🌪️ Micro-turbulences
Every time air hits the internal corrugations, micro-eddies are created. The air's kinetic energy is lost as friction. In engineering terms, this is called Pressure Drop.
Flexible Ducts (Spiral): When they're allowed and the "crime" of the unstretched hose
Flexible ducts (commonly called "spiral" or "flex") are perhaps the most misunderstood material in HVAC installations. Made from a spiral steel wire core wrapped in aluminium and fibreglass insulation - they resemble a giant "accordion".
For a rushed installer, the flex duct is a godsend: pull one hose in 2 minutes instead of cutting sheet metal angles. But that is the recipe for absolute disaster. In this guide we explain why - and how to use them correctly.
1. The law of aerodynamics: Why air hates flex ducts
Inside a rigid duct (sheet metal or PIR panel), the walls are smooth as glass. Air glides freely. Inside a flex duct, the inner wall is covered with corrugations from the wire core - like driving on a potholed dirt road.
📊 3× to 5× greater drop
One metre of flexible duct can cause 3 to 5 times greater pressure drop compared to one metre of smooth, rigid duct of the same diameter. It is the worst choice for long runs.
❌ 6 metres of flex = Disaster
If the installer connects the unit to the diffuser using 6 metres of flex duct, the fan will strain to push air but almost nothing will reach the room. Electricity burns, the room stays hot.
🛣️ Highway vs dirt road
Think of air as a car: on a smooth duct (highway) it cruises freely, while in a flex duct (potholed dirt road) it brakes, bumps and loses speed with every metre.
2. The 4 Golden Rules for flex duct installation
Flexible ducts are not banned. In fact, they are essential for connecting diffusers - but only if the following rules are strictly followed. Proper application eliminates pressure drop and noise problems.
📏 Rule 1: Maximum 1.5 metres
The main rigid duct must reach as close as possible to the diffuser. Flex is used only for the final connection, bridging minor misalignments. Maximum length: 1.5 – 2 metres.
🔄 Rule 2: Stretch to the max
Cutting 2 metres of flex for a 1-metre gap? That is a criminal mistake. When left compressed ("concertinaed"), the corrugations deepen dangerously. It must be fully stretched before cutting and connecting.
↩️ Rule 3: No sharp bends
If you kink flex duct at a 90-degree angle, you throttle airflow by 50%. Bends must be smooth (open arc). For sharp turns, use a rigid metal elbow then attach the flex after it.
🔧 Rule 4: Proper support
Flex ducts are soft - if left unsupported, they sag under their own weight. This changes their geometry and drops the pressure. Support them with wide straps every 1 metre, keeping them horizontal and straight.
3. Compressed vs Stretched: The difference that saves your installation
Stretching flex duct isn't a "detail" - it is the single most critical action during installation. A stretched flex behaves almost like a smooth duct, while a compressed one can double or triple the pressure drop.
📐 Compressed = Air brake
Think of it as a gathered curtain: the material folds, corrugations deepen. Air hits every fold, loses speed and creates noise. The effective cross-section is drastically reduced.
✅ Stretched = Functional
By pulling the flex to its maximum length, corrugations become shallower, the inner surface becomes smoother, and pressure drop decreases by 40-60% compared to the same length compressed.
🔊 Aerodynamic noise
In a compressed flex, air creates a characteristic humming or whistling sound. In a properly stretched one, noise drops below 25 dB(A) - practically silent in a bedroom.
🎯 Correct cutting technique
Stretch first, measure, mark, and cut with a blade at the mark. Never leave excess "just in case". Every extra centimetre of compressed length means lost pressure in the room.
4. Sag-free support: Proper hanging in the ceiling void
A flex duct hanging like a "belly" between two support points isn't just ugly - it is a throttle point for the air. Proper support is just as important as stretching itself.
📎 Wide straps
Use wide installation straps - never wire or thin tape that cuts into the material. The strap cradles the duct without strangling or deforming it.
📐 Every 1 metre
Supports are placed every 1 metre maximum along the flex duct. If the gap is larger, gravity creates a belly and air brakes at the low point.
🔇 Vibration absorption
An important advantage of flex: it absorbs machine vibrations. Like an "anti-vibration connector", it prevents vibration transmission to plasterboard - as long as it isn't stretched excessively tight.
⚠️ When to stop the installer
If you see your installer running metres upon metres of flex duct across the ceiling void, stop them immediately. Only a final 1-1.5 metre connection at each diffuser - the rest must be rigid ductwork.
💡 Flex duct is a necessary evil. Excellent for diffuser connections and vibration absorption - but nothing more. Correct use = cool room. Wrong use = "why isn't the space cooling down?"
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