Document LoyKm7MNGaxMmpEykM77JVDBq

584 CHAPTER 21 i-r1958 Guide' venting back-flow through the pump circuit is provided, r., Where .tempera-- tures higher than normal are maintained in the boiler, the connection of/the return 1ni!fiie boiler-shbuld'follow"'the' manufacturer>;'recbriimendations 'for such seivice. , Where a multiple boiler installation is used, provisions, should be made to prevent the,by-passing of yyater,through a boiler which is not being main?, tained at control temperature. In some cases this provision takes the form of automatic control valves , interlocked with the .boiler temperature control to be open only when the temperature in the particular boiler is being maintained'at operating conditions. In other cases, manual valves are installedirvthe boiler returns and opened only when the burner switch' Fig. 20. Vertical Zoning of.Hot Water Heating System in a 12-Story Building (Provision should be made for expansion in each closed circuit.) which places the firing device at the command of the boiler temperature controlis'-iiFthe 01V position. . J'j" ...Weighted check valves, or flow check valves, should be installed in.the supply piping from the boiler to prevent gravity circulatiyn during periods when the circulating pumps are off. . if the piping is arranged with separate supplies ,to individual zones, a weighted check valve should be installed m the supply piping to each zone. , ij; The Use of Steam with Hot Water Heating Systems K The use of a steam boiler or district steam with a converter has many advantages for tall buildings. The piping for the steam used in this type of a system should follow the' principles outlined in Chapter 20, Steam Heating Systems. In tall buildings the system can be. zoned vertically a shown in Fig. 20, that is, with a given number of floors, per zone and# converter for each zone. This type of design makes the limitation of mffi mum:pressure in the system.possible. . With a converter, type system, tag. volume :of the boiler is removed as a. volume of water causing thermal'.ex-; Hot Water Heating Systems 585; pansion. -Since the boiler in a hot water system may.hold.30 to 50 percent, of the total water volume of the system, the reduction in the,size of the ex-j pansion tank and the problems involved therewith are of major importance.: The use of steam in extended surface coils for heating air will often be, more desirable in a system where steam is available. This type of installa tion will result in less expensive coils, and simplified coil controls. Temperature Control in Hot Water Systems Hot water heating systems permit use of relatively simple automatic temperature control for the purpose of reducing operating costs and pro viding comfortable temperatures. Indoor-outdoor control may be obtained by direct control of the burner by a temperature controller in the boiler water which is re-set according to outside temperature. Such controls are available from several manufacturers for residential and small commercial installations. A separate high limit control should'be used with this type of control. Three-way mixing valves, which blend boiler supply water with return water, or converters with the temperature of the heating water controlled by modulation of the heating source supply may be used on larger installations. Where the mixing valve or converter type control is used, a, thermostat located in the discharge of the valve or converter to control the supply water temperature is re-set according to outside temperature. . Provision for warm-up by the supply of maximum temperature water should be made where temperature of the supply is varied according to out side temperature. This can be accomplished by use of a warm-up period program control or by use of a space low-limit, thermostat set 3 to 5 deg below the room temperature maintained during normal operation. Control can also be accomplished by use of a room thermostat to operate the burner. This type of control is applicable only to residences or very small commercial installations, where there is no great difference in heating requirement throughout the building. . Where a difference in heating re quirement throughout the building does exist, it is better to use a modu lated supply water control with provision for manual or automatic adjust ment of heating unit output in individual spaces. Economical zone control can be obtained by separate return or supply piping for zones, with a zone thermostat to operate zone pumps or auto matic valves. 1 he control of air handling units in a hot water heating system requires careful consideration. Where the piping, required is within economical umits, it is advisable to use a three-way mixing valve and a pump in the eturn from the coil to achieve control. The heating coil in Fig. 19 illusrates the necessary piping arrangment. Control'valves used for air heat- g units should have plugs or discs of special design for close control of a n lhey shu'd be sized accurately for the flow required by the coil at pressure drop as large as the pump and the piping system will permit. wate'ehU^nS s^ou^ be taken to prevent the freezing of water in a hot than*20 tlng system whenever design outdoor, temperatures are lower basis a ' ' ptrt*cularily if the system is to be operated on an intermittent or duF m Sch0ls wilere no operation is required on Saturdays or Sundays must h^ vacat,i..n Periods. Air handling units which heat outdoor air damn aV6f Provision to stop fan operation and close outdoor air intake outdoor ' 6 watcr circulating in the coil drops below a safe level. The to take th11?ta^ce dampers should be interlocked with the fan motor so as bilitv of tle,c oeci Position whenever the fan is not operating. The possi- j water freezing in the mains can be reduced by the installation of an