One side of office warm, other side fine: zone damper stuck
Half the office stayed warm while the other half cooled fine. A contractor had already said the duct was probably disconnected. When only one zone drops out, there is a simpler part worth checking before anyone opens the ceiling.
By Team Snowflake | Reviewed 10 Mar 2026
Case summary
Mitsubishi Electric Ducted10 years oldOfficeStraits View, Singapore
- Concern
- The worry was that the ceiling would need to be opened to repair or replace damaged ductwork.
- Found
- motor-driven zone damper stuck in a near-closed position, restricting cooled air to one zone of the ducted system
- Key check
- Checked damper positions and damper motor response at each zone before assuming duct damage or refrigerant loss
- Result
- Both sides of the office reached the same temperature within the hour. No ceiling panels were removed and no ductwork was touched.
What we were told
One side of the office gets warm air from the ceiling vents while the other side is fine. A previous contractor said the duct to that zone might be disconnected or damaged, and that the ceiling would need to be opened to check.
What we checked
When one zone drops out and the rest stays cold, the problem sits between the main unit and that zone. We checked the damper first, the gate that opens and closes airflow to each area. We did this before considering duct damage or a gas leak, since the damper opens through the access panel with no ceiling work.
Supply air from the main ducted unit measured cold and at normal volume, so the unit itself was working correctly.
Every zone on the unaffected side had full airflow and correct temperature at its supply grille.
At the warm zone, the damper blade sat stuck near closed, which would explain why little cold air reached that side.
Turning the damper shaft by hand met stiffness, a sign of friction rather than an electrical fault.
Through the access panel, dust and light surface corrosion were visible on the damper motor shaft and bearing area.
What we found
The meeting-room damper had stayed in one position too long. Dust and light corrosion built up around the shaft, so the small damper motor could no longer push it fully open. The main unit still supplied cold air, but the stuck blade blocked most of it from reaching that zone.
What fixed it
We cleaned the corrosion and dust from the shaft and bearing area with a contact cleaner, then applied a light lubricant rated for the operating temperature. With the friction gone, we ran the motor through several full open-close cycles to confirm it could drive the blade across its full range without stalling. We then had the thermostat cycle the zone on and off so the motor responded to automatic calls, not only manual ones. The damper opened and closed cleanly each time. Finally, we measured airflow at the affected grilles to confirm it matched the other zones. No motor replacement, no ceiling panels removed, and no ductwork touched.
Outcome
Both sides of the office reached the same temperature within the hour. No ceiling panels were removed and no ductwork was touched.
What this case teaches us
One warm zone points to the damper, not the duct
- If one zone stays warm while the rest cools, the air supply is fine. The problem is usually what blocks air to that zone.
- Each zone has a damper, a small gate the system opens and closes. When it sticks, that zone loses cold air with no duct damage.
- Before agreeing to ceiling work, ask whether the damper was checked. It opens through an access panel in minutes.
Related reading
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