Field-Ready · Lesson 4.2
Hot Water Systems
One boiler, two pumps, OAT reset, and differential pressure control. The sequence that keeps buildings warm.
What Is a Hot Water System
A hot water heating system has three jobs: make hot water, move it, and put the heat where it is needed. The boiler plant controller orchestrates all three from a single controller that reads temperatures and pressures and issues commands to pumps, a boiler, and VFDs.
Make
The boiler burns gas (or uses electricity) to heat water to a target supply temperature. The BAS enables the boiler when heat is needed and holds it off when not.
Move
Pumps circulate water through the loop continuously. VFDs vary pump speed to maintain a target differential pressure as zone valves open and close.
Deliver
AHU and fan coil hot water valves modulate to take heat from the loop. The loop delivers; zone-level sequences decide how much to take.
The sequence you will write for this system has two key calculations that run continuously:
- OAT reset: Calculate the HWST setpoint from outdoor air temperature. Colder outside means higher supply temp setpoint.
- Differential pressure control: Read the loop DP sensor and command the VFD to maintain the DP setpoint. More zone valves open means lower DP; VFD ramps up. Valves closing means higher DP; VFD slows down.
Lead / lag pump logic
Two pumps: one lead, one lag (standby). The lead pump runs whenever the system is occupied and the boiler is enabled. The lag pump starts automatically if the lead pump fails to prove (its current switch does not close within a timeout after the enable command). Pump rotation (swapping lead and lag periodically) equalizes wear between the two.