Can it be solved later? Installation of ventilation systems in existing family houses and apartments

In the corner of the bedroom, along the outer wall, a gray stripe appeared a few weeks ago. At first, you thought it was dust. The next morning, water droplets were running down the window frame. The older child is tired in the mornings, as if they hadn't slept at all. In the apartment, next to a closed window, there is something in the morning that is hard to put into words: it feels as if many people have already used up the air before you.

This is the starting point for most retrofitting ventilation investments. Not a new house, not a major renovation, but an existing home where something has changed. The most common change: the replacement of windows. Ten to fifteen years ago, air leaked out of every opening so much that the building „breathed” on its own. Modern triple-glazed windows were specifically designed to prevent this from happening. The result is good energetically, but the air starts to stagnate.

This article is about what can realistically be achieved in an already standing family house or apartment. Not new construction, not a complete renovation. The question is specific: how can a modern ventilation system be integrated into a building where you do not want to do any demolition. There are three main directions, each suitable for different situations, and the seemingly correct choice is often not the same as what is suggested by Google's first result.

When the existing house no longer „breathes”

The first sign is rarely CO2. Rather, it is humidity. In winter, condensation appears on the inner side of the cold wall, which stabilizes over time in the form of mold. The bathroom mirror does not dry even after showering the previous day. In the kitchen, the glasses fog up while cooking. In the morning, water droplets gather on the bedroom window. A household of four releases about 2-4 kilograms of water vapor into the air overnight: sleeping, houseplants, drying clothes, sometimes showering and cooking also play a role. If this moisture does not find a way out, it returns to the walls.

The second sign is poor sleep. In a closed 15 m² bedroom, the CO2 level can exceed 1500 ppm in an hour with two sleeping people, and by morning it can even rise to 2500 ppm. Headaches, lethargy, concentration problems, frequent morning fatigue: all of these are related to poor indoor air quality. The WHO recommends a level below 1000 ppm for comfortable sleep. A cheap CO2 meter can be obtained today for 10-15 thousand forints, and it usually becomes clear right away whether the air is indeed the problem or something else.

The third sign is odor. When you come home after work, you feel something in the first moments that the person living in the apartment no longer does. This phenomenon is concentrated pollution, typically volatile organic compounds (VOCs): from furniture, cleaning agents, printers, perfumes, cooking. If this smell is strong enough to linger at the door, the system is not exchanging enough air.

These symptoms are not signs of a bad house. They are symptoms of a well-insulated, window-replaced, valuable home today. The good news: there is a retrofitting solution for each of them, and it does not necessarily require demolition.

Three realistic directions for retrofitting

If the symptoms are present and a person starts to look into it, a single article features three completely different worlds: central system, decentralized machines, exhaust solutions with air inlets. These are not different answers to the same problem, but three different investment levels, suitable for different situations.

Decentralized heat recovery machines are the most common retrofitting choice. A wall breakthrough, a power cable, a few hours of work per room. The resident already receives fresh, preheated air in the bedroom that evening. The system can also be gradually expanded: first the bedroom, a few months later the living room, later the children's room.

The retrofitted central (or mini central) system is considered when there is structural possibility: attic space, walkable level above a flat roof, removable false ceiling, or a larger renovation is planned. In this case, one machine serves multiple rooms, and the pipes can be hidden. The investment is larger, but the end result is of almost new construction quality.

The exhaust system with air inlets is the cheapest option. There is no heat recovery, but the demolition is minimal, and it is much better than nothing. In small apartments, rental properties, or when the goal is just to stop mold, this is also a valid solution.

The choice between the three directions is not a matter of brand or product. The question is: how much of the structure can be demolished, how much money is available, and how comprehensive is the goal.

The decentralized machines: one wall breakthrough and a few hours of work

The decentralized (also known as single-room) heat recovery unit is what most articles, forums, and specialty stores mention as a solution for retrofitting. There is a reason for that. With less demolition, almost nothing can be achieved: a single wall breakthrough is needed, typically with a diameter of 160-220 millimeters, plus a standard electrical supply for the unit (most types operate on 230 V).

Two main constructions are competing in the market today. One is the ceramic heat storage, directional change system. In this, a single fan runs outward for 70 seconds, then reverses and draws inward for 70 seconds. The ceramic insert stores heat during one phase and releases it during the other phase. Its efficiency is around 80 percent, and typically two units need to be installed synchronized (push-pull), so that one blows in while the other sucks out. This means an investment of 150-280 thousand forints per machine on the Hungarian market, with a total cost of about 300-500 thousand forints for two units.

The other is the two-fan, heat exchanger version, such as the Zehnder ComfoAir 70 or the ComfoSpot 50 family. A single unit contains two fans, two air ducts, and a heat exchanger. Fresh air comes through one channel, while the used air goes out through the other. The efficiency is 85-90 percent, and a single device can provide continuous air exchange. The entry price is higher (350-600 thousand forints per unit), but the noise level is lower, and no two wall breakthroughs are needed.

The wall passage element is telescopic in most manufacturers' products: it can be adjusted for wall thicknesses between 300 and 500 millimeters. This covers the wall thickness of Hungarian family houses and most panel buildings. Installation typically takes half a day if the location has already been measured and drilling is permitted (for apartment buildings, permission from the homeowners' representative is always required for facade breakthroughs).

It is important, which many forget: the decentralized unit into the living space It needs to be installed, not in wet rooms. That is, it goes into the bedroom, living room, and children's room. The ventilation of the kitchen and bathroom is solved with separate extraction, because there, the local placement of the machine is not a comfort issue, but an efficiency one. In a well-designed apartment, two or three decentralized machines and a smaller bathroom humidity-regulating extractor together provide a sensible ventilation logic.

When a mini central system also fits

The logic is the same as with new construction: one machine, star-point ducting, valves in the rooms. The difference is the routing. In a new house, the duct goes into the ceiling or the false ceiling, the architect leaves space for it in advance. Afterwards, this space is not automatically available; it needs to be created. There are three typical solutions. In a family house, the attic space where the ducts run under the insulation. In an apartment or a house with a roof space, a new, partial false ceiling at a depth of 15-20 centimeters. The third option is the floor structure if the layer structure needs to be replaced anyway (for example, due to new underfloor heating).

The machine can also be smaller than in new construction. There are several compact mini central heat recovery families available on the market today (typically in the 150-300 m³/h air delivery range) that can provide complete ventilation for an 80-120 m² apartment. The efficiency is the same as that of the larger siblings (with an enthalpy heat exchanger, even above 90 percent), and the noise level is barely audible in a well-designed system. The real difference is the size: these machines can be installed in a false ceiling, built-in wardrobe, or on the back wall of a closet, without needing a separate mechanical room.

The price is inherently higher. The retrofitting of a central system for an average family house or a more spacious apartment of 100-130 m² comes out between 1.2 and 2 million forints, including the machine, ducts, installation, and balancing. This is also a significant intervention: planning is required (by a mechanical engineer or a good HVAC contractor), and the work typically takes one to two weeks.

When is this route worth it instead of decentralized? If the entire apartment or house system is needed at once (not just a few rooms), if aesthetics matter (the ventilation valve is less conspicuous than the wall machine), and if the value of the house justifies it. In the case of an expensive, permanently used family home, retrofitting a central system is typically a better investment than five or six decentralized units side by side.

The cheapest compromise: extraction with partial ventilation

There is a third way that manufacturers and specialized companies do not necessarily advertise first because the revenue is lower on it. But it works, and it is precisely for those situations where the previous two solutions are too expensive or too invasive.

This is the logic. In the wet rooms (kitchen, bathroom, toilet), we install a humidity-dependent or timer-controlled exhaust fan. This can be individual, with one smaller unit per room, or a common smaller central exhaust that connects to three rooms. In the living rooms (living room, bedroom, children's room), no unit is needed, but a humidity-regulating slot ventilator is placed in the existing window frame or a separate wall passage. The latter is a hygrosensitive (humidity-sensing) air inlet that automatically opens when the humidity level rises indoors and closes when not needed.

The air flow is natural in this case. A small but constant amount of fresh air continuously enters the living rooms through the slot ventilator. This air passes under the doors into the wet rooms, from where the exhaust fan expels it. The result: a basic level of continuous ventilation that maintains humidity and odor and keeps the CO2 level within an acceptable range.

What is the limitation? There is no heat recovery. In winter, every cubic meter of fresh air must be heated with its own heating energy. In a panel apartment or smaller family house, this can mean an extra heating bill of 30-50 thousand forints annually compared to the previous closed state. In return, the investment can be realized for 250-500 thousand forints, and the demolition is practically zero.

Who is it for? For a rented renovated apartment. For a weekend house. For a smaller panel where the total investment would be disproportionate. For a home where the residents are leaving for a few years and do not want to invest a larger amount. And precisely for those who only want to address the humidity and mold problem at first, not the complete comfort revolution.

Pitfalls that can only be avoided by experience

Every retrofitting has its own little traps. Most are not dramatic, but each one disrupts the system if ignored.

The first is an open combustion appliance. If there is a traditional gas boiler (with an open combustion chamber), gas water heater, or regularly used fireplace in the apartment, then it is not balanced. ventilation system should never be installed. These devices use indoor air for combustion, and an exhaust system creates low pressure, which can cause the backflow of combustion products. This is not a comfort issue, but a life-threatening situation. There is no such problem with a closed combustion chamber combi boiler or district heating, but with a traditional fireplace, consultation with a building engineer is necessary in every case.

The second is the location of the external intake and exhaust. If these are placed too close to each other beyond the facade, the system will draw in its own expelled air. The rule of thumb: at least 1.5 meters distance between the two openings, and preferably at different heights. With decentralized machines, this is almost never a problem, because the two airways separate inside a single wall penetration. However, with mini central and exhaust systems, prior planning is required.

The third is undersizing. A single small decentralized unit is insufficient for a 60 m² apartment. The basis of the design is the actual demand (room size, number of occupants, function), not the price of a cheaper unit. With poor sizing, the system works on paper, but in practice, the symptoms (humidity, odor, fatigue) do not disappear. In such cases, the owner believes that the technology is not working, when in fact the sizing was incorrect.

The fourth is aesthetics, especially for apartment buildings. A small grille, with a diameter of 180-220 millimeters, will be visible on the facade. This is now discreet, paintable, and of high quality, but a permit from the homeowners' representative is required for its installation. Never forget to arrange this in advance, because afterwards, upon a neighbor's complaint, the housing cooperative can have it dismantled.

The fifth is maintenance. Every ventilation system has a filter, and the filter must be cleaned or replaced regularly. For decentralized units, this is a few minutes operation every 3-6 months, which can be done at home with a vacuum cleaner. For central systems, a pair of filters must be replaced annually, which costs between 15,000 and 25,000 forints in material costs. If this is neglected, the system's efficiency deteriorates month by month, and air quality also declines.

In summary

The design of retrofitted ventilation is a compromise. It does not provide what a newly built house designed on a drawing board would offer: the planned route is not available, the ideal mechanical room is missing, and the freedom of execution is also smaller. But the starting point is not the same either. Here, perfection is not the goal, but rather the elimination of symptoms and a well-livable air quality.

In most domestic homes, a well-sized decentralized system already provides 80-90 percent of the comfort that a newly built house would receive from a complete central heat recovery system. There is no condensation on the window in the mornings. The air in the bedroom is continuously refreshed at night. The gray stripe no longer appears in the corner of the wall. The heating bill shows a slight increase, but much smaller than what continuous window opening would cause.

What is worth deciding at this point: is the goal a whole apartment-level solution, or just the most problematic room for now? Both are valid paths. A decentralized unit in the bedroom and a smarter bathroom extractor can visibly improve the situation within six months. Other expansions can wait until later. Conscious renovation does not always happen all at once; sometimes it builds up in steps.

If there are remaining questions, and you are not sure which direction is right for the specific apartment: it is worth sending three things to a specialized company. A floor plan (even hand-drawn), photos of the external facade around the planned breakthrough, and a brief description of when and where the symptoms appear. Serious offers can already be prepared from these, with prices and options. Healthy indoor air is not a luxury. In a modern home, this is the minimum.