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Decentralised MVHR: single-room units for offices and schools

Updated 23 October 2026 · SEO Dons Editorial

What is a decentralised MVHR unit?

HSE's ventilation guidance says that in small spaces and buildings mechanical ventilation fans may be in the room, while larger buildings may use a network of ducts and fans. Decentralised MVHR is the in-room approach with heat recovery added: each unit brings in outdoor air, extracts room air and passes heat between the two. The ecodesign regulation calls the simplest form a non-ducted unit: a single room ventilation unit not intended to be equipped with duct connections. Some classroom units still have short duct connections to the façade, so ask which type is on offer. The wider system choice is on the commercial ventilation and MVHR page.

What do published unit specifications look like?

These are facts from two manufacturers' own pages for their own products, not recommendations. Check the current datasheet and the ecodesign product information before relying on any figure.

Blauberg Civic DB-300Trox SCHOOLAIR-S-HV
MountingCeiling-mountedFloor-standing, 1200 by 2300 by 600 mm, 360 kg
AirflowMaximum 83 l/s (300 m³/h)300, 500 or 800 m³/h, boost 1050 m³/h
Heat exchangerPolystyrene counter-flowRotary, stated 80% heat recovery efficiency
FiltersPanel filters, G4 on extractISO ePM1 60% outdoor air, ISO coarse 90% extract
ControlsDemand control from CO2, humidity and temperature sensors; bypass for natural cooling; BMS integrationOptional air quality sensor; Modbus and BACnet interface options
Noise statedNo dB figure on the product page; see its datasheetSound power level 24 to 53 dB(A)

How many units does a room need?

CIBSE Journal's summary of BB101 reports a school fresh air rate of five litres per second per person, and a purge rate of 8 litres per second per person for new-build teaching spaces. HSE says many guides recommend 10 litres per second per person for commercial buildings. The table is illustrative arithmetic against the two units above (83 l/s, and 222 l/s nominal), not a design.

SpacePeople and rateAirflow83 l/s units222 l/s units
Classroom31 at 5 l/s155 l/s21
Classroom at purge rate31 at 8 l/s248 l/s31 (boost 292 l/s)
Small office12 at 10 l/s120 l/s21
Meeting room8 at 10 l/s80 l/s11

Several units in one room need to act together; Trox's range has master and slave control functions for that. Fewer, larger units cut the filter count but ask more of each wall opening.

Which airflow and CO2 criteria must they meet?

Schools. CIBSE Journal's BB101 summary gives 1,000ppm for mechanical ventilation, no more than 20 minutes above 1,500ppm in a new classroom, an average below 1,750ppm in refurbished teaching spaces, and says mechanical ventilation should not be the sole means of summertime ventilation. The DfE's operating bands are under 800ppm good and over 1500ppm poor. Because the regulation counts one sensor on a non-ducted unit as local demand control, each classroom's airflow can follow its own occupancy. The school ventilation page covers the rest.

Offices. HSE's Approved Code of Practice says the fresh-air supply rate should not normally fall below 5 to 8 litres per second per person. For building work in England the design guidance is Approved Document F, whose Volume 2 gov.uk lists for buildings other than dwellings. A unit meets it by delivering the design airflow to each room at the occupancy assumed, and the commissioning record should show it.

How loud are they, and where do they go?

  • Noise. The regulation requires product information including the casing sound power level for indoor units. Trox gives that and also a sound pressure of 35 dB(A) at nominal flow with 8 dB of room attenuation, which is a different figure. Ask for both, with the room assumptions.
  • Space. CIBSE Journal's non-residential module says ecodesign has made units larger relative to airflow, and that ceiling-void units are usually serviced from the side because ceiling access panels are unlikely to be large enough. A 360 kg floor-standing unit needs a floor loading check.
  • Façade. Each unit needs outdoor air and exhaust openings. Annex I defines mixing as immediate recirculation or short-circuiting between discharge and intake ports, so terminals should be kept apart. Trox's unit closes its dampers on power failure for fire protection, frost protection and to avoid draughts.

What does maintenance involve?

One unit per room means one set of filters per room. Annex III requires a unit with a filter to signal when the pressure drop exceeds the maximum allowed, and Blauberg lists filter replacement indicators and alarm alerts. HSE says to maintain mechanical systems in line with manufacturers' instructions, and the DfE says to check that mechanical ventilation is operational and filters are checked, cleaned and replaced.

  1. Log filter changes by room, from the unit's own signal.
  2. Clean or inspect the heat exchanger as the maker specifies.
  3. Test dampers and actuators, which matters for a unit that closes on power loss.
  4. Re-check each CO2 sensor against the maker's calibration advice.
  5. Route alarms to the BMS so a failed unit is found by data, not by complaints.

No official price list for decentralised units turned up in the sources checked, so this page gives none; the drivers are on the ventilation cost guide.

When is a central system the better choice?

The regulation shows one reason. For a balanced unit with a plate or rotary exchanger, the internal fan power limit is 1,100 W per m³/s, less 300 times half the nominal flow in m³/s, for units under 2 m³/s, and 800 at 2 m³/s and above, before the efficiency and filter corrections. That is about 1.09 W per l/s at 83 l/s against 0.80 for a large unit, so a small unit may legally use about a third more fan power per litre of air.

  • Central suits buildings with ceiling voids and risers, high airflows, one plant room and few filters to service.
  • Decentralised suits refurbishments without duct space, phased works, and rooms whose occupancy differs widely. One failure affects one room.
  • Cooling. A single-room unit moving 80 litres per second carries about 0.8 kW at an 8 K difference, so it does not replace cooling; see MVHR with cooling.

You can send an enquiry through the form on this site.

Sources

Common questions

Do decentralised units need ducts?

The regulation's non-ducted unit is not intended to have duct connections, but products vary. Trox's floor-standing unit has outdoor air and exhaust air connections, so short ducts to the façade are part of that design. Check the unit's installation drawing for the openings and duct runs.

Is a small wall unit a residential unit if it is fitted in an office?

The regulation's definition turns on flow, not on the building. A unit whose maximum flow does not exceed 250 m3/h is a residential ventilation unit, and one between 250 and 1,000 m3/h is residential only if the maker declares it exclusively so. Ask which category the maker has declared and what product data goes with it.

Can decentralised MVHR cool a classroom?

Only a little. The Blauberg page lists a bypass for natural cooling, which uses cooler outdoor air rather than adding capacity, and a unit moving 80 litres per second carries about 0.8 kW at an 8 K difference. BB101, as CIBSE Journal summarises it, also wants windows or vents alongside mechanical ventilation in summer.

How often do the filters need changing?

The maker sets the interval. The regulation requires a filter signal or alarm when pressure drop reaches the maximum allowed, so a unit can tell its maintainer when to act. Both HSE and the DfE say to maintain filters in line with the manufacturer and check them.

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