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NORA Oilheat Technicians Manual

on, the relief valve would open. To fix this problem we have the expansion tank. It is a tank full of air installed in the system, with a flexible diaphragm. When the burner fires, the water expands, pushes against the diaphragm and compresses the air in the tank. When it shuts off, the water cools and the compressed air expands and pushes the water back out of the tank. Originally, all tanks were hollow steel cylinders. These tanks worked on the principle that as the system was filled with water, a cushion of air was trapped in the tank and as the system water expanded, the air compressed. On cooling of the system, the water would contract and the air would decompress. Unfortunately, every time the water cooled it would absorb some air from the tank and carry it to the system. Eventually all the air would be removed and the tank would become waterlogged and require service. The steel tank has been replaced by the flexible diaphragm design. See Figure 13- 18. These tanks are pre-pressurized to 12 pounds per square inch and have advantages over the older design: 1. Smaller size. About 1/3 to 1/2 the size of the older tank. 2. The flexible diaphragm keeps the water and air separated so the cooling water cannot absorb the air from the tank. It cannot become waterlogged unless the diaphragm leaks. System zones Hot water systems are easy to zone or break into separate heating circuits or areas. The two primary ways to provide for zone control are circulators and zone valves. Zone valves are 24 volt valves that provide control to either a circuit or piece of Chapter 13 Heating Systems Air separators remove the air from the water being pumped from the boiler and should be located in the supply piping. Figure 13-17 shows the cross section of an air separator and the installation of one on a steel expansion tank system. Expansion tank All hot-water heating systems need an expansion tank. As water is heated, it expands. We cannot compress water, so in a closed system it has nowhere to expand as it is heated, so the pressure increases instead. If we did nothing to address this, every time the burner came Figure 13-17: Air elimination Figure 13-18: Expansion tank 13-14 Heating Systems Diaphragm Expansion Tank Automatic Vent Air Scoop


NORA Oilheat Technicians Manual
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