Decentralized Ventilation Unit
A small through-wall ventilation device for one room, often installed in pairs with a ceramic heat store. A retrofit option where central ducts do not fit.
What is a decentralized ventilation unit and how does it work?
A decentralized ventilation unit, called decentrálna vetracia jednotka in Slovak, is a small ventilation device that sits in one wall opening and serves a single room or a small zone. Instead of a duct network, it uses a through-wall opening and an internal core that stores heat. Units are normally installed in pairs. One unit brings fresh air into the room while its partner extracts stale air, and the two swap roles on a short cycle, so the same ceramic core is first charged by the outgoing air and then gives its heat back to the incoming air.
The core is usually ceramic. Because it stores heat rather than passing two air streams past each other continuously, it is called a regenerator. This is the main structural difference from a central heat recovery ventilation unit. Each decentralized unit carries its own fan, filter and controls, so every room served is a self-contained ventilation point.
When does a decentralized unit make sense in an existing house?
A decentralized unit makes sense when ducts for a central system cannot be routed without damaging finishes the owner wants to keep. The typical case is a renovation where walls and ceilings are already finished, and running ducts through them would mean major disruption, lost floor space or a layout the building cannot accommodate.
Because each unit needs only a wall opening and a power connection, the work can be staged room by room. The owner can start with bedrooms and living areas and add units elsewhere later. Before committing, a designer should check how airtight the existing envelope is, because a leaky house makes any ventilation strategy less predictable.
How does a decentralized unit differ from a central MVHR unit?
The main differences are where the heat exchange happens, how much air each device handles, and how the maintenance is spread across the building. A central unit serves the whole house through ducts. A decentralized unit serves the room it sits in, from its own wall.
| Feature | Decentralized unit (through-wall, paired) | Central MVHR (ducted) |
|---|---|---|
| Fit into an existing house | One wall opening per unit, little interior work | Ducts through walls and ceilings, often major disruption |
| Heat recovery method | Ceramic core that stores and releases heat in cycles | Continuous exchanger in one central unit |
| Recovery performance | Generally lower than a good central unit | Generally higher when well designed and installed |
| Facade penetrations | One per unit, so one per room served | A small number of intake and exhaust points |
| Filters | Every unit has its own filter to service | Filters in one place |
| Noise | Fans sit in the room being ventilated | Plant usually sits away from living space |
| Interior bathrooms | Cannot serve a bathroom with no outside wall | Can be served by ducted extract |
Central units usually rely on a counter-flow exchanger, where the two air streams run in opposite directions through a permanent core, which keeps recovery more continuous and more even across the house. Decentralized units trade some of that performance for ease of installation. The trade is worth making only when ducts genuinely cannot be installed. If ducts are possible, a central system is usually the better long-term choice.
What are the honest limits of decentralized units?
Decentralized units solve one problem well, but they have limits that owners should understand before buying.
- Noise. The fans and the switching cycle sit in the room you live in. Noise is the most consistent complaint, and it matters most in bedrooms. Ask for measured sound levels at the airflow you will actually run, not at the quietest setting.
- Lower recovery. The ceramic cycle and the room-by-room layout generally recover less heat than a well-designed central unit. Ask for independent test results, not only the manufacturer's figures.
- A facade penetration in every room. Each unit needs its own hole through the external wall. Each opening must be sealed for airtightness and checked for thermal bridging, and the wall must be checked for thickness and structure before drilling.
- Filters in every unit. Each unit has its own filter that must be cleaned or replaced on schedule. Several units mean several filter jobs, and that is easy to neglect.
- No service for interior bathrooms. A bathroom without an outside wall cannot take a through-wall unit. It needs a separate extract solution, usually planned with the designer.
In cold weather, a poorly maintained core can ice up or hold condensate, which lowers its output and can leave damp conditions near the wall opening.
| Question for the supplier | Why it matters |
|---|---|
| Measured sound level at the airflow you will run | Noise is the most common complaint, and figures at the lowest speed flatter the unit |
| Independent test of heat recovery | Manufacturer figures are not always measured the same way |
| Filter type and replacement interval | Sets the upkeep you will carry for every unit in the house |
| Frost protection in cold weather | Icing reduces output and can damage the core |
| Wall thickness and structure at the opening | Decides whether a through-wall unit can be fitted safely |
Where should decentralized units go and how many do you need?
Units belong on an external wall of each room they serve, positioned so fresh air reaches the living zone without short-circuiting straight back to the exhaust. Airflow must match the room, so the unit's rated capacity should be checked against the room's size and use, not chosen by price alone.
Pairs are the norm, and a whole house often needs several pairs. Kitchens and bathrooms usually need their own extract arrangement, planned separately. Supply and extract capacity should be balanced across the house, because unmatched airflows push air through gaps in unplanned places. For that reason, the design should be read alongside the principles of a balanced ventilation system and checked by a ventilation designer, known in Slovak practice as a projektant TZB.
Does a decentralized unit fit a passive house or an airtight building?
A decentralized unit can be part of an airtight renovation, but it does not fit a passive-house concept as well as a well-designed central unit does. A passive-house designer should be consulted before relying on it.
The reason is that a passive house depends on a tight airtight envelope and high recovery, and every facade penetration is a potential weak point. Several small units mean more points to seal, more fans to keep quiet and more places where recovery can drop. For a demanding comfort and heating target, central ventilation usually gives better results, provided ducts can be routed. For an ordinary renovation, a decentralized solution is a reasonable step up from opening windows, not a substitute for good design.
What misconceptions should owners avoid?
- One unit ventilates the whole house. Each unit serves its own room or a small zone only.
- It replaces the bathroom extract. Interior bathrooms are not served by a through-wall unit.
- Once installed, it needs no attention. Filters and cores need regular service in every unit.
Frequently asked questions
- Can decentralized units replace a central MVHR system in a house?
- Sometimes, but only where ducts genuinely cannot be installed. A well-designed central unit usually recovers heat more evenly across the house, so it is the better choice for new builds or deep renovations where ducts are practical.
- Do decentralized ventilation units ventilate bathrooms?
- Not interior bathrooms. A bathroom with no outside wall cannot take a through-wall unit, so it needs its own extract route. A bathroom with an outside wall may be served, but the arrangement should still be checked by a designer.
- How often do the filters need changing?
- Each unit has its own filter, so the job is repeated in every unit. Follow the manufacturer's schedule and check the filters regularly, because a clogged filter reduces airflow and can make the fan louder.
- Are decentralized units quiet enough for bedrooms?
- Often not without care, because the fans sit in the room being ventilated. Ask for measured sound levels at the airflow you plan to use, and compare the result against what you would accept at night.
- Do I need a wall opening for every room I want ventilated?
- Yes, each decentralized unit needs its own through-wall opening, so every room served adds a penetration. Each one must be sealed properly to keep the building airtight and to avoid cold spots at the wall.