Hydronic System Differential Pressure

[Charm City Engineer]

Hello all... Im a controls engineer and I am new here but had a quick question on the theory behind setting a hydronic system differential pressure setpoint. Usually we let the test and balance guru's do the leg work for us and give us the differential pressure setpoint for our systems, but, recently we had a customer request from us to provide the calculations we are using to set the initial differential pressure setpoint. I went around to 3 different mechanical engineers and they all gave me different answers... one said it should be set slightly higher than your largest load (dpv + dpc), one shrugged his shoulders and said its better to figure that out on a live system, and the final one said that it was a pipe calculation. Could anyone with some experience in this area please fill me in?

 

[GT-EGR]

When you size and select a pump you do a pressure loss calculation to see how much is required to fully circulate the flow through the most restrictive part of the system and back around to the pump. This includes all the piping, the valves at the pumps and chiller/heat exchanger, and the losses through the terminals/coils with control valves being served.

If you break out the part of this calculation where the branch comes off the supply main, travels through the valve, and terminal and then back into the return main, that is theoretically the differential pressure you need in the system. It gets more complicated when you do this in real life and the differential pressure sensor is in a different location than your calculation, and you also are trying to figure out which branch valve needs the most pressure as some may have been oversized or some have plugged strainers etc.

So you technically have the calculated value as a starting point, but this needs to be confirmed in reality by the test and balance contractor. So they will get system running and measured at full flow and then in real time look at the differential pressure measurement that provided that flow through the system - and then you set up the controls to maintain that setpoint.

 

[pretendfarmer]

To determine the correct differential pressure set point for your hydronic system, the answer is "It depends". There is usually a lot of confusion regarding the correct value to use.

Here are a few of the factors that determine the set point and sensor location:

For simplicity sake, let's assume you have a single sensor on a direct return hydronic system and that all of the coils have 2-way modulating valves on them. In that case you would pipe the differential pressure sensor/transmitter/transducer across the inlet and outlet of the 2-way valve at the load located furthest from the system circulating pump. You do not need to include the coil pressure drop when installing the sensor. Only the 2-way control valve.

Most modulating 2-way control valves are selected for a 5 psid dropout design flow. You need to verify the design pressure drop for the most remote 2-way modulating control valve at the end of your system. That design pressure drop for that control valve is your initial set point. You don't want the pumps turning any faster than needed wasting energy.

This set point insures that you will always have design flow at the most remote point in your system. However, it does not insure that you have design flow available at every load all the time in your system. If you have high pressure drop loads located closer to the system pump, they may not see design flow when the sensor at the end of the system is satisfied. You may have to nurse the set point up from the original setting. You will be able to determine the amount of increase needed on a design day. If all of the coil design pressure drops are close to each other, it probably won't matter.

If you are installing multiple differential pressure sensors, then a bias control approach is required and the controller will speed the pumps to the point where all of the loads are satisfied. The sensor that is in the lead position may change throughout the day.

If your system is reverse return, then typically the sensor gets installed across the control valve that controls the most critical/sensitive load. The set point determination remains the same as discussed above for the single sensor.

If your control valves are two position on/off valves, the set point selection remains the same as the single sensor direct return system described above.

So your calculation is very simple. Whatever the design pressure drop for the 2-way control on the load that the sensor is installed on is your set point. There is no need for piping calculations and the such. Some people confuse this set point with design pump head. They have no relationship whatsoever. I used to see differential pressure set points operating at 25 and 30 psid when 5 is usually the right set point. The amount of wasted energy at those elevated values is very significant.

Good luck.

 

[nglty]

To remove all doubts about set points, use a head pressure reset as required by ASHRAE 901.
i,e, Poll all valve positions to modulate pump VFD.