Rectangular Duct Standard Sizes and Ancillaries

Why Rectangular Duct Rather Than Round Duct?

Rectangular duct is often the ducting format of choice when installation space is tight. Where a volume of air needs to be moved and you have a rectangular hole to duct it through, the rectangular duct is the space effective solution.

But there is another consideration beyond space. Whereas an unusual duct size might at first sight be the best use of space; it is also worth appreciating the array of standard ancillary equipment that can be connected to the more standard rectangular duct sizes.

What Is ‘Standard Size’ Rectangular Duct?

The trouble with ducting is that, once formed, it takes up a lot of storage space and therefore pre-formed ducting is not something that sits in large quantities on a shelf waiting to be sold. Most ducting is made to order. Rectangular ducting can in fact quickly and easily be folded to any size, within the limitations of the equipment deployed to make it, and ‘Mez’ or ‘Doby’ flanges are easily fitted to each end.

However if there is a standard it is that most catalogue information about rectangular duct recognises height and width variations in 50mm and 100mm increments. Therefore 500 x 300mm would be considered a standard size but rectangular duct measuring 467 x 932mm would not.

You can have either fabricated easily, however you can only get a cracking range of ancillaries for heating, cooling, demisting, filtering and moving air if you go with standard sizes.  The best range of ancillary equipment comes when the standard rectangular duct dimensions falls with in the following sizes:

rectangular duct standard sizes






With the greatest range of accessories for rectangular duct heights ranging from 200 to 500mm it makes this range of standard sizes brilliant for installing into void spaces. This is especially relevant to those smaller and older buildings where you might be looking to retrofit mechanical ventilation to provide those all important air changes to make them safer in this Covid 19 era.

Rectangular Duct Accessory options

Cross section in cm 40-20 50-25  50-30  60-30  60-35  70-40  80-50 100-50
KE 50-30-6 Rectangular fan** n/a Duct fans ranging from 1,000 to 11,000m3/h with 230v and 400v options
n/a EC Duct fans can be delivered with a pre-wired potentiometer (0-10V) which allows you to easily find the desired working point. EC fans are notable for their economical use of energy and excellent ease of control
n/a n/a EC Duct fans insulated. RSI models are thermally and acoustically insulated with 50 mm of mineral wool, with perforated sheet steel on the inner surface. To protect the motor from overheating the RSI fans have integral thermal contacts with external leads for connection to a motor protection device. The fans can be installed in any position and are easy to connect using the DS flexible connections. The casing is manufactured from galvanised sheet steel.
n/a n/a n/a ATEX duct fans. Explosion proof versions comply with EN 50014, EN 50019, EN 1127-1 and EN 13463-1. Improved safety in accordance with EEx e II T3. ?x II 2G Certified according to ATEX
GFL 60-30 Counter flange - GFL - Systemair n/a Counter Flange – Used as a flange for mounting to the duct system. Manufactured from galvanised sheet steel.
LDR 30-15 Silencer LDR Silencer. Easily-fitted silencer immediately before or after the KE, KT, RS and RSI rectangular duct fans. Effectively suppresses noise transmitted to the duct. The silencer should be used together with an insulated fan where there is a requirement for noise suppression both in the duct and in the surroundings as a whole. .
LDR-B 50-30 Silencer, baffle LDR-B Silencer. Rectangular baffle silencer is fitted with built-in baffles and side baffles located outside the fitting dimension and has an external shell of trapezoidal corrugated sheet for stability and reduced risk of natural oscillation. LDR-B is designed for low air resistance with baffle combinations that dampen particularly low-frequency noise well.
n/a n/a Single or double diffusion grilles direct the air using  adjustable blades in a horizontal, as well as vertical direction. Designed for air supply / air extract – for a balanced distribution of air through the entire diffusion grille it is recommended to use a damper or a plenum box.
CGR-UVc-6035-F7+F9 Germicidal chamber without fan for 600 x 350mm rectangular duct with 4 UVc lamps, F7 filter, F9 filter. Ideal for installation in existing air conditioning and ventilation systems. n/a n/a Germicidal chamber without a fan for rectangular ducts equipped with UVc ultraviolet lamps for new builds or easily retrofitted into existing ducts.
CGR-UVc-6035-F7+F9 Germicidal chamber without fan for 600 x 350mm rectangular duct with 4 UVc lamps, F7 filter, F9 filter. Ideal for installation in existing air conditioning and ventilation systems. n/a n/a As above in terms of UVc but with the addition of F7 & F9 filters. NB When retrofitting, the system may need to be ammended to overcome pressure drop due to the filtration section.
CGR-UVc-6035-F7+F9 Germicidal chamber without fan for 600 x 350mm rectangular duct with 4 UVc lamps, F7 filter, F9 filter. Ideal for installation in existing air conditioning and ventilation systems. n/a n/a As above but with a tighter filtration specification.
PGK 100-50-3-2,0 Duct cooler Cold water-cooling battery for rectangular ducts with galvanised steel casing, copper tubes and aluminium fins. Drip pan from stainless steel. MWP 1.0 MPa (10 bar). Two inspection covers for cleaning and maintenance. Droplet separator as an accessory. Air velocities from 2,5m/s.
DXRE 50-30-3-2,5 Duct cooler As above but incorporating a DX coil. DXRE are mostly used for central cooling of the ventilation air but can also be used for individual cooling of the air supplied to individual rooms (zones).
RB 40-20/9-1 400V/3 Duct heat. Electric Heater battery for rectangular ducts. Aluzinc-coated sheet steel with a heating element in stainless steel. The heater has integral overheating protection with a manual reset function. Suitable for control by room thermostat or TTC. 400v ranging from 9 to 80kW
VBR 50-30-4 Water heating batt LPHW heater battery for heating air in rectangular ducts. Hot-zinc-coated casing, heat transmission element with copper tubes and aluminium fins. The water-heating battery can be installed in a horizontal or vertical duct with an optional direction. 16bar WP at 100C water temp
Topvex SF04 HWL - Topvex SF - Systemair n/a n/a Compact Supply AHU for residential and small commercial applications.
n/a n/a n/a n/a n/a n/a Compact Heat Recovery AHU. Topvex FC is a series of efficient ventilation units designed for suspended ceiling installations. The units are especially designed to meet stringent energy requirements with low energy use and high-efficiency heat recovering.
SRK - Shutters ventilation - Systemair n/a n/a n/a n/a Volume control damper with the same flange system as rectangular duct fans. Air tightness class 3. The shutter valve consists of a number of blades which close towards each other, mounted inside a steel sleeve on bushes of glass-fibre reinforced nylon. The linkage is protected so that the valve can be insulated together with the duct. Blades and sleeve coupling are from galvanised sheet steel.
Fire dampers represent passive fire protection, designed with the help of compartmentalization to prevent the spread of toxic gases, smoke and fire. All our standard fire dampers are designed and certified in accordance with the test criteria EIS according to EN 1366 – 2.
Rectangular Smoke Control Damper for one (single) fire compartment with Automatic Activation, CE certified in accordance with the EN 12101-8, tested according to EN 1366-10 and classified in accordance with EN13501-4.
Single or double skin circular VAV (Variable Airflow Volume) controllers are commonly used for return air applications or for supply air applications. Ideal for single zone control with supply and return in Master and Slave setup such as offices, hotel rooms or meeting rooms or larger public or industrial halls where the required cooling and heating load will vary on demand.
Control and shut off damper for rectangular ducts with blade tightness class C4, tested to EN 1751. Ideal where different parts of the system need to be airtight separated from each other. It can be used on air handling units as outer shut-off damper or with modulating actuator as control damper. Supplied in a manual or motorized version with position indicator.
n/a n/a n/a n/a Flexible connections for in-duct fans. The connections fit exactly to the fan and are easily bolted to fan and duct.
FFK 60-30 Filter cassette rect - FFK - Systemair Rectangular Filter Cassettes can be designed for a range of filtration products including bag filters. Filter cassettes can be connected directly on to the duct and should generally be fitted in a flat or vertical position. If fitted vertically, the direction of the air flow must be downwards so that the filter bags do not collapse. The cassette are often fitted with connections for a pressure sensor to be added.


It is the ancillary equipment that makes rectangular duct such a strong contender, yet many in the HVAC trade still try to get round ducts through rectangular holes. Come on – see the light!

If you need any help pulling together the ideal rectangular ancillaries to suit your duct, call us.