Mechanical Ventilation with Heat Recovery in Existing House

 Heat recovery unit in older house

The need to provide a working ventilation system in an existing house strikes its inhabitants usually along with imminent consequences of its insufficient operation or its lack. Thus, if shortly after renovation of the house (e.g. thermal performance improvement including replacing windows and building insulation) moisture/mold on the walls is observed and problems with well-being, tiredness, and breathing start to appear (and windows cannot stay open for a long time because it is winter time), it is high time to think about controlled ventilation.

Inadequate Ventilation and High CO2 Levels

During an epidemic, we spend a lot of time at home trying to efficiently learn and work from home; let us then devote some attention to carbon dioxide (CO2). We produce it ourselves while breathing as well as while consuming oxygen. If we do not supply fresh air to the room, the amount of oxygen becomes insufficient, while with carbon the situation is completely opposite. Such a situation is easy to distinguish from normality, for example, when entering a previously closed, unventilated child’s room after homeschooling...

There is also a so-called Pettenkofer number (from the name of the researcher who recognized the CO2 content as an indicator of indoor air quality already 100 years ago), amounting to 1000 ppm CO2 – it is the limit value above which our well-being, concentration, perception and productivity drop significantly, being slowly replaced with a feeling of breathlessness and fatigue. Is such a situation easy to achieve? For example, after 6-8 hours of sleep (i.e. with rather low respiratory activity) in an unventilated bedroom, the CO2 concentration in the air exceeds 2500–3500 ppm... unless good ventilation is provided.

PN-EN 16798-1: 2019 standard states that the concentration of carbon dioxide in a bedroom in a residential building for the highest category should circulate 380 ppm above outdoor levels. In 2019, scientists measured CO2 level, its average concentration is now 415 ppm. Thus, the CO2 concentration in bedrooms in full comfort conditions should not exceed 800 ppm.

The excess of carbon dioxide appears in rooms with no access to fresh air, or more precisely – in rooms with no ventilation. Ventilation is simply an exchange of air in a room. In perfect condition, exhaust air (contaminated during the use of the house) is removed and replaced with a sufficient amount of fresh air (from outside).

Consequences of poor ventilation

If the ventilation is insufficient, the inhabitants of the house live in an environment polluted by excess moisture, CO2, fungal/mold spores as well as other biological substances (e.g. peeled epidermis, animal hair, mite feces, and bacteria), particulate matter of domestic origin (e.g. particularly hazardous and invisible to the naked eye products formed during frying), dust or cigarette smoke. Such a composition of air causes the following problems:

  • “unpleasant atmosphere”, shortness of breath;
  • malaise, most often manifested by fatigue, shortness of breath, headaches, etc.
  • fungi development (e.g. sprinklers in the bathroom or mold in bedrooms or living rooms), for which good conditions are mainly ensured by high humidity;
  • intensification (and sometimes even development!) of allergic reactions to mold, as well as asthma and other chronic respiratory diseases;
  • greater susceptibility to infections and respiratory diseases;
  • reverse draft, i.e. cold air supply through exhaust grilles (kitchen and bathroom), and in multi-family buildings – exhaust air from neighbors’ home as well...
  • troublesome utility problems – for example, towels/laundry long drying time, unpleasant smell of freshly washed clothes;
  • condensation of water vapor (most often on windows, because it occurs on the coldest surface in the room);
  • swelling of materials made of wood, paper or cardboard (note, the problem may also apply to plasterboard walls!);
  • destruction of the building structure caused by the ingress of moisture into the walls, which may results not only in direct degradation of building materials, but also in the internal development of fungi;
  • serious threat to health and life – especially in rooms with gas heating devices, where a possible carbon monoxide poisoning may occur!

If the occupants of your home are experiencing any of these problems, it is a sign of insufficient ventilation. Described problems appear frequently after renovation or even thermal performance improvement of the existing house – replacement of windows with tight and better insulation of the building...

Disadvantages of Natural Ventilation Systems

Natural house ventilation method, namely taking advantage of the temperature differences between the inside of the rooms and the outside, usually works (or rather usually does not work). In practice, it is constructed in such a way that it allows fresh outside air to flow through the openings (windows, air supply diffusers, air vents) in the walls (this process is called fresh air supply), while the exhaust air is removed by exhaust grilles in kitchens and bathrooms (this process is called air exhaust).

In old houses with leaky windows, gaps in the door or imperfections in the walls, a certain part of the air supply was also provided by these leaks (although at the cost of cooling down the inside temperature and occasional drafts). However, replacing windows with modern, airtight ones as well as insulating the building, although it improves thermal comfort and helps to reduce heating bills, it eliminates ways of fresh air supply process as well.

As it can be read in the Ordinance of the Minister of Infrastructure on technical conditions to be met by buildings and their location (§ 149.1.): the stream of external air supplied to the rooms [...] in the apartments [...] should not be less than 20 m3/h per person envisaged for permanent residence in the building permit design. Unfortunately, it is not possible to ensure such air exchange continuously in a natural way. Natural ventilation may encounter a number of difficulties on its way:

  • no open windows – cooling rooms in winter discourages many people from opening windows, not only for fear of the effects of cold air on health, but also for the wish to save on heating. Indeed, heat losses related to natural ventilation in winter account for as much as 30–40% of the total heat loss for a given building. Window unsealing or even window ventilators are not always correct answers to the problem that can provide a sufficiently large stream of fresh air supply.
  • unfavorable temperature – as a result, e.g. in winter, air is often blown through exhaust grilles instead of escaping through them. Sometimes similar effects can be brought by the wind blowing from an unfavorable direction.
  • no exhaust system – covering the air grills (e.g. due to cold gusts of air), but also the wrong selection / improper use of a bathroom fan – when the device is not working, may cause greater air flow resistance and block the exhaust The lack of an extract grille in the attic also constitutes a significant and frequent problem in houses.
  • lack of air flow within the house – e.g. due to the lack of ventilation openings/gaps in bathrooms doors or toilets, doors being closed tightly/permanently in rooms with window ventilators through which air should flow.

An additional issue related to natural ventilation is the air quality. When windows are opened, the air blown inside the building is not always purely fresh, but frequently contains a vast range of pollutants – suspended dust (the famous particulate matter), mold spores or chemicals (including benzo(a)pyrene). Various researches concerning indoor air quality shows that the concentration of pollutants in rooms is higher than outside due to higher indoor concentration level of said pollutants.

The main problem with natural ventilation, however, is a lack of predictability as well as large dependence on both the external conditions and the habits of the house's inhabitants. Knowing the great importance of ventilation in a healthy and comfortable life as well as in the general condition of the house, it is worth considering the investment in a “real” ventilation system. It is a solution that will operate almost completely autonomously, solving the described problems and ensuring a high quality of everyday life without requiring the inhabitants to take any actions.

How to Achieve Balanced Ventilation

Before the presentation of available technical solutions, it should be emphasized that our aim is to ensure controlled air exchange in all rooms in the house. Therefore, it must be remembered that the ventilation system cannot be replaced by a purifier or an air conditioner.

Neither of these devices will solve the problems described above since the cause is the sheer lack of ventilation, and therefore – air exchange. The reason is very simple – air conditioners/purifiers work with the air that is already in a given room. They neither replace it, nor neutralize carbon dioxide nor produce oxygen...

There is a whole range of products on the market that will help to improve/provide ventilation. The most comprehensive solution is the balanced ventilation system with heat recovery, sometimes called “the heat recovery system”. In this solution, the air exchange is controlled by an electrical mechanical device called a heat recovery unit (HRU or HRV = heat recovery ventilator). It is sometimes said that it is the “heart” of a ventilation system, but it is not the only element of a balanced ventilation system with heat recovery. The vital components of said system are as follows:

  • air intake – a properly secured fresh air inlet in the roof or external wall supplying air to the AHU;
  • air filter (part of the heat recovery unit) – responsible for removing, among others, suspended dust from fresh air, and therefore – improving its quality;
  • air supply fan (part of the heat recovery unit) – a device responsible for fresh air intake as well as redirecting it to the ductwork;
  • ductwork – ventilation ducts (pipes) routed from the heat recovery unit to each room, responsible for supplying fresh air to the rooms (supply ducts) and for collecting exhaust air (exhaust ducts);
  • ventilation terminals – diffusers (air supply elements) placed on the ceiling or on the wall of rooms such as bedrooms or living rooms, and extract grilles/exhaust vents in kitchens, bathrooms, wardrobes, etc., responsible for supplying/receiving air directly to/from the room;
  • exhaust fan (part of the heat recovery unit) – responsible for discharging exhaust air outside;
  • heat exchanger (part of the heat recovery unit) – thanks to which, during winter season, the exhaust air stream from the rooms becomes warm (heated). Thanks to the exchanger, the heat does not escape outside, but it is transferred to the fresh air stream, preheating it – this way heated air flows into the rooms while less heat energy is needed to heat the rooms at the same time;
  • exhaust vent – a properly secured outlet in the roof.

Mechanical ventilation system with heat recovery mvhr

In this solution, fresh air flows silently and without the intervention of residents into the rooms in the house, and exhaust air is removed equally smoothly. Both processes are provided by fans operating continuously, but with different efficiency depending on the needs.

The great advantage of this solution is the heat exchanger. If there was no heat exchanger, in winter the exhaust fan would remove the exhaust (and therefore heated) air, losing irretrievably the heat it contains; similarly in summer, the cold would be lost. The streams of supply and exhaust air in the heat exchanger pass each other (but never mix) – so in a simple heat exchange process, the exhaust air gives off heat to the supply air. This way, it pre-heats or cools it down, using considerably less energy. This heat/cold recovery process is a great advantage of a heat recovery system over other types of mechanical ventilation.

Maintenance costs of MVHR systems

A mechanical balanced heat recovery ventilation system translates, however, into higher maintenance costs for the ventilation itself as well as a bit more “wastage” than in the case of natural ventilation, which in turn does not give the possibility of saving energy.

For natural ventilation, it is sufficient to keep the grilles clean and have the chimney swept once a year (Article 62 of the Construction Law) is a sufficient set of actions.

In the case of a mechanical ventilation system with heat recovery, it is necessary to keep the supply and exhaust elements clean and replace air filters regularly. The annual maintenance costs of mechanical balanced ventilation system include:

  • electricity consumed by fans and a device protecting the exchanger against freezing (when drawing outdoor air with a temperature below, for example -5°C);
  • filter replacements (replaced usually 2–4 times a year, depending on the dustiness of the outdoor air);
  • annual maintenance check + additional service work. What is important, a maintenance inspection is a vital condition for validated warranty, even though such works have to be paid for even during the warranty period.

Additionally, every 10 years, the ventilation ductwork must be inspected and cleaned.

The cost of maintaining such a system ranges from PLN 1,000 to 1,500 per year. This is a lot compared to natural ventilation system, yet there are two points to note. Firstly, health and comfort cannot be converted into money, and secondly – heat recovery in the HRV provides savings on heating, which allows the user to compensate for expenses on the maintenance of the ventilation system.

As a system based on mechanical devices, heat recovery ventilation also has its constraints, e.g. noise generated by fans, especially when operating on the highest gear (might be partially damped by the housing), wearing of mechanical components, etc. More importantly, the mechanical ventilation system does not work during power cuts.

Heat recovery ventilation – Will it work in an already existing house?

Experts that specialize not only in ventilation, but in the entire construction process, indicate that the mechanical ventilation system is a quite large investment, yet least noticeable if planned at the stage of designing a new house.

Building a house with such a system turns out to be slightly more expensive than building a house with a traditional natural ventilation system. When building a house without natural ventilation, ventilation chimneys necessary for air exhausting process are omitted, which simplifies the project a little since there is no need to build them above the roof. The lack of this structural element lowers the price of construction works, which partially compensates for the costs of equipment and materials for the heat recovery ventilation and its installation.

The situation is slightly more difficult with the existing house. Then, the installation of a mechanical heat recovery ventilation means renovation – its scale depends on the type and size of the house, insulation structure, equipment as well as the location of other (e.g. electrical) systems.

There are two main (though unfortunately not the only) problematic issues: heat recovery unit location and duct routing.

Unit location

A typical heat recovery unit for a single-family house is similar in size to a larger washing machine. It must be installed in a room (never outdoors!) where the temperature will not drop below 0ºC. At the same time, the heat recovery unit must be easily accessible, due to the necessity to replace its filters and carry out maintenance inspections.

heat recovery unit

A place often chosen for the location of the heat recovery unit is a non-habitable attic, especially in the case of a one-story house, when it is easy to route the ducts from the heat recovery unit to all rooms. However, the attic may require insulation!

If there is really little space, it is worth considering a compact ceiling-mounted heat recovery ventilator (a flat device is suspended from the ceiling).

The heat recovery unit can also be placed in a utility room, hanging it on the wall, and even hidden in the building. When planning the location of the heat recovery ventilator, it is worth having in mind the noise level this mechanical device produces when working. Therefore, it should be away from e.g. bedrooms or other rooms with higher acoustic requirements.

MVHR Duct Routing

mvhr ducting

On the other hand, the routing of the ventilation ducts may require open pits which will allow to verify the location of other pipes and hammering walls. If the rooms are high, the ducts can run just under the ceiling and be hidden under drywalls.

It is also important to keep the ductwork as short as possible. The longer the ductwork, the more powerful the fan is needed, especially if there are numerous changes in direction (e.g. bypassing other systems).

In unfavorable conditions, it may happen that the fan compression capabilities (i.e. the ability to generate the pressure necessary to overcome the flow resistance through the ductwork) in the heat recovery unit appropriate in terms of air flow is insufficient for effective operation in this system.

Since each house is different (and thus, the heat recovery ventilation system will also be different), the decision to install the system should be preceded by a conversation with the designer, who should provide some suggestions regarding whether it is worth installing a full balanced heat-recovery ventilation system.

If the answer is yes, the designer will also develop the most optimal design for a given building. This system can be installed in many types of houses, but if it is very difficult or technically impossible, there are also other solutions (one of them will be presented below).

How much does mechanical ventilation with heat recovery cost?

The price of the balanced heat recovery ventilation system includes the following:

  • the system design, taking into account the specific nature of the house, the number of inhabitants, the best possible distribution of ducts and the selection of devices;
  • the heat recovery unit – the price depends not only on the quantity of air supplied and extracted, but also on the equipment (e.g. type of filters) as well as automation and control;
  • air intakes and exhaust vents;
  • air ducts (steel or polyethylene) – the price depends on the type of material, the length of the network and the number of fittings (e.g. bends or tees for joining and changing the direction of ducts);
  • ventilation terminals (air valves/air supply diffusers, grilles) – the price depends not only on their number (i.e. the size of the house), but also on the design, which may include, for example, an unusual color or a coating that facilitates cleaning;
  • cost of labor:
    • preparatory work (e.g. open pits for existing systems, wall hammering);
    • HRV delivery and installation;
    • installation of air intakes and exhaust vents;
    • installation of ducts with ventilation terminals;
    • system adjustment and start-up;
    • ductwork installation;
    • “shutting down” the elements of natural ventilation (e.g. bricking up the grilles).

It should be emphasized that the “shut-down” process of natural ventilation mentioned in the last point is vital. Natural ventilation cannot be used in a room where the mechanical ventilation system is installed.

This immediately raises the question of whether windows can be opened when a mechanical ventilation system is in use. Opening the windows, although allowed, should not be intended to perform a ventilation function, and the entire system will return to normal operation only after the windows are closed, because its operation is disrupted by introducing an additional air stream. This is the second, after legal requirements, important reason why there should be no components of the natural ventilation left in the house. Its uncontrolled operation may interfere with the operation of the mechanical ventilation system.

No place for MVHR ventilation system? Single room heat recovery units

Considering the possibilities of installing a ventilation system with a heat recovery unit, it can be concluded that for a given building and for a given investor it is not possible to install such a system.

The problem may be as follows: lack of effective ductwork distribution, lack of space for the heat recovery unit, weakened building structure (e.g. no possibility to make holes in the ceiling for an air intake or an exhaust vent) or, last but not least, financial limitations.

single room heat recovery unit

In such a situation, it is possible to install the so-called decentralized heat recovery units, namely – instead of using one large HRV connecting the entire ductwork, several smaller wall-mounted heat recovery units, individually selected for individual rooms in the house, can be installed in external walls.

A decentralized heat recovery unit (also known as a wall mounted heat recovery unit or wall mounted heat recovery unit), similarly to the centralized HRV, supplies fresh air and extracts the exhaust air.

Fresh air is supplied directly into the room supported by the system through a properly prepared and protected penetration in the wall (diameter: 200 mm), while the exhaust air is extracted from this particular room at the same time.


There are two solutions available on the market:

  • Wall-mounted heat recovery unit with two fans and a small crossflow or counterflow exchanger working as a “miniature” version of the centralised HRU;
  • “push-pull” types of wall-mounted heat recovery units with one fan operating alternately (as an exhaust and as a supply) and with a regenerative ceramic heat exchanger. The system operates in two cycles. In the “pull” phase, the fan extracts (it “pulls out” the air) and the exchanger accumulates heat from the exhaust stream. In the “push” phase, the fan changes direction – it supplies (“push in”) fresh air into which the heat previously stored in the exchanger is released. As the cycles alternate with a slight pause, the air streams do not mix (or more precisely, mix to an acceptable degree). In order to ventilate the entire room more effectively, wall-mounted heat recovery units in the room can be installed in pairs and their operation can be synchronized (one supplies air, the other extracts air).

The wall-mounted heat recovery unit, although small, should perform exactly the same task as the central device – they should ensure a constant air exchange of 20 m3/h for each person using the room.

When selecting and choosing the location of the device, it is worth taking some professional advice. It can be provided by the designer, but this option is also offered more and more frequently by the device supplier itself.

This is especially important because the heat recovery unit should not be selected to fit perfectly, because while working on the highest gear, it produces a sound at a level difficult to accept, e.g. in a bedroom. Moreover, it often consumes a relatively large amount of electricity.

single room heat recovery ventilation

A single room heat recovery unit can help to improve indoor air quality – from a practical point of view, it especially helps with problems with excess moisture in the air. However, it has some limitations, the main of which are low air filtration capabilities.

The fan in the wall-mounted heat recovery unit is small (it has low static pressure) and it was not designed to operate with a very effective filter which high resistance properties. Therefore, only anti-dust filters are suitable for wall-mounted heat recovery unit installations, and the device cannot fulfill the smog prevention function.

More on Wall Mounted SRHR Units in The Ultimate Guide to Decentralised Heat Recovery Ventilation


Centralized heat recovery units and single room HRVs are not the only solutions that provide ventilation in an already-existing home. However, they do provide the greatest opportunities for controlled ventilation and are the only home ventilation solutions to ensure heat recovery, thus reducing heat loss for ventilation.