to the boiler where it can be reheated. It circulates water in a closed loop system by creating a pressure differential, meaning it takes the fluid that comes into it at a given pressure and sends it out at a higher pressure All the circulator has to do is create enough of a pressure differential to produce adequate flow through the system. To produce the necessary pressure differential, an impeller is used. How this impeller is designed and installed is critical to the operation of the circulator, and should be understood by anyone installing or servicing hydronic systems. When the circulator is in operation, water flows into the suction side of the volute—a part of the circulator which is always wider and larger than the discharge side—and then into the eye of the impeller. As the impeller rotates, the vanes “slap” the water from the inside of the impeller to the outside of the impeller. This adds velocity to the fluid. The fluid then moves to the discharge side of the volute, which is smaller and narrower. The collection chamber on the discharge side turns the kinetic energy of the fluid – (energy due to velocity) into pressure. Thus water comes into the circulator at a certain pressure and leaves at a higher pressure. The thickness, diameter and construction of impellers all play a role in the performance of the circulator. The thickness of the impeller determines its flow capacity—the thicker the impeller, the more flow the circulator will be able to produce. The diameter of the impeller is also important. The larger the diameter, the more velocity the circulator can impart to the fluid. The more velocity, the more pressure the circulator can produce. Open vane impellers are used in circulators designed to provide high flow and relatively 10 National Oilheat Research Alliance low head. These types of circulators are called flat curve circulators – as will be apparent when looking at performance curves. Flat curve circulators are used in radiator and baseboard applications which require higher flow rates but have lower overall head losses. Closed vane impellers are used for higher head and medium to high flow circulators. They are designed for lower flow systems that produce higher head losses, which are typically found in most radiant applications. 10 Zones—A zone is an area of the house controlled by a dedicated thermostat. A common residential zoning system has one zone for the first floor and one zone for the second floor. This system requires either two thermostats and two circulators OR two thermostats, two zone valves and one circulator. 11 Zone valves—Zone valves are two-position valves: they are either open or closed and are wired to the thermostats in the house. The valve is usually closed and will open only when there is a call for heat from a thermostat in a particular area of the home. Once the valve is fully open, the circulator will start and water will begin flowing to the rooms that need heat. Figure 12. 12 Flo-Control Valve —Some systems have a dedicated circulator for each zone and do not use zone valves. These systems utilize flo-control valves that are installed in the supply piping to prevent gravity circula
NORA Advanced Hydronics
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