Control Logic and PID

Learning objectives

• >Model a simple hydronic loop with a pump curve and a system curve
• >Implement and tune a PI loop for DP control
• >Implement trim-and-respond DP reset, lead/lag rotation and alarms
• >Estimate pump energy savings with the affinity laws

Sessions

Three 2-hour weeknight sessions plus a weekend block. Check each one off as you finish it.

1. SESSION 1· 2 h Build a simple hydronic loop model: pump curve, system curve, and a valve that varies with load (Python or Node-RED)
2. SESSION 2· 2 h Add a PI loop holding DP through pump speed, and tune it
3. SESSION 3· 2 h Add trim-and-respond DP reset based on the most-open valve
4. WEEKEND· 4 h Add lead/lag rotation, failover and alarms. Trend a load drop and estimate the savings with the affinity laws

Answer in your notebook first, then reveal the answer and grade yourself honestly.

1. Q1Pump speed drops from 100% to 70%. By the affinity laws, roughly what share of full power is used? Why are real savings usually lower?

   [ reveal answer ]

   About 34% (0.7³). Real savings are lower because of fixed DP setpoints, static head, and VFD and motor efficiency losses.

   ✓ I got it ✗ Missed it

2. Q2What symptom tells you a loop's proportional gain is too high?

   [ reveal answer ]

   The loop oscillates or hunts around setpoint.

   ✓ I got it ✗ Missed it

3. Q3What happens if the integral time is too short? Too long?

   [ reveal answer ]

   Too short: overshoot and oscillation. Too long: a slow return to setpoint.

   ✓ I got it ✗ Missed it

4. Q4Why rotate lead and lag pumps?

   [ reveal answer ]

   To equalize wear and run hours, and to prove the standby pump works.

   ✓ I got it ✗ Missed it

5. Q5Why can't you reset the DP setpoint all the way to zero?

   [ reveal answer ]

   The most demanding branch still needs enough pressure to reach design flow.

   ✓ I got it ✗ Missed it