By Chad Edmondson
In our last blog, we cautioned against using pressure independent control (PIC) valves when attempting to apply area control with reset. But why is that a problem? Aren’t PIC valves the greatest thing since microwave ovens? Let’s look at the below system using PIC valves:
PIC valves are, in fact, pretty cool. They do not require a separate balancing valve. They are great at keeping zones operating within their control range despite changes in pressure. Remember, the key to PIC valves is that a change in pressure across the valve will not cause a change in flow if we are within the valves control range. But when it comes to the following ASHRAE Standard, they might not the best choice:
“Individual chilled water pumps serving variable flow systems having motors exceeding 5 hp shall have controls and/or devices (such as variable speed control) that will result in pump motor demand of no more than 30% of design wattage at 50% of design water flow. The controls or devices shall be controlled as a function of desired flow or to maintain a minimum required differential pressure. Differential pressure shall be measured at or near the most remote heat exchanger or the heat exchanger requiring the greatest differential pressure. The differential pressure setpoint shall be no more than 110% of that required to achieve design flow through the heat exchanger. Where differential pressure control is used to comply with this section and DDC controls are used the setpoint shall be reset downward based on valve positions until one valve is nearly wide open.”
18.104.22.168 Hydronic Variable Flow Systems
ASHRAE 90.1 – 2013
ASHRAE is saying that when DDC controls are used to control chilled water variable speed pumps, they want us to monitor valve position and continue to lower the control setpoint until there is just enough head to keep one valve nearly wide open. However, there is one BIG problem when applying this reset to PIC valves. The problem is a change in differential pressure doesn’t impact the flow or valve position through a PIC valve. That is because a built-in pressure regulator compensates for those increases and decreases in pressure, so the valve position doesn’t change even though we have reset the control head. The only time the valve position will change based on control head reset is when have lowered the differential pressure enough so that the valve is no longer in its control range. When this happens, we are no longer in control and can have a big coil flow miss. That’s not really helping ASHRAE’s mission to save energy by downward reset of the differential pressure setpoint. PIC valves offer excellent controllability, but be careful if you are applying reset!
So, you might ask what you are supposed to do if you want to use the PIC valves when you have a chilled water system with DDC controls requiring variable speed pump control? We have a solution for you. Stay tuned for our next blog on DemandSet control.