Document 33Or2DK7xXo51Np5pw7YBmkn

188 CHAPTER 13 1965 Guide And Data Book coil valve V-t. This serves as a low limit' for the fan Economical zone control can be obtained by separate discharge temperature during heating operation. When V-t return or supply piping for zones, with a zone thermostat is closed, thermostat T-l then gradually opens maximum to operate zone pumps or automatic valves. The piping for outdoor-air damper D-t and closes return-air damper DS each zone should be isolated to avoid a common return lino proportionately. At 65 F outdoor temperature, D-t is wide because, in many cases, the return water from one zone is open and DS is closed. Exhaust-air damper D-4 opens and warmer than required for the supply water to another zona do9es in synchronization with D-t. At about^70; F out; The oontrol of air-handling units in a hot water heating door temperature when the outdoor air is no longer useful for system requires careful consideration. Where the piping re natural cooling, T-t closes D-t and D-4 and opens DS. quired is within economic limits, it is advisable to use & Reheater coil valve VS is controlled by' submaster three-way mixing valve and a pump in the coil circuit (see thermostat TS in the fan discharge. This thermostat is auto Fig. 8, Chapter 11 of the , 1964 Goins And Data Book). matically readjusted over an appropriate' range of ten*-' This arrangement provides a constant Sow to the coil while perature by thermostat T-4 so that the discharge'' tem the temperature of the water to the coil is varied by changing perature matches the space requirements. When"thfe"space the mixture of supply water and return water. If it is impracti temperature ' tends to rise above a' predetermined value,' cable to use separate three-way' valves and pumps for each TS has already been readjusted downward far ehbugh to keep coil, it is necessary, when varying the flow to a hot water coil, reheat valve V-5 closed. T-4 then controls cooling coil valve' to use control valves with plugs or discs designed for close con V-t directly, to maintain the desired space temperature. trol of flow.. They should also be accurately sized for the flow Humidistat H operates humidifier valve 'V-4 to' maintain required' by the coil at a pressure drop as large as the pump' the desired humidity in the conditioned space. It' is usually and the piping system will permit. Face and bypass dampers' adjusted to maintain 30 to 40 percent relative humidity, to vary the flow of air over a'coil with constant water flow ami therefore keeps the humidifier off during cooling opera can be used in the control of air-handling units.- tion when the humidity is higher. Precautions should be taken to prevent the freezing of water in a hot water heatiog system whenever design out TEMPERATURE CONTROL IN HOT , WATER SYSTEMS door temperatures are lower than approximately 20 F, par ticularly1 if the system is to be operated on an intermittent basis as in schools where'normal operation is not required Hot water heating systems permit use of relatively simple' on Saturdays or Sundays or during vacation periods. In automatic temperature control for the purpose of reducing selecting a means of freeze protection, consideration should operating costs and providing comfortable temperatures. be given to the type of system as'well as'outdoor tempera-' On installations not requiring different supply-water tem ture. For instance, an air-handling unit to heat outdoor air peratures simultaneously, and where the boiler is not used must have the coil protected for 32 F, whereas supply main* to heat domestic hot water, outdoor reset control of the -\ and radiation usually are not subjected to freezing tempera supply-water temperature may be obtained by direct con tures until the outdoor temperature drops considerably be trol of the burner from a thermostat that is located in the . low 32 F for extended periods. Intake dampers on air- boiler water and is reset in accordance with outdoor tern- : handling units that heat outdoor air should be interlocked peratures. Separate high limit and low limit controls should . with the fan motor, so as to close whenever the fan is not be provided. running. Provirion should be made to either close the outdoor- In systems nwng converters for a hot water supply, a = air intake, stop fan operation, or both; if the water circulating controlling thermostat actuated by'the leaving water tem- in the coil drops below a safe temperature level. The pos perature modulates the heat source to the ;converter.- The sibility of water freezing in the mains can be minimized by set point of this supply-water thermostat can be readjusted using an outdoor thermostat to provide continuous circula in accordance with outdoor temperature. tion of the water whenever the outdoor temperature drops In some systems, a boiler or converter supplies water at a ; below approximately. 30 F. Another arrangement uses an constant temperature, and three-way mixing valves'are used interval timer. and outdoor thermostat to provide inter to supply water at different temperatures for various uses ; mittent cycling of the circulating pump whenever, the out or to multiple zones. In these cases,-a three-way mixing ! door temperature drops' below some predetermined tempera valve, under control of a thermostat in the valve discharge, ture. A thermostat in the return water at the boiler can be blends boiler water with return water to provide supply used to start the burner.or 'other heat source if the tempera water at the desired temperature. The control setting of ; ture of the return'water drops to about 40 F.' - this thermostat in the valve discharge can- be- reset in ao- --cordance with outdoor temperatures. On systems using outdoor-reset supply water and lowered ZONE CONTROL OF STEAM AND WATER - SYSTEMS . night or week-end temperatures, provision should be made* - This section is concerned with zone controls for steam for warmup on restoration to day or normal operation. This ' or hot water heating systems. Refer to Part II of this can be accomplished by a program control, a warmup space thermOSt&t, or some nimilar mpn of providing higher than chapter for a general discussion of zoning. Most zone control systems are designed to supply heat normal water temperatures during the warmup period. to a zone at a rate equal to the heat loss. This rate of A measure of control can be accomplished by use of a heat'input may-be established by room thermostats, by room thermostat to operate the burner. This type of control controllers responsive to outdoor conditions, or by a combi is applicable only to residences or very small commercial nation of both. The outdoor controller responds to tem installations, where there is no great difference in Keating perature and may respond- to solar radiation and wind requirement throughout the building. Where a difference in velocity and direction. Features which may be added to the heating requirements throughout the building does exist, it is advisable to control the supply-water temperature with provision for automatic control or manual adjustment of heating unit output in'individual spaces. -- basic zone control system include: 1. Means for automatically or manually varying the heat input to the zone to compensate for variations in internal beat gain. The` automatic 'arrangement' requires a thermostat at a Automatic Control consists of e conveniently located switch. ,, for rriniuning a lowered economy temperature or shutting off the heat at night or at other times when tberoneis unoccupied. This may be accomplished manually or by means of a dock. - An arrangement providing rapid warm-up following a neriod at the nonoccupancy temperature. - During this warmfull heat may be admitted to the system or the heat Lmiut may be controlled at a higher-than-normal rate. The IJirm-UD may be accomplished by a manual switch or may be art of the dock program. A warm-up thermostat located Sthin the zone-may be arranged to terminate.the warm-up ocnod independently of the switch or dock when the zone approaches the desired occupancy temperature. 4. A switch to provide full heat or no beat inde pendently of other controls and to permit manual selection of the various clock functions. 5. Means for getting off the heat completely when the out door temperature reaches a point where heat is no longer needed. . 6. A high-limit thermostat in the zone to shut off the heat completely when the zone becomes overheated. Steam:; Zone Distribution In continuous-flow steam systems the quantity, of steam supplied to tlie zone is varied in accordance with the de mands of the control system. To obtain equalized distri bution of the steam, metering orifices are generally required on the inlets to all radiators or convectors. Supplementary controls for low heat output are often desirable. The follow ing two arrangements are common: 1. Varying the difference in pressure between the supply and return which results in partially filling the heating units in accordance with demand. This method can be used on atmospheric or vacuum return systems. 2. Varying the pressure in the system while maintaining a constant differential pressure between the supply and return. This results in the heating units being filled with steanrat a temperature which is proportional to demand. A vacuum pump maintaining a vacuum slightly below the supply pressure is re quired, ana vacuums as high as 20 in. Hg during miM h^otJng weather are common. In intenrnttaU systems, steam at full pressure is supplied intermittently to the zone, the length of the on and off periods being varied in accordance with the demands of the control system. Two common arrangements are: 1. A timing device controlled by an outdoor thermostatic element. This can be used to vary the on-off periods of boiler operation or the opening and closing of a steam valve as a function of outdoor temperature. 2. A control responding to an outdoor element and an element attached to a radiator or convector. This control vanes the length and frequency of the on-off intervals in such a way that the radiator or convector temperature is varied in accordance with outdoor temperature. For proper operation of zone control equipment, the heating system must be carefully designed and installed, and maintained in good operating condition. Vents, traps, vacuum pumps, and valves must be carefully inspected and repaired or replaced when required. Piping must be of ade quate size, graded, and dripped. Return piping must be vented and any pockets or lifts must be removed. Water: Zone Distribution A common method of obtaining zone oontrol of hot water systems consists of varying the temperature of water supplied to the zone inversely as the outdoor temperature variea. This may be accomplished by a submaster ther mostat in the zone supply line, readjusted by an outdoor master thermostat, or by a single controller having one 189 measuring element in the supply-line, and another compen sating element outdoors. On single-zone installations the controller may actuate the burner directly: as shown'in Fig. 32. If the boiler is also used for domestic hot, water, the controls may be arranged as in Fig.'33. In this case/the boiler is controlled at a fixed temperature'and the zone controller is arranged, to operate a valve to proportion hot water from the boiler with cooler return water, to obtain the desired zone water temperature. On multiple-zone in stallations, the boiler is usually controlled ,at a fixed' tem perature, and each zone is controlled by a-separate valve and outdoor compensated'controller. Each zone should' be equipped with a separate pimp, because, in many cases, the return "water from some rimes is warmer than the re quired supply water for other zones." By n^ng individual zone pumps, the designer can separate the. returns andthus prevent overheating that would otherwise occur.' Fig. 32 .... Control of Hot Water Heating Without Domestic Hot Water Service Rg. 33 .... Control of Hot Water Heating With Provisions for Domestic Water Heating To compensate for variations in internal load, the action of the zone control may be n>ndifivi by a room thermostat in each zone which either starts and stops the zone pump, mndifip-q the relationship between outdoor temperature and water temperature, or operates an automatic valve in the zone supply or return line. Ech zone may be further sub divided by providing a separate thermostat and valve for each subdivision. Another zone control method for hot water systems congists of iruntaining a constant water temperature and varying the volume of water supplied to the zone by means of a room thermostat which either starts and stops the zone pump or operates a two-position valve in the zone supply or return Kne. Whenever flow control valves are employed in water circuits, it is necessary to employ a relief valve or pressureregulating valve in a bypass connection around the pump.