Let’s break down HVAC (Heating Ventilation Air Conditioning) heating dual pump logic. It is typically used in hydraulic heating or chilled water systems. This breakdown will help you understand both the control sequence and the logic design.
Overview
In a hydraulic HVAC system with two pumps (Pump A and Pump B), dual pump logic ensures:
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Continuous operation even if one pump fails.
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Equal wear between pumps (via lead/lag alternation).
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Automatic backup operation.
This logic is often implemented in a BMS (Building Management System) or a PLC (Programmable Logic Controller).
Typical System Components
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Pump A (Lead Pump)
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Pump B (Lag Pump)
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Differential Pressure Sensor or Flow Switch
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Temperature Sensor (supply or return)
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Start/Stop Signal (from heating demand or schedule)
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Pump Status Feedback (run/fault)
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Manual/Auto Selector Switch
System Stop Conditions
System stops if:
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No heating demand
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Manual stop command
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Fault or safety trip signal
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Both pumps stop together (or lag pump stops first if both were running)
Additional Enhancements
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Soft start delay: Avoid both pumps starting simultaneously.
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Run hours equalization: Rotate pumps based on runtime difference.
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Fault reset logic: Automatic retry after timeout.
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Manual override: For maintenance mode.
Control Logic — Step by Step
1. System Start Conditions
System starts if:
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Heating mode is active (e.g., boiler ON or heating valve open)
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Enable signal is TRUE (from BMS schedule or thermostat)
2. Lead Pump Operation
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The lead pump (say Pump A) starts.
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Differential pressure (ΔP) or flow rate is monitored.
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If ΔP ≥ Setpoint → system stable → keep Pump A only.
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If ΔP < Setpoint → start Pump B (lag pump) after a delay (e.g., 30–60 seconds).
3. Pump Failure Handling
If a lead pump fault occurs:
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Immediately stop the faulty pump.
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Start the lag pump.
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Send an alarm or fault signal to BMS.
4. Auto Alternation
To ensure equal wear:
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Alternate lead/lag status every:
- Fixed time period (e.g., every 24 hours or week), or
- Each time the system restarts.