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American Society of Heating and Ventilating Engineers Guide, 1934 Sizing Gas-Fired Healing Plants While gas-burning equipment can be and usually is so installed as to be completely automatic, maintaining the temperature of rooms at a pre determined and set figure, there are in use installations which are manually controlled. Experience has shown that in order to effectively overcome the starting load and losses in piping, a manually-controlled gas boiler should have an output as much as 100 per cent greater than the equivalent standard cast-iron column radiation which it is expected to serve. Boilers under thermostatic control, however, are not subject to such severe pick-up or starting loads. Consequently, it is possible to use much Table 1. Selection Factors for Gas Boilers Equivalent Cast Iron Steam Radiation (Squabs Feet of 240 Btu Each) Selection Factor (Per Cent) 500 800 1,200 1,600 2,000 3,000 4,000 and over 56.0 54.0 51.0 48.0 45.0 42.5 40.0 lower selection or safety factors. A gas-fired boiler under thermostatic control is so sensitive to variations in room temperatures that in most cases a factor of 25 per cent is sufficient for pick-up load. The factor to be allowed for loss of heat from piping, however, must vary somewhat, the proportionate amount of piping installed being con siderably greater for small installations than for large ones. Consequently, a selection factor for thermostatically controlled boilers must be variable. Table 1 gives selection factors to be added to the installed steam radiation under thermostatic control. They have been established by experience and are recommended by the American Gas Association. The same factors may be used in determining the gas demand for which conversion burners installed in steam or hot water boilers should be set Multiplying the equivalent direct heating surface (radiation) by 240 and adding the appropriate percentage from Table 1, and then dividing by the heat value of the gas and by the heating efficiency (see discussion of heating efficiencies in Chapter 29), gives the proper hourly rate of gas consumption. However, inadequate boiler heating surface for gas burning, often encountered in coal-designed boilers converted to gas, may necessitate operation at a lesser demand, resulting in much slower pick-up and less margin of safety for piping loss. Appliances used for heating with gas should bear the approval seal of the American Gas Association Testing Laboratory. Installations should be made in accordance with the recommendations shown in the publica tions of that association. Ratings for Gas Appliances Since a gas appliance has a heat-generating capacity that can be pre- 394 Chapter 28--Automatic Fuel Burning Equipment dieted accurately to within 1 or 2 per cent, and since this capacity is not affected by such things as condition of fuel bed and soot accumulation, makers of these appliances have an opportunity to rate their product in exact terms. Consequently all makers give their product an hourly Btu output rating. This is the amount of heat that is available at the outlet of a boiler in the form of steam or hot water, or at the bonnet of the furnace in the form of warm air. The output rating is in turn based upon the Btu input rating which has been approved by the American Gas Asso ciation Testing Laboratory and upon an average efficiency which has been assigned by that association. In the case of boilers, the rating can be put in terms of square feet of equivalent direct radiation by dividing it by 240 for steam, and 1505 for water. This gives what is called th^ American Gas Association rating, and is the manner in which all appliances approved by the American Gas Association Laboratory are rated. To use these ratings it is only necessary to increase the calculated heat loss or the equivalent direct radiation load by an appropriate amount for starting and piping, and to select the boiler or furnace with the proper rating. The rating given by the American Gas Association Laboratory is not only a conservative rating when considered from the standpoint of capacity and efficiency, but is also a safe rating when considered from the standpoint of physical safety to the owner or caretaker. The rating that is placed upon an appliance is limited by the amount of gas that can be burned without the production of harmful amounts of carbon monoxide. This same limitation applies to all classes of gas-consuming heating appliances that are tested and approved by the Laboratory. Gas boilers are available with ratings up to 14,000 sq ft of steam, while furnaces with ratings up to about 500,000 Btu per hour are available. (See Chapter 23). Installation Features One feature of the piping installation that adds to the satisfactory service rendered by gas boilers is provision for adequate and rapid venting of the air from steam heating systems. If air leaks into the steam dis tribution system during the period that the gas is turned off, and then vents out slowly when the thermostat calls for heat, the result will be a further cooling of the premises between the time that the thermostat calls for heat and the time that steam reaches the radiators. A freely venting steam or vapor system gives maximum economy and minimum temperature variation. When gas boilers are attached to existing heating plants, it is good practice to check the effectiveness of the venting devices and if necessary to replace them with more effective ones that will prevent the return of air into the heating system, and also to check the tightness of the piping. Frequently when a coal boiler is already installed in a home, it is expedient to leave the coal boiler in place, and to cross-connect the gas boiler with it. Where gas heating is new to the community, it pro- A value of 160 for the heat emission of hot water radiators is used by many engineers. The actual heat emission, however, depends on the temperature of the water and of the surrounding air. See Chapters 30 and 33. 395