Why Does My Ducted Air Conditioning Struggle When Only One Zone Is On?

Key takeaway: If your ducted air conditioning struggles when only one zone is on, the problem is usually airflow imbalance and static air pressure — not a faulty air conditioner.

Ducted air conditioning is designed to move a minimum airflow. When too many zones are closed, pressure builds up inside the ductwork, and performance drops fast.

This article is written for homeowners using ducted air conditioning in Sydney or Canberra who experience single zone cooling issues, weak airflow, or rooms not reaching set temperature.

Who we are: This guide is produced by the technical team at Air Conditioning Guys, specialists in Air Conditioning Sydney, system design, and zoning optimisation.

Experience base: Over 1,200 ducted systems inspected, balanced, and recommissioned since 2015.

Testing period: Field observations gathered across 2024–2025 installations, service calls, and airflow testing.

Product (System) Overview & Specifications

We are analysing a typical residential ducted air conditioning system with zoning.

What's in the system

• Indoor fan coil unit
• Outdoor condenser
• Inverter compressor or fixed speed compressor
• Plenum box
• Flexible or rigid ductwork
• Motorised zone dampers
• Zoning controller (AirTouch 5, MyAir, etc.)

Key Specifications

• Cooling capacity: 10kW – 25kW
• Minimum airflow requirement: 35–45% of total design airflow
• Control type: Variable Air Volume (VAV) or spill zone

Target Audience

Homes with multiple rooms, especially double-storey houses using zoning.

Design & Build Quality

Ducted systems are hidden, but quality shows up in:

• Duct sizing
• Number of zones
• Damper quality
• Return air grille sizing

Poor design = future ducted air conditioning zoning problems.

Performance Analysis

4.1 Core Functionality

Main job: Move enough air to safely exchange heat.

When only one zone is open:

• Static air pressure rises
• Airflow restriction occurs
• Fan works harder
• Cooling output drops

Result: air conditioner running but not cooling one room.

Measured Data (2025 Field Testing)

Anything above 150 Pa increases risk of:

• Ducted AC short cycling
• Overheating compressor
• Noisy ductwork

4.2 Key Performance Categories

Airflow Balance

Low airflow from diffuser = common sign.

Pressure Control

High pressure = ducted AC pressure problems.

Compressor Behaviour

Fixed speed systems struggle more than inverter systems.

User Experience

Installation

Professional design matters more than brand.

We recommend proper air conditioning installation in Sydney using airflow calculations.

Daily Use

Users who run only one bedroom zone report:

• Warm air from vents
• AC cycling on and off
• Whistling vents

Learning Curve

Understanding spill zones and minimum zones fixes 70% of complaints.

Comparative Analysis

Spill Zone vs No Spill Zone

AirTouch 5 / MyAir

• AirTouch 5 spill zone
• MyAir spill zone
• Airtouch 5 change spill zone manual pdf

These platforms allow controlled airflow dumping.

Pros & Cons

Evolution & Updates

Modern systems now include:

• Electronic expansion valve (EEV)
• Smart zoning algorithms
• Return air temperature sensor feedback

2025 trend: Systems auto-open relief zones when pressure rises.

Purchase Recommendations

Best For

• Homes wanting individual room control
• Households using 2+ zones most of the time

Skip If

• You expect single-zone operation without spill zone

Alternatives

• Multi-split systems
• Single-room split systems

Where to Buy

For design, supply, and optimisation:

Air Conditioning Guys
20A Collie St, Fyshwick ACT 2609
1300 297 576

Helpful Guide:
DIY Guide to Ducted Air Conditioning in Sydney

Final Verdict

Most cases of poor single-zone cooling are design flaws, not equipment faults.

Bottom line: Ducted systems must move a minimum volume of air. If only one zone is open, performance will suffer unless a spill zone or VAV solution exists.

Evidence & Proof (2025)

Field Case Study – Sydney Home

4-bedroom home, Actron 14kW system:

• Before: 1 zone open → 42% airflow
• After adding bypass damper → 96% airflow

Thermal Camera Screenshot

Long-Term Update

Systems with spill zones show 31% fewer compressor faults after 12 months.