As zone valves close, a system curve intersects a pump curve at higher and higher pressure differentials. Higher pressure differentials can lead to higher flow velocities, which, in turn, can lead to velocity noise. One way to deal with velocity noise is to install a pressure-differential bypass valve, which prevents flow when all heating zones are calling. As zone valves close, increasing pressure differential, the bypass valve opens to allow excess pressure and flow to pass back to ...
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