Document 4J5kkvjwVZBpOOQ9zK0v0OQkR

1054 CHAPTER 41 1958 Guide y' plaster is drying out, the range and rate of temperature changes (either increase or decrease), should be kept low. Vermiculite or other insulating plaster must not be used, since it causes cables to overheat. For laminated ceiling construction, cable is stapled to non-metallic lath as for plaster, then a second layer of non-insulating ceiling board is applied with a layer at least f in. of plaster or similar compound to sandwich the cable firmly in place and improve heat transfer. Thermostats should be of high sensitivity type with a manual switch and have other characteristics like those described for conductive rubber panel installations. Electric Wall Panels Cable embedded in walls similar to ceiling construction is occasionally found in Europe. Because of possibility of damage due to nails driven for hanging pictures or from building alteration, most codes prohibit such panels in the United States. Interference with radiation due to furniture placement and interior decorations reduce the desirability of wall panels. Electric Floor Panels Resistor cables of the same types which are used for ceiling panels are employed also for floor panel systems in concrete slabs. The thickness of the concrete slab is usually made a minimum of 3 in. when placed directly on soil. Precautions to be observed in construction of floor slabs for panel heating are covered in Chapters 12 and 23. Non-insulating cement finish If in. thick is poured and trowled above the cables. They are stapled at the specified spacing to wood nailing strips laid on the rough concrete slab. It is desirable that cables be secured in place either with a f to J-in. layer of cement wash or thinned grout before the final lf-in. cement or terrazzo finish is applied, or by tacking at 2 ft intervals with daubs of cement, plaster-of-Paris, strips of masking tape or other non-conductive material. When the slab is made of light-weight insulating concrete, cables may be stapled directly to the slab. Alternate special anchors are available to hold the cable at proper spacing during pouring, and are held by nails driven by hand or powder impact drivers. With monolithic finish, nailing strips are omitted and the cable is strung on frames with nail spacers; frames are removed when enough concrete has been poured to hold the cable in place. If desired, magnesite flooring, asphalt tile or wall-to-wall carpeting may be used on the floor. Insulating type concrete may be used below cable, but must not be used around or over cable. The minimum spacing between cable passes is 1 in., and clearance of 1 in. must be maintained between the heating cable and other metallic bodies embedded in the floor. Non-heating leads run in rigid conduit between the wall outlet box and floor, terminating through bushings beneath the sur face of the final coating. Cable circuits are tested for continuity of circuit and for insulation resistance of at least 100,000 ohms to ground before and after final pour. Central Hot-Water Systems Heating systems of hot-water type using conventional radiator or con vector units, discussed in Chapter 21, may be operated electrically by means of heat exchangers containing immersion elements (resistors) 0 Electric Heating 1055 the required capacity. Control of the resistor circuit by a thermostat within the exchanger is usually desirable. The water holding capacity of exchanger should be made sufficient to minimize frequency of cycling of the heavy electric load from the resistor, and for safety. Resistors may be interlocked with the circuit of the water circulating pump, so that they can be energized only when circulating pump is in operation under control of the room thermostat. For off-peak operation, where rate schedules for electric power make this desirable, a water heating tank of large storage capacity is employed. The system may be desijgned for a maximum water temperature of 250 to 275 F at pressures up to 75 psig. An automatic valve is required to provide 140 to 160 F temperature at the pump, by mixing hot water from the tank outlet with cool water from the return main. Another, method for heat delivery employs the flash principle, withdrawing water, at .high storage temperature into a low-pressure separating chamber where steam is obtained as a result of the pressure reduction; however, power for pump ing is substantially greater with this steam-accumulator method. In practice, tanks up to 1200 gal with immersion elements of 25 to 30 kw have been used for installations with design heat requirements up to 90,000 Btu per hr. Thermal insulation on the tank piping must have low heat transmission in order to cut down the daily and seasonal standby losses. Design of such off-peak systems requires careful analysis of all factors, both thermal and economic. High investment, space required for equip ment, and other practical considerations tend to limit the application of this type of electric heating. Central Warm-Air Systems These systems usually employ heaters consisting of resistors mounted in a frame or housing, as mentioned under Electric Heating Units earlier in this chapter. In many cases, arrangement of the supply and return duct system can be planned advantageously to facilitate future addition of equipment for summer air conditioning. Conversion of Existing Heating Systems Changeover of existing conventional fuel-type house heating systems to electric operation, by substituting resistors in place of the fuel equipment in the combustion chamber of a warm-air furnace or boiler, is seldom de sirable. Excessive thermal losses from such central equipment and lack 01 adequate refinements in the distribution facilities and controls, will usually cause high operating cost. However, where heat delivery into the system is accomplished by a steam or hot-water coil assembly located within a duct system, replacement or supplementing with a bank of resistors like noseemployed in electric unit heaters is sometimes advantageous. A case oi this kind occurs where standby heating is needed to maintain safe or comfortable temperature in isolated rooms, during emergency conditions r when the normal electric service for lighting and motors is not in use. Heat Pump Systems he^^616 summer a*r conditioning is wanted in addition to heating, the at-pump type of all-year system, which utilizes a refrigeration com- numV?r Uf1't 'n. conjunction with a central-plant installation, is finding a forVi + aPP^cations- It reduces electric input demand and consumption the ndTln.to about one-half or one-third that with the resistance method, reduction depending mainly on temperature level of the heat source