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DucoBox Silent and Focus: Demand-Controlled MEV Explained

Most homes are ventilated on a timer or a light switch. The fan runs whether the room is occupied or not, at a rate chosen on a drawing before anyone moved in. Demand-controlled mechanical extract ventilation does the opposite: it measures what is actually happening in each room and moves only the air that situation needs. DUCO builds two units for this job. The DucoBox Silent is the central unit for houses; the DucoBox Focus is the compact version for flats and retrofits where plant space is tight. Both work the same way, and both are only as good as the valves and ducting attached to them. What mechanical extract ventilation actually does An MEV system puts one fan unit in the loft, cupboard or utility space and runs ducts from it to the wet rooms: kitchen, bathrooms, WCs, utility. The fan pulls stale, humid air out of those rooms continuously. Fresh air enters through trickle vents or window grilles in the habitable rooms and crosses the dwelling on its way to the extract points. It is a single-fan system, so there is no heat recovery - that is what MVHR is for. What MEV gives you instead is simplicity: one unit, one duct run per terminal, far less plant, and a much lower install cost. In the UK this is a System 3 solution under Part F of the Building Regulations. Silent or Focus: which unit fits The DucoBox Silent is the full-size central unit. It takes multiple extract branches, carries the acoustic casing that gives it its name, and suits houses where the duct network reaches several wet rooms across two or three floors. The DucoBox Focus is a smaller casing designed for apartments, maisonettes and retrofit jobs where the only available space is a cupboard ceiling or a bulkhead. Fewer branches, smaller footprint, same control logic. The deciding factors are usually the number of wet rooms, the total duct resistance, and whether the void you have will physically take the unit with the service clearance it needs. Demand control is the whole point Left on its own, a central fan is a blunt instrument. DUCO's control valves turn it into a responsive system. Each valve sits at the extract terminal in the room it serves and modulates the opening based on what it senses: Humidity valves for bathrooms and utility rooms, which open as relative humidity rises after a shower or a wash cycle and close again once the moisture has cleared. CO2 valves for rooms where occupancy is the driver, opening as carbon dioxide builds and settling back when the room empties. Manual or timed valves where a simple boost is all that is wanted - a WC, for example. The unit holds a low continuous background rate and only ramps where a valve has asked for more air. Nothing is extracted from an empty bathroom at three in the morning, and nothing is under-extracted when four people are in the kitchen. Noise: where the decibels come from MEV gets a bad name from badly installed systems, not from the technology. Three things create the noise people complain about: Fan speed. A fan running flat out because the ducting is too restrictive will always be audible. A DC fan held at a low background speed will not. Air velocity at the terminal. Undersized ducting forces air through faster, and the hiss happens at the grille rather than at the unit. Structure-borne transmission. A unit bolted hard to a joist above a bedroom ceiling transmits through the fabric regardless of how quiet the casing is. Anti-vibration mounting, generous duct sizing and a silencer on the branches serving bedrooms deal with all three. Ducting decides whether it works The unit is specified from a duty point - a flow rate at a pressure. If the installed ducting has more resistance than the design assumed, the fan has to work harder to hit the same flow, which costs noise and electricity. Semi-rigid radial ducting such as DucoFlex keeps resistance predictable because each terminal gets its own uninterrupted run back to a plenum, with no in-line joints to leak or collapse. Flexible duct pulled around tight bends does the opposite. If you are going to spend money anywhere in an MEV system, spend it on the ducting. Commissioning and compliance A System 3 installation has to be commissioned and the measured flow rates recorded. That means setting the background rate, checking each terminal against the design figure with a calibrated instrument, and adjusting valves until the dwelling as a whole meets the minimum whole-house rate and the individual wet-room rates. Keep the commissioning sheet. It is the document building control asks for, and it is also the baseline you compare against if the system is ever questioned later. Running cost and maintenance Because the fan spends most of its life at a low background speed, a demand-controlled MEV unit uses a fraction of the energy of a system that runs at full rate on a timer. The saving is in the hours, not the peak. Maintenance is modest: clean or replace the unit filter on schedule, wipe the extract grilles, and keep the condensate route clear if the unit is positioned where condensation can form. Sensor valves need no routine calibration, but they do need to be left unobstructed - a shelf or a towel rail in front of a valve defeats the sensing. Specifying a DucoBox system A workable specification needs four things: the number and type of wet rooms, the duct route and the resistance it implies, the valve type for each terminal, and the space available for the unit itself. Get those right and the rest follows. PureClima supplies the full DUCO range - DucoBox Silent and Focus units, CO2 and humidity control valves, DucoFlex radial ducting, plenums, silencers and terminals. If you have a floor plan and a wet-room schedule, send it to sales@pureclima.co.uk and we will put a component list against it.