Residential Ventilation Ducting Systems

When designing and installing residential ventilation systems the ducting chosen can have a big impact on how the system performs overall. It is essential to select an appropriate ducting system that will suit the layout of the building, the service area available, and the unit it is connected to.

Numbers?

There are a number of options when it comes to ducting, and all will work when used correctly. Ducting is key to achieving compliance with Part F of the building regulations. No matter what unit is being installed, or how efficient or powerful it may be, if connected to a poorly designed ducting system it can be almost impossible to achieve compliance and the good indoor air quality (IAQ) required for a residential property.

What ducting options are there?

There are a wide variety of options of ducting systems that can be used and are appropriate to use with either MVHR or CMEV units. Each ducting system will have its own merits and benefits in specific use cases. Types of commonly used ducting systems include,

Round PVC ducting – 100mm, 125mm, 150mm
Rectangular PVC ducting (flatpack) – 204x60mm, 220x90mm
Semi Rigid Ducting – 75mm, 90mm
Metal Spiral Ducting
Thermal ducting

Over the last number of years, in residential settings, the round and rectangular ducting systems have been the most prevalent.

The flat pack rectangular type ducting is especially popular in apartments where service areas can be relatively tight. Flat pack duct generally comes in 2m lengths with a number of fittings such as,

90-degree bends (horizontal & vertical)
45-degree bends (horizontal & vertical)
Straight connectors
Rectangle to round adaptors (horizontal & vertical)
T pieces

The above fittings allow designers to work out appropriate routes though a property.

The round type ducting systems are very similar to the flatpack ducting in terms of approach to design and fittings. As this duct is round it can take up more space in a service area. This means it can be more commonly used in housing rather than apartments where an attic allows for more space for installation.

A newer type of ducting system that has become much more prevalent in the residential ventilation market in recent years is semi rigid ducting. This ducting comes in 50m long coils and is connected to a manifold adjacent to the MVHR/CMEV unit. A typical semi rigid ducting system includes,

75/90mm semi rigid ducting
Extract/Supply Manifold
125mm Valve plenum

Semi rigid ducting systems, such as the Brookvent Airduct System, have become very popular in housing developments using MVHR especially. These systems can be faster and easier to install for ventilation contractors.

How to choose the right system

In most cases the type of ducting system that is selected will be decided upon due to the constraints of the specific property.

For example, flatpack ducting is most common in apartment developments due to the often-constrained service cavities available under floor slabs. These service cavities must also accommodate other services such as electrical, refrigeration pipes, sprinkler systems, etc. so ensuring all services can be contained within the cavity will often dictate what ducting system can be chosen.

When MVHR ducting systems are to be installed in a service cavity there will quite often be a need for supply/extract ducting to cross over. As the service cavity may be quite shallow, only a flatpack ducting system would be feasible to fit within the cavity.

The case of housing, it can often be a bit more flexible in terms of choice of ducting system used as there tends to be more space.

In these cases, it often comes down to designer preference, or what they are more familiar with. In CMEV type systems in housing, rigid duct is often the most common system specified. These systems are interesting as they often mix flatpack and round ducting to achieve good detailing.

Round ducting may be used in the attic space where there is space to do so but when ducting needs to drop to the ground floor a switch to flatpack may be considered. It is completely acceptable to mix and match both ducting types to achieve the required details.

Speed of install is a big consideration for all stakeholders in a construction project, as is quality of install. This is quite often why semi rigid ducting systems are specified. As the ducting comes in coils 50m long, and as such there are considerably less joints required. This has two major benefits; installation time can be greatly reduced, and possibility of leakage is greatly reduced. Installation time can be reduced greatly without having an impact on either quality or cost.

Due to the smaller duct size however, more ducting needs to be run and this can have an impact on feasibility due to available space for services.

All systems have their benefits but must be chosen to suit the building they are being installed in.

airduct Semi Rigid Range

The importance of design

No mechanical ventilation system, MVHR or CMEV, can work correctly if connected to a badly designed or improperly specified ducting system.

A designer should be able to adequately calculate what size ducting is required to move air to/from each air valve. Designers should start with an airflow design to ensure they know how much air is required in the property and in each room. This allows them to calculate what size ducting, or how much ducting is required. The goal is to keep static pressure in the ducting as low as possible.

Once a designer knows how much air is required and what type of ducting system it is feasible to install, they can then start to design the ducting routes. When designing the ducting system, a designer must co-ordinate with designers of the other services such as plumbing and electrical to ensure there are no clashes with other services. Other considerations for designers are,

Keeping duct runs as straight as possible
Positioning of valves in appropriate locations
Shorter duct runs where feasible
Position of lighting or smoke alarms in a ceiling
Fire considerations – avoiding protected corridors where possible
Insulation details
SAP listing of Semi Rigid Ducting

Insulation is a very important consideration in design of systems, where a warm air duct passes though a cold space, or a cold air duct passes through a warm space, insulation is required. If insulation is not correctly specified this can cause serious issues and water damage in the long term.

Quality of installation

Even with proper choice or duct and good design, a residential ventilation system can be let down by poor installation.

Installers must ensure they install to a design, using the correct material. Installers must ensure they use appropriate tapes and mastics. In rigid PVC ducting systems best practice is to seal all joints with mastic, tape, and a screw to ensure no air leakage can occur.

In semi rigid systems the installer must ensure all joints have been gasketed and correctly secured, that the ducting is secured at regular intervals and pulled tight with no peaks or troughs in the duct runs.

Simple QA documents can be used during installation of ducting systems to ensure quality of installation has been achieved and that the residential ventilation systems will perform as designed.