Both systems bring in filtered fresh air and recover heat from the air they extract. Central MVHR uses one unit and ducts to every room; it usually recovers more heat, filters better and runs quietly from a utility room or hot press. Common reversing wall units work in pairs without ducts and suit retrofits where ducts will not fit. In a new airtight Irish home, central MVHR is the usual choice; under Part F, the system is commissioned and then validated by an independent competent person.
New homes in Ireland are built airtight to meet the nearly zero energy building (NZEB) standard of Part L, and an airtight home needs a planned ventilation system. Mechanical ventilation with heat recovery (MVHR) supplies fresh air and keeps the heat that would otherwise leave with stale air. This guide compares central and decentralised systems on heat recovery, air quality, noise, maintenance and compliance with Part F of the Building Regulations.
How does central MVHR work?
A central MVHR unit sits in a utility room, hot press or ceiling void. Ducts take fresh air to the living room and bedrooms and extract stale, humid air from the kitchen, bathroom and utility room. In the unit, the two airflows pass each other in a heat exchanger without mixing, so the outgoing air warms the incoming air. Fans run continuously at a low setting and can be boosted for cooking or showers.
Because the whole house runs through one unit, the airflows are balanced, filters can be fine, and the fans sit away from the rooms you live in.
How do decentralised ventilation units work?
Decentralised units are fans built into the external walls of individual rooms. Most work in pairs: one unit blows air out while the other draws air in, and they swap direction at a fixed interval. A ceramic or similar store in each unit absorbs heat from the outgoing air and returns it to the incoming air. There are no ducts, only wall openings.
Paired reversing units are quick to fit into an existing house, and each device has a fan, filter and wall opening. Other decentralised designs supply and extract air simultaneously through a heat exchanger; some can also serve an adjoining room. The number of devices depends on the ventilation design.
Central or decentralised: how do they compare?
| Central MVHR | Decentralised units | |
|---|---|---|
| Layout | One unit, supply and extract ducts | Wall devices; number and placement depend on the design |
| Heat recovery | Counter-flow heat exchanger; certified units reach high efficiencies | Reversing heat store or continuous-flow heat exchanger; performance depends on equipment and operating conditions |
| Air quality | Fine filters at one point; balanced supply and extract | Smaller filters per unit; airflow per room |
| Noise | Unit outside the living rooms; ducts with silencers | Fan in the room's wall, audible in quiet bedrooms |
| Installation | Duct routes planned with the house | Core drill per unit; no ducts |
| Maintenance | Filters changed at one unit | Filters changed at every unit |
| Best for | New builds and deep retrofits | Retrofits where ducts cannot be fitted |
In a new home, ducts can be planned with the house. Central MVHR keeps filter changes at one point and fans out of bedrooms. Compare the certified heat recovery of the actual units: the Zehnder ComfoAir Q450 in the larger BioBuilds homes reaches 88% in its Passive House Institute certificate.
Compare whole-home installed quotes at the same design airflow: include ducts and silencers for central MVHR, or all wall devices, openings and controls for a decentralised system. Include commissioning, independent validation, filter replacement and fan electricity costs.
What does Part F require for a new home?
Technical Guidance Document F (2019) sets out how a new dwelling meets the ventilation requirement of the Building Regulations. For mechanical ventilation, it expects the system to be designed by a competent designer, installed, balanced and commissioned by a competent installer, and then validated by an independent competent person, for example one registered on the NSAI ventilation validation scheme. Validation checks the installed airflows against the design and the minimum rates of Part F, and its certificate forms part of the building control file.
Plan this from the start: the validator measures the system as installed, so the commissioning record and design airflows need to be ready at the end of the build.
What ventilation do BioBuilds homes have?
BioBuilds installs central Zehnder MVHR: the compact Zehnder EVO in Wanderlust, and the Zehnder ComfoAir Q450 with the ComfoClime Q module in Serenity, Sanctuary and Bloom. Zehnder rates the ComfoAir Q450 at up to 96% heat recovery; in the Passive House Institute's component certificates, the Q450 reaches 88% and the EVO 84% effective heat recovery, both above the 75% the institute requires. BioBuilds fits Zehnder's standard filter set: ISO ePM1 65% (F7) on the fresh air and ISO Coarse 65% (G4) on the extract air, so the supply filter holds back at least 65% of the finest particles (PM1).
Ventilation is included in the Turnkey finish and is an option in Exterior Finished, where it costs €9,500–€30,500 depending on the model, including 23% Irish VAT. Both finishes are built on BioBuilds' Passivhaus-certified building system. To see the Irish price with your chosen finish, configure your home.
How do you maintain MVHR?
- Filters. Change them on the manufacturer's schedule, and more often near busy roads or farmland. Central MVHR needs one set; decentralised systems need one per unit.
- Valves and grilles. Clean them from time to time without changing their settings, which the validation measured.
- Settings. Use the boost mode for cooking and showers, and a lower setting when the house is empty.
- Service. A periodic check of the unit, condensate drain and airflows keeps the system efficient.
