Document emv1bxpD0RjZM0BVjXqVD84vq

Heating Ventilating Air Conditioning Guide 1939 140 F or as high as 180 F and may be readily adjusted to meet particular requirements. With this type of control it is impossible to overheat the hot water supply which is an important safety consideration in some installations. This type of system may also be conveniently used during the non-heating season with the operation of the fuel burning device controlled by the water heater thermostat. (See Chapter. 37). During the heating season the water heater temperature control functions as a low limit control. When an indirect water heater is applied to a gravity hot water system, it is necessary to provide a valve in the supply to the heating system to prevent the flow of hot water from the boiler when heat is not required in the house: This valve may be controlled from a room thermostat and the automatic fuel-burning device controlled from the water heater thermostat. To prevent circulation in a forced hot water heating system flow control valves may be installed in the flow and return lines which act merely as check valves when the circulating pump is not operating. In this arrangement the pump is controlled by the room thermostat and the automatic fuel-burning device is controlled from the water heater thermostat. STORAGE CAPACITY AND BOILER ALLOWANCES The amount of storage provided in the hot water tank or heater is somewhat a matter of choice but is usually made ample to carry over the peak shortage which is likely to occur and is based on the assumption that only 75 per cent of the storage capacity will be available, as it has been found that if more than this amount is withdrawn from storage, the tank is so cooled down as to make the balance useless.. The general rule may be cited that the less the heating capacity the greater must be the storage, and the greater the storage the less may be the heating capacity down to a point where the heating capacity will fail to be sufficient to heat up the tank storage during the periods of small load. Example 5. A heater to supply 500 persons will have an average daily use of about 500 X 40 gal = 20,000 gal and this is an average of ^'^24 ^ = gph but the peak hour will require Mo of 20,000 = 2000 gal and the shortage during the peak hour, if the heating capacity is made to suit the average hourly use of 833 gal, will be 2000 -- 833 = 1167 gal so that the storage capacity, based on 75 per cent being available from this u 116T capacity without cooling the tank excessively, will be q = 1556 gal. Should it be desired to reduce the size of storage tanks and to use a greater heating capacity, it is only necessary to increase the heating capacity to say 1200 gph which then gives 2000 -- 1200 = 800 gal as the shortage during the peak hour, and the necessary storage will be = 1067 gal; or the heating capacity can be increased to 1500 gal, leaving a shortage of 2000 -- 1500 = 500 gal. Good design requires that the heating capacity be made as small .as possible without introducing undesirable amounts of storage, as the heating capacity directly determines the load on the source of heat. 818 Chapter 43. Water Supply Piping and Water Heating As indicated in Example 5, the heating load is proportional to the heating capacity, and the boiler capacity must be increased for higher heating capacities and may be reduced for smaller heating capacities with greater storage. It may be assumed that a boiler capacity of about 4 sq ft of equivalent steam heating surface (radiation) must be provided for every gallon of water heated 100 F or from 50 F to 150 F, which is the temperature rise most commonly assumed and required.. On this basis it will be seen that the various conditions cited in Example 5 will require additional boiler "capacity as follows: Heating Capacity (Gph) 833 1200 . 1500, Additional Boiler Capacity (Sq Ft EDR) 3332 4800 . 6000 ' From this it is apparent; that it is less costly to provide ample storage and to reduce boiler capacity than to diminish the storage and supply a greatly increased boiler capacity to compensate. ' The boiler allowance value of 4 sq ft of equivalent steam radiation for each gallon of water heated through a temperature range of 100 F is based on an hourly heating rate. When reduced heating capacities are desired for economic reasons of boiler design and selection, engineers frequently recommend that the heating rate be extended over a period of two hours in which case the boiler allowance value would be reduced to 2 sq ft of equivalent steam radiation. Similarly any other heating rate may be established and a corresponding value of boiler allowance deter mined. Reliable information based upon the installations of several'heaters in existing heating systems indicates varying arbitrary values of boiler allowances to be used. When these values are selected for usage, a careful analysis of the varying factors involved in determining these values should be considered so that the proper heating allowances may be provided. ESTIMATING HOT WATER DEMAND BY FIXTURES In buildings where the occupancy is doubtful and only the number of plumbing fixtures can serve as a basis for determining the probable hot water demand, the problem is not so simple owing to the fact that a fixture gives no information as to how heavy a service may be demanded from the fixture and this amount of service is really the governing factor in making an estimate of the probable hot water demand. Table 13 may prove of some value in this respect as it gives the maximum assumed quantity of hot water per hour which will be demanded of any fixture and then gives a percentage of this amount which may be assumed as probable in different types of buildings. Table 14 gives approximate hot water re quirements in various types of buildings. Example 6. Let it be assumed that an apartment house with 20 apartments has 20 baths, 20 lavatories, 20 kitchen sinks and 20 laundry trays; what is the probable maxi mum hourly demand for hot water? Actual requirement for 100-deg temperature difference =* 100 X 8.33 240 water heated 819 3.48 sq ft per gallon of