HVAC Controller – Permanent Ventilation Mode

Reported Issue

HVAC unit remained in ventilation mode and would not enter heating or cooling operation.

Background

• Two faulty units were supplied.

• One unit had previously been sent to a third party for nearly two years with no resolution.

• Documentation provided included service manuals and limited internal schematics.

Diagnostic Approach

• Service manual review identified multiple conditions required for HVAC enablement.

• RS232 communications established using TerraTerm after identifying non-standard serial settings.

• Controller logic and fault states were interrogated in real time.

• Input pins were individually simulated according to schematics.

Root Cause

• A Power Detect input was permanently inactive.

• The signal passed through a solid-state relay, which failed to energise.

• Visual inspection revealed no LED activity on the relay.Component-level inspection identified a burnt resistor, resulting in incorrect resistance.

Resolution

• Resistor replaced.

• HVAC functionality fully restored.

• Second unit required extensive rework due to prior damage and missing components.

• Unit was rebuilt using recovered and replacement parts.
  
  

Outcome

• Fault identified within 48 hours.

• No OEM spares available – repair capability established internally.

• Reverse engineering initiated to support long-term overhaul and spares manufacture.

• Mechanical DIN-rail standoffs were redesigned using fire-resistant materials.