National Oilheat Research Alliance 25 According to the chart, it should be fired at .68 gph for 83,000 Btuh, which is slightly over the Btuh required to properly heat the building. Universal Hydronics formula Once the boiler size has been determined based on heat load calculations, it is necessary to determine the right amount of heat that must be delivered to each room. In doing this work, examining piping sizes, pumps, circulators and the heat transfer in each room is necessary for this analysis. Key to this is the use of the Universal Hydronics Formula. The Universal Hydronics formula states that Gallons per Minute (gpm) equals British Thermal Units per Hour ( Btuh), divided by Designed Temperature Drop (Delta T—also written as ΔT) times 500. gpm = Btuh/(ΔT x 500) • gpm is gallons per minute—the flow rate required to deliver a specific amount of heat • Btuh is the Btu per hour requirement at a given point in time—the heating load • ΔT is the designed temperature drop across the piping circuit. In a series loop, fin tube baseboard zone, the design ΔT is 20—that’s the manufacturer’s rating for the baseboard itself—meaning that if the water enters the baseboard zone at 180°F, it should return to the boiler at 160°F, 20 degrees cooler. In most residential radiant floor heating systems, the design ΔT is usually 10 degrees— meaning the water enters the radiant loop at 130° F and returns at 120° F. This 10 degree ΔT is important in radiant because it ensures an even, comfortable floor surface temperature throughout a room. A wider ΔT might mean a noticeable drop in floor surface temperature near the end of the run. The final element of the equation is 500— that’s a shortcut representing the weight of one gallon of water (8.33 pounds), times 60 minutes in an hour, times the specific heat characteristic of the fluid (1 for 100 percent water). It takes 1 Btu to raise the temperature of 1 pound of water 1 degree in 1 hour. 8.33 x 60 x 1 = 499.8 (500) Virtually everything in hydronics—from pipe sizing to circulator selection—stems from this formula. For example, if the need is to distribute 70,500 Btuh (assuming piping to a zone of baseboard), the formula is: gpm = Btuh/ ΔT X 500 gpm = 70,500 / 20 X 500 gpm = 70,500/10,000 gpm = 7.1 To properly heat this area, 7.1 gallons of hot water per minute needs to be moved. Pipe sizing guidelines The “rules of thumb” are as follows: Pipe sizing guidelines • 2-4 gpm = ¾" • 4-9 gpm = 1" • 8-14 gpm = 1¼" • 14-22 gpm = 1½" One inch pipe will suffice for this application. There are a number of charts available, from the Hydronics Institute section of AHRI that are shown at the end of this chapter on pages 30, 31 and 32. Head In hydronics, pressure is typically referred to as “head pressure” or “head.” Head is the total mechanical energy content of a fluid at a given point in a piping system. In hydronics, it is used to express pressure loss in the piping system. When water flows through the system,
NORA Advanced Hydronics
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