When you have decided to specify a Mechanical Ventilation with Heat Recovery (MVHR) system for your home, it can quickly become confusing which type of ducting to use.
There are two main types of MVHR ducting to choose from, explained below:
MVHR ventilation ducting in a “branch” layout
This is the classic ventilation system, also used in commercial buildings such as offices, schools and supermarkets. Ducts, normally made of steel, begin with a large diameter from the MVHR unit, and run around the property “branching” off into smaller ducts as they duct air to each room. Rigid steel ducts will start at between 200mm – 125mm diameter at the MVHR unit, gradually reducing to 125mm and 100mm diameter ducts to each room.
Here’s an example MVHR branch design, by Enhabit:
Because ducts branch to different rooms from a single main duct, it’s necessary to place large drum sound attenuators at each room valve to prevent the risk of “cross talk”, because otherwise you could hear people talking through the ducts from one room to the other.
Branch systems require a lot of ceiling, wall or floor space to run the heavily-sized ducts. The sound attenuators are also quite large, although “slim” versions are available – for an extra price.
MVHR systems in a radial or “octopus” duct layout
MVHR systems in a radial format have a single (or sometimes two) ducts that run to each room valve from a manifold above the MVHR unit. The ducts are semi-rigid plastic manufactured in 50 metre lengths, so a continuous sealed duct from unit to valve. This produces more ducts altogether, but they are smaller than in a branch system at 75mm external diameter. This means that – at most – any necessary ceiling voids will be only 75mm.
Here’s an MVHR radial duct layout, by Enhabit:
Radial systems negate the need for acoustic attenuation at each room valve to prevent cross-talk, because each valve returns to the manifold, which in itself attenuates sound.
Because radial layouts use semi-rigid plastic ducting there is no need for hot cutting works (eg, cutting steel ducts with an angle grinder) and there is very little heavy lifting.
A radial system generally uses less space than rigid steel ducting – apart from above the unit at the point where all of the ducts meet with the manifold (the “octopus” part).
Rigid steel ducting is still used in a radial system for the ducts from the unit to the two extract/supply manifolds, and to and from the exhaust/intake ducts. These ducts also have drum silencers to stop breakout noise from the fan.
MVHR Radial systems are much easier to install than branch layouts
Radial ducting will be much easier to install for a local builder or self-builder. Approximately 50% of Enhabit’s clients in the last couple of years have self-installed their radial ducting system, and all of them have mentioned how straightforward it was (with our supporting guidance documents). It requires almost no cutting with a circular saw or heavy lifting of steel ducts. Radial is semi-rigid and double-walled, and it’s cut with a common Stanley knife and push-fit into the plenums.
Radial is a single length, so there will be no breaches or leaks and no need to tape the joins between steel ducts (time-saving). This means they’re guaranteed airtight from MVHR unit to room valve. Branch ducts have lots of joins between lengths of steel ducts. If not taped properly they can form air leaks. Any leaks in the ducts will mean the fans have to work harder, wasting energy.
In all of our radial layout installations we’ve seen the design flow-rates met, with very little pressure loss (a common misguided criticism of radial ducting systems is that they can’t supply air to the furthest rooms – we’ve proven this incorrect on many of our MVHR projects).
Radial MVHR duct systems are cheaper than branch layouts
We’ve found that radial system materials are about 30% cheaper than the equivalent branch systems, because steel ducting is more expensive. Installation costs are also much cheaper for radial systems because it’s so easy to cut and fit, as mentioned. Please contact me at Patrick [a] enhabit.uk.com (replacing the [a] with an @ symbol) for a free quotation.
Finally, radial ductwork is easier to clean and maintain long term than branch, because they have smoother bends and longer duct runs for pipe cleaners.
So when should MVHR branch duct layouts be used?
Branch duct layouts are appropriate when the building has lots of rooms which come off a central corridor, for example, student accommodation with lots of bedrooms, or for properties which requires very high flow-rates. They can be appropriate for homes which share a similar layout, but normally the lower cost, ease of installation and smaller ducting of a radial system will “win out” during the detailed design phase of an MVHR system.