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564 CHAPTER 24 ^ 1952 Guide vefit is-not required if the main is-above the heater in. which case.air can be eliminated through the piping system. ;. ' BOILER CAPACITY FOR STEAM UNIT HEATERS : The capacity, of the boiler should be based on the rated capacity of the unit heaters at the lowest entering air temperature and highest fan speed that wifi occur, plus an allowance .for pipe Une' losses. It is unwise to iristall a single unit heater as the sole load on any boiler, particularly if the unit heater motorris started and stopped by thermostatic Control. The wide and sudden fluctuations of load that occur under such cpnditions would require closer, attention to the boiler than is usually .possible, in a small installation.Where oil or gas fuel is used in the boiler, it is possible, by means of a pressure operated switch to control the boiler, in response to this Unit Heaters and Unit Ventilators 565 hot water may-be employed, as the heating medium. ; Unit ventilators are intended primarily for use.in schools,, meeting rooms, offices, or other ap^ plications where the density of occupancy indicates the need for ventilation. In normal operation, the discharge air temperature from a unit ventilator is varied in accordance' with the room demaridk Where A heating effect is required, the air. delivered is above room temperature. /Where the heat generated within the room' by occupants, sun, etc.,is sufficient to cause overheating, the .air delivery temperature mUt;be below, that.of the. rOoms. It is customary to equip unit ventilators with, control devices that prevent the delivery, of air at a temperature, that will cause cold drafts. Unit ventilators may be of the heating element or damper controlled type, con structed on the blow-through (Fig.-9)-'or .draw-through prmciple as [illus trated in Fig. 10. .......... WINDOW Fig. 8. Method op Connecting Unit Heateb to Hot Wateb System rapid fluctuation. In most cases, and particularly where the boiler is coal- firedi .it. is advisable to use: two. or more smaller units instead, of one large unit heater. . : ... ' ... Steam pressures tbelow 5 lb Can be used with safety for recirculating unit heaters ;when their heating surfaces are designed for those pressures;. and when proper provision is made for returning .the .condensate. If units receive air that may be at a temperature below-freezing, a steam pressure of not less than 5 lb should be maintained in the heating element, or a con-e- sponding differential in pressure between the Supply and return piping should be maintained, by means of a vacuum. UNIT VENTILATORS '. A unit ventilator is essentially a. modified' unit heater. In addition to heating, it hais the, additional function of introducing outdoor air for ven tilation and cooling, The typical unit is arranged to introduce outdoor air and recirculated air to "the room in varying quantities and is equipped with a system of control that permits both the heating and eooling effect to be varied while the fanfare operating continuously. Eitheristeamor forced Fig. 9. Typical Blow-Through Type Unit'Ventilatob Showing One op Many Arrangements of Dampers and Heating Elements .. Ratings of Unit Ventilators ; ' , : Unit ventilators are customarily cataloged.with two ratings : the anemom eter rating and the standard air rating.. The anemometer air rating is peculiar to school house ventilation and was for many years the standard by which school officials checked and specified unit ventilator capacities. It originated as a convenient field measurement for checking, air. quantities, and is the basis of rating for ventilation requirements under many state and local building codes-. -Th anemometer rating (air capacity) is obtained by averaging the air velocities, obtained, by an anemometer held over equal subdivisions of. the outlet, grille at a distance of 2 in. from the grille, and multiplying the average velocity by . the gross area of the grille. (The average velocity is sometimes obtained by moving' the anemometer over the outlet 2 in. from its face.) The anemometer rating is based on the final temperature of the air leaving the grille while the unit is delivering outdoor air and room air in the proper proportion and the heating element is supplied with steam or hot water as specified. , The standard air rating is obtained in accordance with the A.S.H.V.E.