One Zone Not Cooling: Zoned Systems vs. Separate Systems

Short answer

If one zone won't cool while the rest of the house does, first find out what kind of setup you have. Some homes have one AC with zone dampers in the ducts, run by a zone control panel. Others have two or more separate systems, each with its own outdoor unit and thermostat. In a zoned home, a stuck damper, a failed damper motor, or a zone thermostat that isn't calling is common. In a home with separate systems, the "zone" that won't cool is its own AC with its own repair. Call (702) 738-6976 to schedule.

Step 1: Zoned system or separate systems?

• Count the outdoor units. One outdoor unit with two or more thermostats usually means a zoned system. Two outdoor units, each matched to a thermostat, usually means separate systems.
• Look for a zone panel. Zoned systems have a control board, often near the indoor unit, with a light for each zone. You can look at it without opening anything.
• Separate systems: if the upstairs unit runs but doesn't cool, treat it like any AC problem. See AC running but not cooling.

Many two-story homes in Southern Highlands have more than one system, so check this before assuming it's a damper.

Safe homeowner checks (zoned systems)

Don't open the zone panel or ductwork, and don't remove damper motors.

1. Is that zone's thermostat calling for cooling? Set to Cool, screen working, and set to call for cooling. A blank screen often means dead batteries or a wiring issue; see thermostat not working.
2. Watch the zone panel lights. When only the warm zone calls, the panel should show that zone open (on many panels a green or lit LED). Note what you see for the technician.
3. Feel the vents. Little or no air at several vents in that zone, while another zone blows strongly, points to a damper or duct problem. Air that's moving but not cold points to the equipment or that zone's thermostat.
4. Check that vents and returns in the zone are open and not blocked by furniture.
5. Did it start after thermostat work? A new or replaced thermostat that isn't set up for the zone panel is a common cause. Mention it when you call.

What usually causes it

• Damper motor (actuator) failed, so the damper stays closed.
• Damper stuck in the duct even when the motor is powered.
• Zone panel output or wiring fault for that one zone.
• Bypass or airflow problems. When only one small zone calls, the system can move too much air into too little duct. Some zoned systems use a bypass damper or staged equipment to handle this; when that isn't working, airflow and cooling suffer.
• Duct problems in that branch, such as a disconnected or crushed run. See ductwork problems.

Zone problem or system problem?

• Only one zone is warm, others cool fine: most likely the damper, actuator, zone thermostat or that branch of duct.
• Every zone is a bit warm, worst upstairs: more likely the system itself (low charge, dirty coil, outdoor unit) or heat load on the top floor. See two-story temperature balance.

FAQ

Why is one zone hot when the AC is running? In a zoned home, that zone's damper may be closed, its motor may have failed, or its thermostat may not be calling. In a home with separate systems, the AC serving that area has its own problem.

How do I know if I have zone dampers? One outdoor unit with more than one thermostat, plus a zone control panel near the indoor unit, usually means a zoned system.

Can I open a stuck damper myself? No. Dampers and motors are inside the ductwork and wired to the zone panel. Note what the panel lights show and call (702) 738-6976 to schedule.

My new thermostat broke one zone. Is that possible? Yes. A replacement thermostat that isn't wired or set up for the zone panel is a common cause after thermostat work.

Upstairs is always warmer even with zoning. Is something broken? Not always. Upper floors gain more heat. If the upstairs zone never reaches what it's set to, or has weak airflow, have the damper and airflow checked.