Figure 4 Figure 5 mum operating pressure, the relief valve is designed to open and release water from the system, reducing the pressure. Most residential hot water systems are equipped with relief valves that open at 30 PSI, but which may actually release and weep around 27 psi. See Figure 3 on previous page. The relief valve MUST be properly sized. The rating MUST meet or exceed the boiler’s DOE heating capacity rating. See Figure 4 above. 3 Low water cut-off—LWCOs protect boilers against damage and are installed at or above the boiler’s minimum safe water level. They utilize an electronic probe—when water is in contact with the probe, it completes a circuit between the probe tip and the boiler surface or a tee. If the water level drops below the probe, the circuit opens and shuts off the burner. Figure 5. 4 Air elimination device—Air elimination devices are used to help control air bubbles in the heating system. If a system has too much air, it can’t transfer heat efficiently and can be very noisy. A properly designed and installed hydronic system should be virtually silent and very efficient. There are a number of different types of devices available to remove 6 National Oilheat Research Alliance air from the circulating water including air scoops and air separators equipped with automatic air vents. Air scoop The inside of an air scoop is larger than the pipe feeding into it. When water enters this larger area, its velocity slows down and the pressure decreases slightly, causing the air bubbles to separate from the water. Vanes inside the device “scoop” the bubbles into an upper chamber where they pass out through an air vent at the top of the scoop. (Figure 6) It’s important to understand that the scoop will only work if there is a minimum of 18 inches of straight pipe on the inlet side. The air bubbles need to be on the top of the flow as the water enters the scoop, otherwise the vanes won’t catch the bubbles and the scoop won’t remove the air. The 18 inch minimum length is needed to create laminar flow, meaning the air will float to the top. If the scoop is installed closer to an elbow, the flow will be turbulent, the air will be mixed with the water and the scoop will be unable to remove the air. Figure 6 Pressure Relief Valve Rating Must Match or Exceed the DOE Heating Cap. Air Scoop
NORA Advanced Hydronics
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