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526 CHAPTER 36 1959 Guide other. The connection from the larger device should be rea sonably low, and that from the smaller should be up near the ceiling, so that each device can be serviced as well as possible, regardless of the treatment of the other. Even where such precautions are taken, there is, under unusual conditions, some possibility of flow of combustible gases from one appliance into another. Reverse flow of cooled gases has been demonstrated in a chimney at very low rates of flow* Under certain similar quiescent conditions, acci dental discharge of combustible gases from a defective device into a chimney could result in flow of these gases counter current into the combustion chamber of another device at tached to the eamft chimney. If ignition occurred in this sec ond device, an explosion could result. Excessive height in a chimney does no harm but muw for controlling the draft are more than ordinarily if the chimney is too tall. Coal-buming devices often have air leaks around the firebox, and the. draft doors sometimes fit so poorly- that the fire cannot be controlled at a low rate. The amplest remedy for such cases is the barometric damper which admits air into the flue pipe and thus reduces draft. Where a chimney serves a fireplace, it is important that no other heating device be connected to it unless the fire place is effectively sealed. - REFERENCES 1R. H. Sherlock and B. A. Stalker: A Study of Flow Phe nomena in the Wake, of Smokestacks '(University of Michigan, Department of Engineering Research Bulletin No. 29, 1941). * E. F. Miller and James Holt: Notes on Power Plant Design (Massachusetts Institute of Technology, 1930). * L. F. Moody: Friction factors for pipe flow (ASME Trans actions; Vol. 66, 1944, p. 671). 4 R. T. Kent (ed.): Mechanical Engineers' Handbook (John Wiley A Sons, New York). *G. A. Hooi and N. G. Johnson: Handbook of Budding Con struction (McGrawrHill Book Co, New York, 1929). * R. S. Dill, P. R. Achenbach, and J. T. Duck: Observed per formance of some experimental chimneys (ASHYE Trans actions, Vol. 48, 1942, p. 351). r L. B. Schmitt and R. B. Engdaht: Performance of residential chimneys (ASHVE Transactions, Vol. 55, 1949, p. 241). *P. R. Achenbach and S. D. Cole: Performance of fourteen masonry chimneys under steady state conditions (ASHVE Transactions, Vol. 55, 1949, p. 129). *R. D. TVnlman and W. H. Shenkle: A theoretical and ex perimental investigation of the performance of some short flues under steady-state conditions (Thesis at Massachusetts Insti tute of Technology, June 1951). "P. R. Achenbach: Physics of chimneys (Physic* Today, Vol. 2, December 1949). " R. K. Thulman: Performance of Masonry Chimneys for Houses (Housing and Home Finance Agency Technical Paper No. 13, August 1949). " CS101-43 Od-Buming Space Heaters Equipped with Va porizing Pot-Type Burners, CS75-42 Automatic Mechanical Oil Burners Designed for Domestic Installations, CS(E)104-43 Worm Air Furnaces Equipped with-Vaporizing Pot-Type ButTi er*, CS109-44 Solid-Fuel Burning Forced Air rumacest CSl 13-44 Od-Buming Floor Furnaces Equipped with Vaponxmg PotType Burners (National -Bureau of Standards Commercial Standards). " American Standard Installation of Gas Piping and Gas Appliances in Buddings (American Standards Association, Z2130-1950). "Comfort Healing (American Gas Association, 1938, p. 71). "C. G. Segeler: Sizing of Chimneys and Flues Used for Gas Appliances (American Gas Association; Sixth International Gas Conference, 1955). " N. D. Mitchell: Fire hazard tests with masonry chimneys (Notional Fire Protection Association Quarterly, October 1949). . "S. Konso: Chimneys and draft (Winter Air Conditiorttng, National Warm Air Heating,and Air Conditioning Aftm-M-jarirm 1939, Chapter 32). CHAPTER 37 ESTIMATING FUEL CONSUMPTION FOR SPACE HEATING flares of fuel Estimates; Efficiency of Utilization; Calculated Heat Loss Method.- Computation and Application, Short Methods for Estimating Heat loss; Degree-Day Method: Computation and Application, Unit Fuel Consumption per Degree Day, Estimating Consumption for Various Fuels, Degree Day as an Operating Unit; Industrial Degree Days; Maximum Demand and food Factors IT IS often necessary to estimate the anticipated heat re the heat in the fuel is used. Efficiency can be defined in quirements and fuel consumptions of beating plants for a variety of ways for various purposes, and the values either short or long terms of operation. These quantities can in any given case can vary widely depending on the con be much more difficult to calculate than design heat loss or ditions. For estimating purposes, as will be shown later, it required system capacity, since they involve essentially the is the efficiency of utilization of the fuel over the calculation summing up of.the net result over the period in question of period which is wanted. This is distinct from furnace efficiency the influence of many factors which may vary greatly with which expresses the heat delivered by a furnace unit as a per time. It will seldom be possible to predict with any great ac centage of the heat provided in the fuel and which is the curacy the way in which all the factors involved will vary throughout the prediction period. In addition to this, the cal culations required to-take all such variations into account become very involved. For these and other reasons records of past operating experience, when these are available, pro vide the most reliable and usually the most accurate basis for the prediction of future requirements. Records of past heat requirements or of fuel consumption of a particular building are a better basis for estimates Than are averages of records from similar buildings. In the absence of past records for a particular building the data from sim efficiency normally used in describing the performance of a unit under rated, or stated, load conditions. It has been shown, at the University of niinnia for example,1 that in the case of a dwelling with an inside chimney, as much as 35 percent of the heat delivered to the chimney which is nor mally considered to be a loss so far as efficiency of the fur nace is concerned may be delivered to the house through the heated chimney walls. This recovered heat amounted to as much as 12 percent of the heat required by a house. Much of this recovered heat may be available for heating. It has been said in connection with the same study that at times ilar plants in the same locality may have to be used. Averages of consumption figures taken from many types of plants in only about 50 percent of the heat utilized by a house was supplied by radiators, while the other 50 percent of the many types of buildings in various localities can only pro duce an average estimate which may prove to be inaccurate when applied to a particular building. Where unusual operating conditions exist due to factors heat utilized was supplied from such sources as chimney, piping, boiler jacket and smoke pipe, people, lights, etc. A summary** * of many tests in two research residences at the University of Illinois using many fuels and systems give such as excessive ventilation, abnormal inside temperatures, and beat gains from external sources, or where, in tire ease of values of 67 to 90 percent for overall house efficiency (the ratio of the heat loss from the structure to the heat input proposed buildings of unusual design, no information is avail able regarding former consumption, it is necessary to estimate fuel consumption from the computed heat losses. In preparing fuel consumption estimates it is well to real ize that any estimating method used will produce a more reliable result over a long period operation than over a short period. Nearly all of the methods in common use will -to the unit for an average day). The efficiency of utilization of the fuel burned can at times be relatively very high for houses. It can also vary quite widely from one house to another because of variations in chimney location and design, in the utilization of beat lost to the basement from the heating unit and smoke pipe, and for other reasons. The efficiency of utilization may also vary give trustworthy results over a full annual heating season, and in some cases such estimates will prove consistent within themselves for monthly periods. As the period of the estimate is shortened, there is more chance that some factor not tAVon into account directly in the estimating method will deviate from its long-time average value, and thus lead to serious error in the predicted heat requirement. EFFICIBnICY of utilization Any method of estimating fuel requirements based pri marily on calculation of heat losses must, of necessity, aJrq take into account considerations of the efficiency with which with time of year as the average load on the furnace varies. Some of the reasons for this in the case of intermittentlyfired oil furnaces have been demonstrated.4 In buildings other than houses the efficiency of utilization of the heat supplied in the fuel may also vary widely. There will, in general, be less tendency in many larger buildings toward effective use for treating purposes of the heat which is normally lost from chimney, furnace, and piping surfaces, and efficiency is seldom likely to be as high as in the case of. many bouses. The average fuel consumption of various types of ap proved gas-fired equipment has been obtained from a large number of beating systems* Corresponding efficiencies of 527