Heat Exchanger Not Heating: Troubleshooting Guide

Quick Diagnostic Flow

1. Check the hot side inlet temperature: Is the heating medium (steam, hot water, thermal oil) at the correct temperature? If not, the problem is upstream (boiler, heater, or distribution).
2. Check the hot side flow rate: Is the heating medium flowing at the designed rate? A blocked strainer, closed valve, or pump failure reduces flow.
3. Check the cold side flow rate: If the process fluid flow is too high, it passes through too quickly to be heated. If too low, it may be fouling or a blocked passage.
4. Check both inlet and outlet temperatures: Calculate the actual heat duty (Q = m × Cp × ΔT) and compare to the design duty. A significant shortfall indicates fouling or a flow problem.
5. Check the control valve: If the hot side is controlled by a modulating valve, verify it's opening correctly.

Common Causes

• Fouling (most common): Scale, biological growth, or particulate buildup on the heat transfer surfaces reduces the heat transfer coefficient. The hot side can't transfer heat to the cold side efficiently. Fix: clean the heat exchanger (see our Heat Exchanger Fouling guide).
• Air-bound (on the hot side): If air is trapped in the heating coil (common in hot water systems), it blocks flow and creates an insulating layer. Fix: vent the air from the high point of the heat exchanger.
• Steam trap failure: On steam-heated exchangers, a failed-closed steam trap floods the steam side with condensate, reducing the effective heat transfer area. A failed-open trap wastes steam but may still heat. Fix: test and repair the trap.
• Control valve stuck closed: The modulating valve on the hot side isn't opening. This can be a stuck valve, a failed actuator, or a controller issue. Fix: verify the valve position matches the controller signal.
• Flow imbalance: If the process fluid flow is too high relative to the heating medium flow, the exchanger can't keep up. Fix: verify both flows are at design rates.
• Bypass valve open: A bypass valve around the heat exchanger may be partially open, allowing process fluid to bypass the exchanger. Fix: verify the bypass valve is fully closed.
• Internal leakage: A tube leak or plate perforation allows the hot and cold fluids to mix internally. The process fluid exits at the wrong temperature, and the hot side flow appears to increase. Fix: pressure test and repair or replace the damaged component.
• Insufficient heating medium supply: The steam pressure or hot water temperature is below design. Fix: investigate the boiler, hot water heater, or steam distribution system.

Calculating Actual vs Design Heat Duty

To determine if the heat exchanger is performing as designed:

Where: Q = heat duty (kW), m = mass flow (kg/s), Cp = specific heat (kJ/kg·K), ΔT = temperature change (K)

Calculate Q for both the hot side and the cold side. They should be approximately equal (heat lost by hot side = heat gained by cold side). If they differ significantly, there's an external heat loss or measurement error. Compare the actual Q to the design Q — if actual is below 70% of design, the exchanger needs cleaning or repair.

Stall Condition (Steam-Heated Exchangers)

A stall condition occurs when the steam pressure inside the heat exchanger drops below the backpressure in the condensate return line. When this happens, condensate cannot discharge from the exchanger and floods the steam space, reducing heat transfer. This is common on modulating control valve applications at low loads.

• Symptom: the exchanger heats fine at high load but not at low load
• Fix: install a pumping trap (pressure-powered condensate pump) to discharge condensate against backpressure
• Alternative: reduce the condensate return line backpressure by repairing failed-open traps
• Alternative: install a vacuum breaker at the exchanger outlet to allow condensate to drain by gravity