Hydronic systems are dynamic. Pressures change continuously in the system as valves open and close and pumps start, stop, and change speed. With any change in pressure, there is a change in flow through any device, even if the heating or cooling load is constant.

On an air handler, when the flow changes, so does the heat transfer rate. The thermostat picks up that change and attempts to reset the valve for the proper flow for the new pressure condition. With any conventional control valve, as pressures change, the actuator strokes to return the valve to the right position to provide optimum flow for the load.

This takes time. Typical commercial quality globe valves are especially poor and simply unable to modulate adequately in all flow conditions. With high pressure differential, and below 50% flow, these valves tend to behave like two-position on/off valves. This dramatically reduces heat transfer, causing excess flow, and low delta T. Next >>