Frequently asked questions about mechanical ventilation with heat recovery

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Common questions about heat recovery ventilation – a comprehensive guide for homeowners and professionals

Heat recovery ventilation is one of the most frequently discussed topics in the construction of new single-family homes and renovations. At the same time, it is an area where the most half-truths, inaccuracies, and marketing promises circulate, which have little to do with real-life practice. This article compiles the most common questions asked by customers – from complete beginners who have just read their first article about heat recovery to technically skilled builders dealing with specific installation details. The answers are compiled based on hundreds of real customer cases and consultations.

What exactly is heat recovery and why should I care about it?

Heat recovery is essentially a simple principle: the air you extract from your home (warm, moist, exhaled) transfers its thermal energy to the fresh cold air coming from outside. This happens in a heat exchanger without the two air streams mixing. The result? Fresh outdoor air flows into your home, but it is preheated – and you don’t pay for heating air that you would otherwise completely exhaust through a chimney or a window gap.

Why is this important now? Modern low-energy and passive houses are designed to be as airtight as possible – minimal thermal bridges, triple glazing, an airtight building envelope. This is great for energy consumption, but it creates a problem: natural ventilation through gaps is practically non-existent. Without active ventilation, CO₂, moisture, odors, and in the worst case, radon, quickly accumulate in such a house. Heat recovery ventilation solves both problems at once – it ensures fresh air and minimizes heat loss.

Centralized or decentralized system – what is the difference?

This is probably the most common question when choosing a system. In short: a centralized system has one larger unit (usually located in a technical room, basement, or attic) and distributes air throughout the house via a network of ducts. A decentralized system consists of several smaller units – each serving one room or a pair of rooms and mounted directly into the wall.

A centralized system is suitable for new builds, where the ductwork can be planned from the beginning and built into the structure. It makes sense for houses of about 100 m² or larger, where it is economically viable to invest in one powerful device with better efficiency. A decentralized solution, on the other hand, is ideal for renovations of existing houses and apartments, where running ducts would either be impossible or very costly. Each unit operates independently and the air does not mix with other rooms.

Practical example: a customer in a panel apartment from 1978 did not want to tear down the ceilings. We installed four units of decentralized ventilation unit HRC E – electronic version (Master, pr. 160 mm) – two in the living room and two in the bedrooms. Each unit alternates between air supply and exhaust (usually every 60–70 seconds), with a ceramic insert storing heat from the exhaled air and transferring it to the fresh air in the next cycle. The efficiency of such a system is lower than that of centralized counterflow heat exchangers (around 75–80 %), but for an apartment without existing ventilation is practically the only reasonable option.

More on this topic can also be found in the article Centralized vs. decentralized heat recovery units – what is better in our Knowledge Center.

What is the real efficiency of heat recovery and how can I verify it?

Manufacturers of heat recovery units typically state the efficiency of the heat exchanger in the range of 75 % to 95 %. However, these figures are measured under precisely defined laboratory conditions – at specific temperatures, air flows, and zero leakage. In a real installation, the actual efficiency is always lower for several reasons:

Leakages in the ductwork: every improperly joined part or missing seal means mixing of the air streams or heat loss to a cold area (e.g., the basement).
Unbalanced air flow: if the supply and exhaust air flows are not balanced, efficiency drops. This is also one of the most common problems customers describe as "heat recovery is not working well".
Clogged filters: dirty filters increase resistance, reduce airflow, and in extreme cases cause condensation in the wrong places.
Extreme outdoor temperatures: in strong frost (below –15 °C), freezing of the heat exchanger is a risk, the system must switch to bypass or preheating mode, which temporarily reduces efficiency.

How can you verify efficiency in practice? The simplest way: measure the temperature of the outdoor air (T_out), the temperature of the air supplied into the house (T_in), and the temperature of the air in the house (T_house). Efficiency = (T_in – T_out) / (T_house – T_out) × 100 %. If it is –5 °C outside, +21 °C inside the house, and the supplied air is +18 °C, the efficiency is (18–(–5))/(21–(–5)) × 100 % = 23/26 × 100 % = approximately 88 %. This is a good result.

What pipe sizes are suitable for ventilation ducts?

Choosing the right pipe diameter is one of those details where design errors can be painfully felt later — in the form of noise, insufficient airflow, or unnecessarily high fan power consumption. Basic rule: the larger the diameter, the lower the resistance, the quieter the system. But a larger diameter takes up more space in ceilings and walls.

Most commonly used pipe diameters in residential systems:

Ø 63 mm: connections to individual outlets, short branches into rooms, maximum flow of approx. 20–25 m³/h per branch
Ø 75 mm: medium branches, flow up to 35 m³/h
Ø 90 mm: main distribution branches, flow up to 60–70 m³/h
Ø 125 mm and 160 mm: main inlet/outlet openings on the unit, in larger systems

IVAR.PROFI-AIR CLASSIC systems operate with modular distribution cabinets, where the branches are in diameter 63 mm and the central opening is 90 mm. For connecting these cabinets and extending the ducts, for example, the modular distribution cabinet coupling – 90 mm (classic) is used, which allows simple extension of the entire distribution system without the need for special tools. Maintaining air tightness in each connection is critical to the quality of sealing – in this system, the tongue-and-groove circular seal for 63 mm (classic) is used, which creates a reliable seal when inserted and prevents air leakage even under pressure differences typical in installations (up to 80 Pa).

If in some branch the system fails to deliver the required flow – for example, due to a longer route or a larger number of branches – the solution is not to immediately change the entire unit, but to use the booster for 75 mm or 90 mm (classic). This auxiliary fan is mounted directly into the duct and locally increases the pressure in the branch, thus balancing the hydraulic imbalance without affecting the other branches.

A more detailed discussion of dimensions and sizing is covered in the article What pipe diameter do I need for ventilation in the Knowledge Center.

Is heat recovery really quiet? Where does noise occur and how to eliminate it?

Heat recovery is often described in promotional materials as "quiet" or "noiseless". This is basically true for a properly designed and installed system – but practice shows that noise is one of the most common complaints from customers after installation. Where does the problem arise?

Fan of the unit itself: every heat recovery unit has a fan – centralized systems usually have two (supply and exhaust). Modern EC motors are very quiet (typically 20–35 dB(A) at low speeds), but if the unit is placed directly next to a living area without acoustic insulation, you can hear it. Solution: a technical room with doors, or anti-vibration hangers for suspended units.

Noise from air flow in the duct: if the duct is undersized (too small a diameter for high flow), the air in it will be noisy. Air speed should not exceed 2.5–3 m/s in main branches and 1.5–2 m/s in connections to outlets. At speeds above 4 m/s, the noise in the duct is audible through the outlets and through the walls.

Clicking in decentralized units: ceramic inserts when switching the direction of airflow can produce a click or a slight "thud". This is normal, but customers are not always warned about it. Most people get used to it, or the switching frequency can be reduced.

Acoustic transfer between rooms: ducts carry air, but also sound. If acoustic silencers are not installed, you can hear adjacent rooms through the ventilation. This is not critical in houses, but it is a serious problem in apartment buildings and hotels.

More on this topic can be found in the article Noise from heat recovery units – causes and solutions in the Knowledge Center.

What to do with condensate – where does it form and how to drain it?

Condensate is a by-product of heat recovery that is most often overlooked when designing a system. It forms in the heat exchanger on the exhaust side – the warm, moist air from the house cools here and the moisture condenses. The amount of condensate depends on the outside temperature and the relative humidity of the air in the house.

In practice, this can be quite a lot: at temperatures around 0 °C in a house with standard humidity of 50–55 % RH, a centralized unit for a 150 m² house can produce 1–3 liters of condensate per day. This has to go somewhere – into a sewer trap, and that with a trap (to prevent bad odors from returning). If the condensate has nowhere to drain or the trap dries out, the system will find its own way – and that is usually not a pretty sight.

In decentralized units with ceramic inserts, condensate is almost non-existent – the ceramic absorbs moisture and returns it to the house during the intake phase. This is actually an advantage: the system partially regulates indoor humidity without the need to drain the condensate.

What is the correct air exchange rate for a single-family house?

Basic hygiene standards (STN EN 15251, EN 13779) recommend ensuring a minimum of 0.3–0.5 air changes per hour for living areas. For passive houses, the PHPP standard recommends 30 m³/h per person or 0.3 times the house volume per hour, whichever is higher.

What does this mean in practice? A house with a living area of 150 m² and a ceiling height of 2.6 m has a volume of approximately 390 m³. For 0.3 air changes per hour, you need an airflow of: 390 × 0.3 = 117 m³/h. For a family of four, 30 m³/h per person equals 120 m³/h. Both values are therefore very close to each other.

In practice, systems are designed with regulation, as the need for ventilation is not the same throughout the day. During the day, when people are at work and the house is empty, 50–60% of maximum airflow is sufficient. In the evening after returning home, while cooking or when the house is more occupied, the performance is increased – either manually or automatically based on a CO₂ sensor, humidity sensor, or motion sensor.

Watch out for a common mistake many designers make: they design the system for maximum airflow and set it to this value permanently. The result: unnecessarily high electricity consumption, dry air in winter, and unnecessary noise. Heat recovery should run at 60–70% capacity most of the time and only at full capacity occasionally.

What happens during a power outage?

Heat recovery systems are dependent on electricity – they do not function without power. This is a fact that must be accepted. What does this mean during a power outage? In a tightly sealed building without the possibility of natural ventilation (triple glazing without micro-ventilation), CO₂ and humidity will start to accumulate during a prolonged outage. Solution: windows can always be opened. A short-term outage (hours) is not a hygiene issue – CO₂ quickly dilutes if the house is ventilated through the window at least once every few hours.

For sensitive customers or those living in areas with an unstable grid, backup UPS solutions exist, but their use in conjunction with heat recovery units is rather exceptional and economically justifiable only in specific cases (e.g., gardens with autonomous power supply, cottages).

How long does it take for the investment in heat recovery to pay off?

This is a question where one must be honest and not paint too rosy a picture. The payback period depends on many factors: gas/electricity prices, number of heating seasons, house size, original state of ventilation and thermal insulation, and, of course, the total installation cost.

As a reference: a well-designed heat recovery system in a new build with an area of 150 m² typically saves 1,500–2,500 kWh of thermal energy per year compared to ventilation through windows. At a natural gas price of 0.10 €/kWh (GJ equivalent), this is 150–250 € per year. The cost of a complete installation of a centralized system with ducting ranges from 4,000 to 10,000 € including labor and materials. The math therefore suggests a 20–50 year payback period purely from energy savings.

Why then do people buy heat recovery systems? Because the primary benefit is not energy, but air quality, comfort, and health. People with allergies, asthma, or young children value the constant supply of fresh filtered air much more than hypothetical savings. And in a passive house or a house of class A0/A1, heat recovery is practically mandatory – no other solution works there.

What is the maintenance effort and what should be checked regularly?

Heat recovery systems belong to relatively low-maintenance equipment, but they cannot be left completely unattended. Basic tasks:

G4 filters (coarse filter): check every 2–3 months, replace or clean every 6 months – every 3 months in case of higher air pollution (near roads, pollen season)
F7/M5 filters (fine filter, anti-pollen): replace once a year, ideally before the heating season
Heat exchanger: visual inspection once a year, cleaning according to condition – if the fins are dusty, efficiency drops significantly
Condensate siphon: check water level before the season, refill if necessary
External grid: check whether it is clogged with leaves, snow, cobwebs, or bird nests
System balance: recommend measuring airflow at individual outlets and readjusting balance every 2–3 years

The external aesthetic grid is the first point of contact between the heat recovery system and the external environment and is one of the most neglected components. For IVAR.HRC systems, for example, there is an external aesthetic grid for e.g. 160 mm available, which, in addition to its aesthetic function, prevents rain, insects, and birds from entering directly to the heat exchanger. A clogged grid dramatically increases the resistance at the inlet, the fan works under higher load, and the system efficiency drops.

The topic of regular care for the system is discussed in more detail in the article Maintenance and cleaning of heat recovery units – how often and how to do it properly in our Knowledge Center.

Can I install heat recovery myself?

Technically yes, practically – it depends on your skills and exactly what you are doing. Decentralized units like HRC E are designed so that a single skilled person can install them – a hole is drilled in the wall, gaskets are inserted, the unit is mounted, and it is connected to a 230 V socket. Nothing mysterious. This is also covered in the article Installation of a decentralized ventilation unit HRC step by step in our Knowledge Center.

A centralized system with an extensive duct network is a different category. The ducts themselves can be installed by a skilled builder or technically capable homeowner. However, system design – duct sizing, outlet placement, balancing – requires experience. A design error is difficult to correct after ceilings and walls are closed. Electrical connection of the main unit should be done by a licensed electrician. Final system balancing (setting the required airflow at each outlet) is also work for a professional with a measuring device (anemometer or flow meter).

Recommendation from practice: with a decentralized system, you can easily install it yourself if you're skilled. With a centralized system, at least commission the design and final balancing to a professional – you'll save your nerves and the result will be functional.

What is a bypass and when is it used?

A bypass is a bypass damper that allows fresh air to flow past the heat exchanger – without heat transfer. It is used in three main situations:

Summer night cooling: in summer, when it is cooler outside at night than inside, it is beneficial to bring in cool outside air directly, without heating it with the warm indoor air. The bypass opens a path outside the heat exchanger, allowing you to use free night "air conditioning". This is one of the most valuable benefits of a bypass in real operation.

Preventing freezing of the heat exchanger: at temperatures below –5 °C (depending on humidity), condensation and subsequent freezing on the cold end of the heat exchanger is a risk. The system temporarily switches to bypass or uses preheating of air (electric or ground heat exchanger), thus protecting the heat exchanger.

Ventilation at moderate annual temperatures (spring, autumn): when the outside temperature is close to the inside temperature, heat recovery would only unnecessarily transfer heat back and forth. The bypass makes the system more efficient.

Not every heat recovery unit has a bypass – it is missing in cheaper models. When selecting a system for a warmer climate or if you emphasize summer cooling, a bypass is practically essential.

Heat recovery and radon – is it true that heat recovery helps?

Yes, and this is one of the benefits that is relatively rarely discussed. Radon is a naturally occurring radioactive gas that enters buildings from the ground. In older houses with leaky floors and natural ventilation, it disperses quite well. In new airtight buildings, it can accumulate to dangerous levels (above 300 Bq/m³ is considered risky).

Forced ventilation with heat recovery constantly exchanges air in the interior, effectively reducing radon concentration. If you suspect elevated radon levels (geological maps of Slovakia are available), heat recovery is one of the recommended measures along with proper insulation and sealing of the floor.

Common mistakes in the selection and installation of heat recovery

From practice, we know that the most common mistakes occur during selection rather than during installation itself. Here is an overview of the most painful ones:

Undersizing the capacity: the customer buys a unit with a maximum flow of 200 m³/h for a house that needs 250 m³/h. The unit runs constantly at full capacity, is noisy, has a shorter lifespan, and despite that, does not provide enough fresh air.
Inappropriate placement of outlets: supply outlets directly in the living zone (e.g., above the sofa or above the bed) cause unpleasant drafts even at low air speeds.
Neglecting acoustic insulation: a unit without noise dampers installed in a technical room with lightweight drywall walls – the customer hears the fan from the living room.
Lack of system balancing: after installation, the unit is simply turned on and no one measures the actual flows at the outlets. Result: in some rooms there is too little fresh air, in others there is an excess.
Ignoring pressure loss on filters: neglected filter replacement can double the pressure loss on the filter, which the unit compensates by increasing speed – noise increases and efficiency drops.
Heat recovery without a ground heat exchanger in a cold climate: in areas with long and harsh winters, freezing of the heat exchanger without preheating of the air (ground heat exchanger or electric preheating) can cause long system outages.

If you plan to design the system yourself, also read the article How to properly design an air distribution system with heat recovery and How to choose a mechanical ventilation system with heat recovery for a family house in our Knowledge Center – it will save you time and money.

Heat recovery and healthy living – what do the numbers say?

An adult exhales about 20–25 liters of CO₂ per hour. In a well-sealed house without ventilation, the presence of 4 people can raise the CO₂ concentration from the normal 400 ppm (outside air) to 1 500–2 500 ppm within a few hours. At 1 000 ppm, most people start to feel tired and reduced concentration. At 2 000 ppm, the symptoms are pronounced – headache, drowsiness, reduced performance.

Heat recovery ventilation designed for hygienic air exchange (30 m³/h per person) keeps CO₂ levels typically below 800–900 ppm even with full house occupancy – that is in the comfort range. It is a measurable, objective advantage, not just a marketing promise.

Filters of class M5 and F7 capture most pollen, coarser dust particles, and some biological contaminants. For allergy sufferers, this is a real change in quality of life when combined with airtight windows – the interior becomes a refuge during pollen season.

Most frequently asked questions (FAQ)

Do I have to keep the windows closed all year round with heat recovery ventilation?

No, it is not mandatory – but heat recovery works efficiently only in a relatively airtight building. If you keep the windows open for a long time, the system still does its job, but the energy savings are lower (some of the warm air escapes through the windows without transferring heat to the heat exchanger). In practice, people open the windows occasionally – for intense cooking, when they want to ventilate quickly – and this does not significantly interfere with heat recovery. On the contrary, in summer bypass mode, open windows at night can complement the bypass logic.

My heat recovery unit keeps freezing – what am I doing wrong?

Freezing of the heat exchanger at temperatures below –5 °C is a normal physical phenomenon, not a malfunction. The problem arises if the system lacks anti-freeze protection – either it does not have a bypass or the anti-freeze activation temperature is set incorrectly. Temporary solution: reduce the supply air flow during heavy frost (the unit then works less with extremely cold air). Permanent solution: installation of a ground heat exchanger (geothermal preheating) or an electric preheater before the heat exchanger. The article Common faults in heat recovery units and how to eliminate them in our Knowledge Centre describes this situation in more detail, including parameter settings.

Is heat recovery worth it for an old brick house from the 80s?

It depends on the condition of the house. If the house has undergone insulation and window replacement and is relatively airtight, heat recovery makes sense even in an older building. If the house still has original windows with putty and grooves and leaky doors, natural air infiltration is so high that the heat recovery has nothing to do – the air is exchanged naturally and the system would be running unnecessarily. Recommendation: first insulation and window replacement, then heat recovery. For older houses undergoing renovation without the possibility of running ducts, decentralized units are ideal – they are installed in existing walls without major construction work.

How much does heat recovery operation cost – electricity?

Modern centralized EC units for a 150 m² house consume 60–120 W of electricity at medium performance. Over a year with 24/7 operation, this amounts to 525–1 050 kWh, which at an electricity price of 0.20 €/kWh is about 105–210 € per year. Decentralized units are smaller in power – each 2–8 W – but they work in pairs and together consume 10–20 W, which is 88–175 kWh per year with 24/7 operation, i.e. 18–35 € per pair. Electricity consumption is therefore not negligible, but compared to the saved thermal energy, it is usually lower.

Can I connect air conditioning or additional heating to the heat recovery system?

Yes, there are systems that allow this directly. Centralized heat recovery units with air water heater are powered by a boiler or heat pump and can directly heat the air in the unit without the need for an additional heating system (so-called air heating system). This solution is very popular in Austria and Germany for passive houses. In summer, cooling can be connected in an analogous way. However, the capacities must be well calculated – ventilation alone can provide a maximum heating capacity of 10–15 W/m², which is sufficient for passive houses, but not for older buildings.

Where should the external exhaust of the heat recovery system be located?

The external exhaust (fresh air intake and exhaust of stale air) should be at least 1 meter away from any other opening (windows, doors), at least 2 meters above ground level, and on a side without direct sunlight (to prevent air from overheating in summer). The exhaust of stale air and the intake of fresh air must not be close to each other – the minimum distance is 1–1.5 m, even more for heat recovery units with low efficiency. In case of incorrect placement, there is a risk of "short-circuiting", where the exhaled air is immediately sucked back in. This topic is also covered in our article Roof vs. façade exhaust for heat recovery – what to choose and when.

Conclusion: Heat recovery as a long-term investment in the quality of living

Mechanical ventilation with heat recovery is a technology that truly changes the quality of daily life when properly designed and installed. It is not a cheap solution and the return on investment in terms of pure energy savings may be long – but the added value in the form of fresh air, low CO₂, pollen filtration, radon protection, and comfortable climate without drafts from windows is invaluable for many families. The key to success is proper design, quality installation, correct balancing, and regular maintenance. Each of these steps is equally important – a good heat exchanger in a poorly installed system will not help, nor will a perfectly installed system with clogged filters.

In the category mechanical ventilation with heat recovery, you will find components for both centralized and decentralized systems – from seals and connectors through boosters to complete units. If you are unsure which components you need for your project, other articles in this Knowledge Centre will help you make the right decision without unnecessary compromises.

Do you have a question about this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us – we are happy to help.