Ventilation is the part of your home you only think about when something is wrong: the unit in the attic is getting noisier, there is condensation on the windows, or the bathroom stays damp for a long time after a shower. Yet ventilation largely determines how healthy the air in your home is. This article explains which ventilation systems there are, when a ventilation unit or wtw unit (warmteterugwinning, heat recovery) is due for replacement, what maintenance is needed and how often, what the rules say and what a good installer or heating engineer does when replacing or balancing the system.
Why ventilation matters, and even more after insulating
In an occupied home, moisture and pollutants enter the air all day long: from breathing, cooking, showering, drying laundry, cleaning products and new furniture. Milieu Centraal (the Dutch environmental information centre) says ventilation is necessary for a healthy and comfortable home free of damp and mould, and points out that there is often more particulate matter indoors than outdoors. The advice is to ventilate continuously: day and night, all year round, including in winter. Milieu Centraal calls opening a window wide for a while after cooking or showering "airing"; that is a supplement to ventilation, not a replacement for it.
In older homes, part of the ventilation happened unnoticed through gaps, seams and single glazing. If you insulate, seal draughts or have new glazing fitted, you make the house more airtight, and that unplanned ventilation disappears. Milieu Centraal is clear about this: if you insulate, it is even more important to have a good ventilation system. The moisture you produce then has to go outside via the ventilation; otherwise it condenses on the coldest spots that remain, such as external corners and thermal bridges at window frames. Read more about this in Having cavity wall insulation installed and Replacing glazing: HR++ or triple glazing. If you already have problems with condensation or mould, Recognising damp and mould in the home helps you establish the cause first.
The best approach is therefore to include insulation and ventilation in a single plan. If you do that, you improve your energy label without picking up a damp problem as well, as explained in Improving your energy label: which measures really count?
Which ventilation system do you have?
Installers and manufacturers often refer to ventilation systems with the letters A to D. Three systems are mainly found in homes; system B, with mechanical supply and natural extraction, is hardly ever used in homes.
System A: natural ventilation
Fresh air comes in through vents, top-hung windows and gaps, and stale air leaves naturally through ducts and openings, without a fan. According to Milieu Centraal, houses built before 1975 have natural ventilation. That is often not enough, certainly after insulating. Milieu Centraal recommends having trickle vents fitted at the same time when replacing windows or frames, and mentions self-regulating or wind-pressure-controlled vents: they close a little in strong wind and open further when there is little wind, which limits draughts and heat loss. A window frame specialist or glazier can fit such vents in the frame or above the glass.
System C: mechanical extraction, natural supply
With mechanical ventilation, a central ventilation unit, usually in the attic, extracts stale air through vents in the kitchen, bathroom and toilet. Fresh air comes in through vents or top-hung windows in the living room and bedrooms. According to Milieu Centraal, about four in ten houses in the Netherlands have such a system. It only works well if the supply is open: keep the vents open and make sure there is a gap under the internal doors or a grille in the door, so that air can flow through the house. Milieu Centraal mentions at least one and a half centimetres of space under the door.
System D: balanced ventilation with heat recovery
With balanced ventilation, a unit supplies as much fresh air as it extracts stale air, both through ducts and vents. Most systems have a heat recovery unit (wtw): the heat from the outgoing air warms the incoming air, so that you ventilate with little heat loss. Supply vents are in the living room and bedrooms, extract vents in the kitchen, bathroom and toilet. According to Milieu Centraal, central balanced ventilation is mainly used in new-build homes. If your home has balanced ventilation, trickle vents in the windows are not needed and can even reduce efficiency; make sure this is taken into account when you replace windows or frames.
According to Milieu Centraal, since 2016 balanced ventilation units have had to have a bypass: when the outside air is cooler than the inside air, the fresh air goes around the heat exchanger, so that in summer you can cool the house with outside air at night.
Decentralised heat recovery: per room
In an existing home without ducts, a central system D is often difficult to install. An alternative is decentralised balanced ventilation: one unit per room, usually in the external wall, which supplies air, extracts air and recovers heat on its own. Milieu Centraal calls this suitable for existing homes. For a house with natural ventilation, Milieu Centraal mentions a demand-controlled unit with heat recovery in the external wall of the living room as a good improvement.
Demand-controlled and CO2-controlled
Both mechanical extraction and balanced ventilation can be demand-controlled: sensors measure the CO2 level and sometimes the humidity, and the system ventilates harder or more gently based on that. If no one is home, the unit automatically runs more economically. That saves energy and you do not have to change the settings yourself. As a guideline, Milieu Centraal says a CO2 level below 750 ppm is good and above 1200 ppm unhealthy, and that a humidity between 40 and 60 per cent is good.
When is a ventilation unit due for replacement?
There is no fixed replacement age, and as long as the unit works properly, it does not need to go. Signs that replacing it is sensible:
- An old AC motor. Look at the unit's rating plate. If it says AC, wisselstroom (alternating current) or 230 V, it has an AC motor. According to Milieu Centraal, ventilation units from before 2010 usually still run on alternating current. A unit with a DC motor (direct current, also called an EC motor) is more economical and quieter. Controls via an app or remote control usually go with a DC unit.
- Noise. A unit that hums, rattles or whistles, or noise that reaches the bedroom through the ducts. Sometimes cleaning, rebalancing or a silencer in the duct helps; with a worn motor or bearing, replacement is often the better choice.
- Too little extraction. The bathroom stays steamed up for a long time or the kitchen smells of food for hours, while vents and ducts are clean.
- A heat recovery unit without a bypass. Older balanced ventilation units sometimes have no bypass. Retrofitting one is possible but, according to Milieu Centraal, often expensive; with an old unit, replacement may make more sense.
- Plans for a heat pump. Milieu Centraal also mentions replacing the unit if you are considering an exhaust air heat pump, which uses heat from the extracted air for heating or shower water. You can read more about heat pumps in Having a heat pump installed: hybrid or all-electric.
Since 1 January 2016, ventilation units have had an energy label, which also shows the noise level. According to Milieu Centraal, units for mechanical extraction usually have label B or C; the most efficient label is A+. Milieu Centraal gives 45 dB as the maximum noise level, and 40 dB for units that came onto the market in 2018 or later; under the European ecodesign rules, those limits apply to units without ducts, such as decentralised units in the façade. For other units, compare the noise level on the label.
Maintenance: what you do yourself and what the technician does
Dirty filters, vents and ducts make the fan work harder, cause more noise and reduce the air exchange. Milieu Centraal gives the following guidelines per system.
Mechanical extraction (system C)
- Grilles and vents: clean thoroughly at least once a year. You can remove vents and clean them with soapy water. Then put them back in exactly the same place and in the same setting; mark them beforehand, because a vent that has been turned upsets the distribution of the air.
- Unit and motor: have them serviced by a technician about every two years.
- Whole system: have it checked about every four years and rebalanced if necessary.
- Extract ducts: have them cleaned at least once every eight years.
Balanced ventilation with heat recovery (system D)
- Filters: replace with clean filters twice a year and vacuum them at least once in between. If you live on a busy road, more often. Newer controls show a light when the filters are due.
- Vents: have them cleaned by a technician about every two years. If you do it yourself, clean them one at a time and mark the vent and its holder.
- Unit and bypass: have them serviced about every two years; have the whole system checked about every four years and balanced if necessary.
- Ducts: have them cleaned by a specialist company about once every eight years.
With balanced ventilation, Milieu Centraal points out that the installer must rebalance the system after every service. If you opt for a maintenance contract, Milieu Centraal recommends a company with the Ventilatiekeur quality mark.
Use: never switch it off
According to Milieu Centraal, you should never switch off mechanical ventilation, except for maintenance or in the event of a disaster nearby when the government advises closing windows and doors. In the event of smoke nuisance from a fire, the fire brigade also advises closing windows and doors and switching off the ventilation; switch it back on once the advice has been withdrawn. With three settings, you use setting 1 when you are out, setting 2 when you are at home, including at night, and setting 3 when cooking and showering. Put the ventilation on the highest setting when there are many people in the house or when you dry laundry indoors.
What the installer does when replacing or installing a new system
Swapping a unit seems simple, but a properly working system takes more than a new appliance on the same ducts. Make sure the installer:
- determines the capacity. How much air needs to be extracted and supplied per room? That depends on the rooms, their floor area and what is in them, such as a cooker. A unit that is too small runs at full power all the time and is noisy; one that is too large is unnecessary.
- balances each room. After installation, they adjust the vents so that every room gets the right amount of air, and measures it to check. Ask for the measured values per vent; they are useful later during maintenance.
- pays attention to noise. A unit mounted free of vibration, silencers in the ducts and a unit that does not have to run on the highest setting to ventilate enough prevent you from switching the ventilation off because it is a nuisance. Milieu Centraal also mentions mounting the unit with sound-damping material, or a closable cupboard around it.
- connects the condensate drain on a heat recovery unit. Condensation forms in the heat exchanger; it is usually connected to the sewer via a trap, and in an unheated attic that drain should be frost-free. Follow the manufacturer's installation instructions for this.
- checks the supply. With system C: are there enough vents in the living spaces, do they work and are they accessible? Are there gaps under the doors? With system D: are there still vents in the windows that would be better closed?
- positions the controls and sensors properly, and explains to you how the settings, the filter indicator and, if present, the bypass work.
If a new connection to a circuit in the meter cupboard is needed, or a socket has to be added in the attic, an electrician can take care of that; many ventilation installers do this themselves. Who to call for which part of the installation is explained in Plumber, installer or heating engineer: who do you call for what.
What do the rules say?
The Besluit bouwwerken leefomgeving (Bbl, the Dutch Building Decree) sets minimum requirements for ventilation. For existing homes, these include that every habitable room must have a ventilation provision with a minimum capacity per square metre of floor area, and that a toilet and a bathroom have their own extraction that goes directly outside (Articles 3.67 and 3.71 Bbl). For a toilet that is at least 7 dm³/s and for a bathroom 14 dm³/s; a habitable room with a cooker must be able to extract at least 21 dm³/s directly outside. In addition, a habitable room must have a purge ventilation provision, such as a window that can be opened, so that it can be aired quickly.
With a renovation, the rechtens verkregen niveau (legally acquired level) generally applies: the quality after the renovation must not be worse than before, with the requirements for existing buildings as the lower limit and the new-build requirements as the upper limit. For installations, the Informatiepunt Leefomgeving (IPLO, the government's information point on environmental and planning law) makes a distinction: when building installations such as mechanical ventilation are completely renewed or installed entirely from scratch, the new-build requirements apply (Article 5.4, paragraph 3 Bbl); with partial renewal, the legally acquired level applies. IPLO mentions mechanical ventilation as an example, but does not say whether replacing only the unit counts as partial renewal. If you are installing a completely new system, explicitly ask the installer to design it according to the new-build requirements. If you are unsure which level applies to your situation, the municipality is the point of contact.
If you live in a flat with a shared extract duct or a collective ventilation system, consult the vereniging van eigenaren (VvE, owners' association) in advance: the unit in your own attic or storage room must suit the building's duct.
Subsidy: only together with insulation
Since 1 January 2026, energy-efficient ventilation has been covered by the Investeringssubsidie duurzame energie en energiebesparing (ISDE, the Dutch sustainable energy and energy saving investment subsidy) for homeowners, but only if you combine the ventilation with one or more insulation measures. According to RVO (the Netherlands Enterprise Agency), three types qualify: a central mechanical extract fan with at least two CO2 sensors, central balanced ventilation with heat recovery and decentralised balanced ventilation with heat recovery, each with requirements for capacity and, with heat recovery, for efficiency. Humidity sensors, motion sensors or a multi-position switch do not count as CO2 control.
Further conditions according to RVO: the ventilation was installed on or after 1 January 2026, by a (building) installation company and not by you; the unit is new; the home was built before 1 January 2019, or the omgevingsvergunning (environmental permit) was applied for before 1 July 2018; and you apply for the subsidy after installation, within 24 months of the installation of the ventilation and of the insulation measure. If the unit is not on the reporting code list, you must include the technical documentation with your application and you run the risk of rejection. Check in advance whether the unit is on RVO's meldcodelijst ventilatie-units (reporting code list for ventilation units). Combining insulation with ventilation alone does not increase the subsidy amount for the insulation. The conditions change regularly; read the current rules on rvo.nl before you sign a quote.
What belongs in the quote?
- make, type and capacity of the unit, the energy label and whether the motor is DC;
- with heat recovery: the efficiency, the bypass and the reporting code if you want to apply for ISDE;
- whether the unit is demand-controlled, with how many sensors and which ones;
- what is being done to ducts, vents, silencers, trickle vents and the condensate drain;
- balancing per room and an overview of the measured values;
- disposal of the old unit, the guarantee and the maintenance arrangements, with the interval for filters and inspection.
In summary: ventilate continuously, and certainly after insulating or fitting new glazing. Clean vents and grilles yourself, replace heat recovery filters on time and have the unit, ducts and balancing checked periodically. Replacing an old AC unit with an economical DC unit is often a simple improvement. On VerbouwPro you post your job free of charge and heating engineers and installers from your region respond.