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NORA Oilheat Technicians Manual

Chapter 13 Heating Systems Figure 13-27: Indirect storage type water heater Chapter 13—Heating Systems 13-21 from a vacuum being exerted onto the tank is important since this can lead to a tank implosion. If a vacuum occurs, the vacuum relief valve automatically vents the closed system to the atmosphere and allows air to enter and prevent conditions that could damage the water heater. A back flow preventer is a device much like a vacuum relief valve that will open a vent line to the atmosphere when it senses a vacuum. A back flow preventer should be installed in the cold water feed line above the top of the heater. If there were a leak in the cold water feed to the tank and all the fixtures were closed in the house, a vacuum would be pulled on the system. If the tank or system is not equipped with a back flow preventer or vacuum relief valve, damage to the tank may occur. A tempering or mixing valve is used to control the temperature of the water being delivered to the fixtures. In tankless coil applications, the temperature of the water that has been sitting in the coil immersed in hot boiler water has the potential to scald on an initial hot water draw at the fixtures. As a hot water draw continues, the temperature of the water that has passed through the coil will not be as hot . In order to avoid the potential for scalding and to balance the temperature of the water supplied during a draw, a tempering valve is used. A tempering valve has three ports for piping connections, one for the hot discharge side of the water heater, one for a cold water connection, and one for the hot water supply to the fixtures in the house. The tempering valve is a simple device that contains an element that senses the temperature of the water being supplied from the water heater and then mixes an appropriate amount of cold water to provide domestic hot water at the desired temperature. These valves are often adjustable so the temperature of the water may be regulated to satisfy individual preferences. The elements in these valves are susceptible to liming from the minerals in fresh water and require periodic replacement. Rebuilding kits are readily available. When installing a tempering valve they should be installed at a level of 8"-12" inches below the heater so that they will be protected from a heat build-up while not in use. The dielectric fitting protects the water heater or storage tank against the galvanic reaction caused by the use of dissimilar metals and stray current corrosion. These fittings come in many sizes, materials and types and can also be used in place of “pipe thread to solder” adapter or union. Pressure reducing valve and an expansion tank. Although these items are mentioned here as “optional,” they may be required by local codes. Since the domestic hot water heater or storage tank is a closed system it should be protected by an expansion tank specifically designed for the purpose. These expansion tanks should always be installed on the cold water line, before the tank but after all service valves and pressure reducing devices. The use of these tanks will prevent nuisance relief valve discharges and premature tank failures due to excess pressure build-up. Hot Water Outlet Temperature and Pressure Valve Outlet Tank Shell Boiler Inlet and Outlet Connections Cold Water Inlet/Drain


NORA Oilheat Technicians Manual
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