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