Hot Water Systems
Lesson 4.2
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.
Make: The boiler burns gas (or uses electricity) to heat water to a target supply temperature.
Move: Pumps circulate water through the loop continuously. VFDs vary pump speed to maintain a target differential pressure.
Deliver: AHU and fan coil hot water valves modulate to take heat from the loop.
The sequence has two key calculations:
- OAT reset: Calculate the HWST setpoint from outdoor air temperature.
- Differential pressure control: Command the VFD to maintain the DP setpoint.
Lead / lag pump logic: Two pumps: one lead, one lag (standby). The lag pump starts automatically if the lead pump fails to prove.
Point Map
| Point | Description | Category |
|---|---|---|
| HW_SupplyTemp | Hot Water Supply Temperature | AI |
| OAT | Outdoor Air Temperature | AI |
| Diff_Pressure | Loop Differential Pressure | AI |
| Pump1_Status | Pump 1 current switch proof | BI |
| Pump2_Status | Pump 2 current switch proof | BI |
| VFD1_Spd | Pump 1 VFD speed command 0-100% | AO |
| VFD2_Spd | Pump 2 VFD speed command 0-100% | AO |
| Boiler_En | Boiler enable relay | BO |
| Pump1_En | Pump 1 enable relay | BO |
| Pump2_En | Pump 2 enable relay | BO |
Software points: Sys_Enable (BV), Rotate_Pumps (BV), HWST_SP (AV), DP_SP (AV).
Simulator Notes
Boiler Setpoint Signal: Scaled 0-10 VDC over 120-180 degrees F. The boiler reads this voltage and sets its firing target accordingly.
OAT Reset: OAT <= 20 degrees F results in HWST-SP max 180 degrees F. OAT >= 60 degrees F results in HWST-SP min 140 degrees F.
Pump Rotation: The incoming pump starts and proves first. Both pumps run briefly in parallel to prevent flow interruption. Once proof closes, the outgoing pump shuts down.