 70 F with other types of heat. This is due to the re-radiation of energy from the walls themselves, which allows you to hold a lower air temperature for the same comf ort level. Architecturally, radiant systems are desirable, because the heating source to the room is not visible. In addition, zone control or individual room control is available. Hydronic radiant floor (HRF) systems usually r un at lower water temperatures than other types of hydronic systems. In applications where the tubing is imbedded within a concrete slab, as in Figure 10-1, the water temperatures range from 80 to 135 F. For applications where the tubing is attached to the underside of the floor above, as shown in Figure 10-2, the water temperatures may be as high as 165 F. NOTE: A minimum of 2 in. of air space between the bottom of the hose and the foil-faced insulation is required in this application (not shown in the illustration below). Clamps Staples Brackets Pressure test kit Supply and return manifolds FIGURE 10-2. Tubing attached to underside of the floor (w ater temperatures reach 165 F) 5 750-275
RSES Materials used in radiant heating You will find that most tubing is cur rently made either of a plastic or of rubber -based series of compounds held together by various means. Figure 10-3 shows a layered view of typical tubing which is Entran Onix. The different layers of the tube are required for different purposes. The external walls of the tubing are designed to handle the environmental conditions of the outside world, including minor job site abuse. The inner walls are designed to be inert to the temperatures and pressur es of the water system. Often, a protective oxygen barrier is used between the other layers of the tubing. This barrier prevents oxygen from permeating through the tube walls and into the water. The tubing is flexible and comes in rolls as long as several hundred feet. The length helps to keep the number of field connections under the floor to a minimum, because leaks occur where dissimilar materials are joined, such as splices. The tubing is connected to manifold assemblies, as shown in Figure 10-4. These assemblies provide the headers to which the supply and return water will enter or leave the tubing. The manifolds may include balancing valves, air vents, drain ports, zone controls, or combinations of these. Refer to the manufacturer s literature for details. The manifolds are then connected to the supply and return pipes of the system. Heat loss Durel inner tubing Fluid channel AlumaShield oxygen barrier Contour extrusion layer HiGuard industrial cover FIGURE 10-3. Typical tubing Aramid fiber reinforcing A. Commercial manifolds HEATWAY HEATWAY Although standard methods may be used for determining radiant heat loss, they may over-calculate actual heat loss for the reasons mentioned in the first several paragraphs. As a result, the system may need to be designed with outdoor air reset to pr ovide greater flexibility, or it may be designed to use the man ufacturer s heat loss software. B. Swedged manifolds FIGURE 10-4. Typical manifold assemblies If a manufacturer s software is used, then it will be necessary to pro vide complete and accurate information as to R-values (a measure of thermal resistance) of the wall, windows, and doors, as well as the correct dimensions of the rooms being heated. Also, large infiltration rates such as those found in Entran Onix, AlumaShield, Durel, and HiGuard are registered trademarks of Heatway. 6
LESSON 10 Calculated heat loss for zone one at 68 F indoor* Heat loss Infiltration loss Total loss Description Units R-value (Btuh) (Btuh) (Btuh) Exposed walls 159 ft 2 17.00 599 281 880 Windows 20 ft 2 2.04 568 88 656 Doors 21 ft 2 2.5 496 288 784 Unheated floor area 12 ft 2 11.00 69 0 69 Exposed ceiling area 269 ft 2 19.00 908 0 908 Skylights 1 ft 2 1.61 35 7 42 Exhaust fans 1 354 354 Recessed light fixtures 2 646 646 Manual infiltration 0.7 cfm 5 5 Totals 2,675 1,659 4,334 Project totals: Total heat loss = 4,334 Btuh; total back and edge loss = 371 Btuh; Outside air temperature = 3 F. TABLE 10-1. Typical heat loss computation for a zone-one slab application fireplaces or exhaust fans should be considered. Here a gain, the manufacturer can assist in computing these values. Typical heat loss computations are shown in Table 10-1 for a zone-one slab application. Temperatures and spacing To provide the proper amount of heat to the room, you must calculate the design floor temperature. This is the temperature of the floor at the coldest outside air temperature. In most applications, the maximum temperature of the floor is 85 F. Temperatures higher than this may cause discomfort to the occupants of the room. If the heat loss of the room e xceeds the heat output of the floor at 85 F, then it may be necessary to install auxiliary heat to the r oom. Once the floor temperature is calculated, the tube spacing and water temperature is computed. Maximum floor temperatures may be achieved by increasing water temperatures until the floor temperature is satisfied, or by decreasing the spacing between the tubes. Larger spacing means lower first cost, as the total amount of tubing used on the project is decreased. The larger spacing may also mean that the supply water temperature becomes too high, which could cause damage to the slab. Shorter spacing will decrease the required w ater temperature, but it will also result in mor e tubing. The manufacturer of the 7 750-275
RSES tubing can help determine a good balance between these tw o extremes to provide satisfactory performance. The chart in Figure 10-5 provides a good picture of the relationship between temperature and spacing. This chart shows that the required floor temperatur e is to be just over 77 F to provide 26 Btuh per square foot of flooring. This would provide for a 68 F indoor temperature with an outdoor temperature of 5 F. If you spaced the tubing 12 in., then a mean water temperature of 125 F would be required. If you spaced the tubing 6 in. for a 20 F temperature dif ferential, then the mean water temperature would be 117 F, with a corresponding water temperature of 127 F. The dark line at the 85 F mark refers to the maximum floor surface temperature. In this case, 85 F is well beyond the requirements to heat the room. Tube diameter may also effect heat transfer and water temperatures, but to a much smaller degree. Usually, the tubing diameter is selected to k eep pressure drops and velocities within acceptable limits. Radiant floor heat output intensity, Btuh/ft 2 50 45 40 35 30 25 20 15 Waiting room: Slab; 3 8 in. Entran 3d Total floor R-value = 1.0 Indoor design temp. = 68 F Outdoor design temp. = 5 F Back and perimeter heat loss intensity, Btuh/ft 2 3.3 4.4 6 in. 9 in. 12 in. 90 87 85 82 79 77 74 70 Effective floor surface temperature, F 10 66 5 Comfort limit Required output Design point 61 0 59 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 Mean water temperature, F FIGURE 10-5. Relationship between temperature and spacing of tubing 8
LESSON 10 INSTALLATION Imbedded systems Imbedded systems are those in which the entire tube is b uried within the concrete or thin coat slab. Therefore, the entire cross-sectional area of the tube is in contact with the slab. Figures 10-6 to 10-11 (F igures 10-9, 10-10, 10-11 are found on the next page) show several common imbedded-type systems. Insulation Tubing Steel deck Structural slab FIGURE 10-6. Slab on grade Foil-faced insulation facing up and 2-in. minimum air space between the foil surface and the steel deck 2-in. minimum of concrete above the top of the tubing Use extruded polystyrene insulation board on the edge of the slab and optionally under the slab Tubing FIGURE 10-7. Slab over steel deck If the space permits, use extruded polystyrene insulation at the perimeter of the new slab Use poly-fiber material in the concrete slab to add crack resistance HEATWAY HEATWAY HEATWAY Tie the tubing to rewire or to poultry netting FIGURE 10-8. Slab over existing slab 9 750-275