How to choose a mechanical ventilation system with heat recovery for a family house

We are still preparing products.

But you can look at other categories.

How to choose a mechanical ventilation system with heat recovery for a single-family house

Mechanical ventilation with heat recovery is now one of the basic elements of modern low-energy and passive houses. At the same time, it is increasingly implemented in the renovation of older single-family houses, where natural ventilation – i.e., air flow through gaps in windows and doors – has stopped working after replacing the windows with new, airtight ones. In practice, we see this on almost every project: homeowners pay thousands of euros for plastic windows, the house becomes airtight, and after one or two years, problems with mold, condensation on windows, and stuffy air in bedrooms begin.

Selecting the right heat recovery system is by no means simple. The market offers a wide range of products, technical parameters can be confusing at first glance, and mistakes in selection – whether underestimating performance, choosing the wrong duct layout, or ignoring noise levels – will accompany you for the next twenty years. This article will guide you through the entire decision-making process step by step, from basic concepts to specific recommendations for different types of houses.

Why heat recovery is important – and when you really need it

Standard STN EN 15251 and Slovak decree No. 364/2012 Z.z. set the minimum air exchange in living spaces. For bedrooms, it is recommended to have at least 25 – 30 m³/h per person, for the living room 30 – 50 m³/h, for the bathroom and toilet 50 – 100 m³/h of extracted air. These values are simply impossible to ensure reliably and for a long time with natural ventilation – especially not in today’s airtight building envelopes.

Heat recovery ventilation is not only about air hygiene. It is primarily about energy balance. A modern heat recovery unit with an efficiency of 85 – 93 % will return most of the heat from the exhaust air back into the fresh incoming air. Simply put: a ventilated house does not have to be a cold house.

When do you really need heat recovery? In practice, a simple test applies: if your house has new airtight windows or was built after 2010 with an emphasis on thermal protection, mechanical ventilation with heat recovery is essential for you – regardless of whether the standard formally requires it or not. Without it, you risk health issues and structural damage.

Centralized vs. decentralized system – the basic choice

This is the first and most important decision. Both types have a legitimate place in the market, but their deployment conditions are different. A detailed technical comparison can be found in the article Centralized vs. decentralized heat recovery units – which is better in the Knowledge Center, here we summarize the essentials for selection.

Centralized system

One larger ventilation unit is installed in a technical space (boiler room, attic, garage, basement) and distributes air through a duct network to all rooms. Fresh air is supplied to living rooms (bedrooms, living room, children's rooms), and exhaust air is drawn from wet and waste rooms (bathroom, toilet, kitchen). This layout is standard in new constructions.

Advantages of a centralized system: One device to maintain and service. Quiet operation in living spaces (the unit is far away). Possibility of integrating heating, cooling, pollen filter, humidification. Long lifespan with proper maintenance (15 – 25 years). Low noise in living spaces – the unit hums in the technical space, not in your room.

Disadvantages of a centralized system: Extensive duct network that must be planned before the rough construction. Duct installation is a major problem during renovation. Higher installation cost. One point of failure – if the main unit fails, nothing works.

Decentralized system

Smaller units are installed directly in the walls of individual rooms. A classic example is the decentralized ventilation unit HRC E – electronic version – 05 Master with a diameter of 160 mm, which is installed through a core in the perimeter wall. These units typically work in pairs or groups and alternate the phase of air supply and exhaust (phase heat recovery units with a ceramic heat accumulator).

Advantages of a decentralized system: Ideal for renovations – only one 160 mm diameter hole in the wall is needed. Flexibility – you can ventilate only selected rooms. Lower installation cost in existing buildings. Individual units operate independently.

Disadvantages of a decentralized system: Noise is directly in the room – depends on the type and settings. Lower theoretical heat recovery efficiency (about 70 – 85 %) compared to a centralized counterflow unit. More devices to service and replace filters. Electricity is needed for each unit.

How to calculate the required performance of the ventilation system

This is where most mistakes are made. Either the system is designed too weakly (the house is not sufficiently ventilated) or too strongly (unnecessarily large and noisy unit, unnecessary over-ventilation and overheating).

The basic calculation is based on the prescribed air volume flow. For a single-family house, a rough estimate applies: 30 m³/h per person, minimum 0.3 times the internal volume of the house per hour. For a family of four in a house with a living area of 150 m² and an average ceiling height of 2.7 m, this is:

House volume: 150 × 2.7 = 405 m³
According to people: 4 × 30 = 120 m³/h (minimum hygienic flow)
According to volume: 0.3 × 405 = 121.5 m³/h
Practical recommendation for comfortable ventilation: 150 – 200 m³/h with regulation

In practice, it is better to design the unit with a reserve of 20 – 30 %, because during intense cooking, showering or with a larger number of guests, you need a higher flow. At the same time, the unit must never run continuously at 100 % capacity – this shortens the life of the fans and increases noise.

With decentralized systems, the flow per unit typically ranges from 15 – 60 m³/h. For example, a unit with a diameter of 160 mm (such as the HRC series) will cover one larger room or two smaller bathrooms with proper pairing. More on dimensioning pipe diameters can be found in the article What pipe diameter do I need for heat recovery ventilation.

Heat recovery efficiency – what do the numbers in the catalog mean

Manufacturers list the heat recovery efficiency and this number is a mystery to most buyers. It is important to know the difference between thermal efficiency and enthalpic (moisture) efficiency.

Thermal efficiency (sensible heat recovery efficiency) expresses how much of the air heat is transferred from the exhaust air to the fresh air. A value of 85 % means that if it is 0 °C outside and 22 °C inside, the incoming air will be approximately 18.7 °C before any additional heating. This value is measured under specific laboratory conditions and in real operation it may be 5 – 10 % lower.

Enthalpic efficiency also includes moisture transfer – relevant for membrane heat exchangers or rotary heat recovery units. For single-family houses, counterflow plate heat exchangers with thermal efficiency of 85 – 93 % are common.

Watch out for confusion: some manufacturers list efficiency at very low flows, where the heat exchanger works ideally. At nominal flow, efficiency can be 5 – 8 % lower. When comparing products, always check at what flow the efficiency was measured.

Types of heat exchangers – what is inside the unit

The heart of every heat recovery unit is the heat exchanger. In single-family houses, we encounter three main types:

Counterflow plate heat exchanger (cross-counter flow)

The most widespread and for single-family houses the most suitable type. Fresh and exhaust air flow in opposite directions, which maximizes the temperature gradient and thus also efficiency. It achieves 85 – 93 % thermal efficiency. The disadvantage is the risk of freezing at temperatures below –5 °C – the unit must have frost protection (preheating of air or a bypass with a defrost cycle). For Slovak climatic conditions, this is a common occurrence several times a year, so always check what kind of frost protection the specific unit has.

Crossflow heat exchanger (cross flow)

A simpler construction, lower efficiency (65 – 80 %), cheaper. Suitable for milder climates or less demanding applications. It is practically not used in new units for single-family houses today, because the counterflow heat exchanger has only a minimal price premium with significantly better performance.

Rotary heat exchanger (rotary heat exchanger)

A slowly rotating aluminum foil rotor alternately absorbs and transfers heat. The advantage is the possibility of moisture transfer (enthalpic heat recovery) and natural defrosting. It achieves 80 – 88 % thermal efficiency. The disadvantage is a small air leakage between channels (cross-contamination) – it is not suitable for houses where air microbiological purity is critical or where the exhaust air contains a lot of odor (e.g. intense kitchen fumes are partially returned).

Ceramic regenerator (used in decentralized units)

Used precisely in decentralized wall units such as the HRC series. A ceramic storage insert first absorbs heat from the exhaust air (exhaust phase), then the flow direction is reversed and the insert heats the fresh air from outside (supply phase). The cycle typically lasts 60 – 90 seconds. Efficiency 70 – 82 %. The advantage is a very simple construction without moving parts in the heat exchanger and natural frost resistance.

Duct distribution – star vs. collective topology

With a centralized system, you must decide how to organize the duct distribution. There are two basic schemes:

Star topology (distributed from a dividing plenum box): From the central unit, one or two main ducts of larger diameter (150 – 200 mm) lead to distribution boxes. From the boxes, flexible hoses (diameter 63 – 90 mm) lead directly to individual rooms. Each branch is independent, the pressure is well balanced. This is the preferred topology today. For this type of ducting, components such as the coupling of the modular distribution box for IVAR.PROFI-AIR CLASSIC with a diameter of 90 mm are used, which allows modular expansion of the distribution box according to the number of branches.

Collective (serial) topology: A main pipe of a larger diameter runs through the whole house, and individual rooms are branched off from it. Simpler pipe installation, but more difficult hydraulic balancing. In a larger house, there is a risk that the last rooms along the route will be poorly supplied with air. This topology is used less frequently in practice.

A key component for any topology are sealing and connecting elements. Leaky ducts are one of the most common problems in real installations – air escapes into wall and ceiling cavities, system efficiency drops, and moisture problems may occur. For tight connections in ducting, it is important to use high-quality seals, such as a tongue-and-groove gasket for a diameter of 63 mm, which ensures reliable sealing even after repeated assembly and disassembly.

Pressure loss occurs in long or complex duct runs. If the route exceeds approximately 15 – 20 m or contains more than 3 – 4 bends, it is necessary to use an airflow booster. For PROFI-AIR CLASSIC systems, for example, there is a booster for a diameter of 75 mm or 90 mm, which compensates for pressure losses in long runs and ensures sufficient airflow in each room without the need to dimension the entire system for higher pressure.

Unit and exhaust placement – practical limitations

For a centralized unit, you need a technical space with a minimum area of 1.5 – 2 m² and a free access height of at least 0.8 m for service. The unit must have access to outside air (inlet) and exhaust air from the house. These two larger diameter pipes (typically 150 – 200 mm) pass through the perimeter wall or roof.

Pay attention to the relative position of the inlet and exhaust openings. The minimum distance between them should be 1.5 – 2 m to prevent the re-ingestion of exhaust air. On the south side of the façade, summer sun can unnecessarily preheat the incoming air. Inlet should preferably be oriented to the north or northeast, ideally at a height above 0.5 m from the ground and out of the reach of snow and flooding.

A detailed comparison of roof and façade exhausts with specific recommendations for different situations can be found in the article Roof vs. façade exhaust for heat recovery – what to choose and when. Here we only emphasize that the aesthetics of the exhaust are not only about appearance – a proper grille protects the duct from birds, insects, rain, and snow. For the outdoor exhaust of HRC units, for example, an external aesthetic grille for IVAR.HRC with a diameter of 160 mm is used, which is designed to minimize aerodynamic resistance, maintain the correct pressure profile of the unit, and visually blend with the façade.

Filters, noise, and control – what people ask most often

Filters and air quality

Every heat recovery unit must have filters on both sides – for the incoming and outgoing air. Standard filters used are class G4 (coarse dust) on the exhaust side and F7 or higher (fine dust, pollen) on the incoming air side. Some premium units offer H13 (HEPA) filters as an optional add-on for allergy sufferers.

Clogged filters are the most common cause of system failure – they reduce airflow, increase motor power consumption, shorten their lifespan, and paradoxically worsen air quality. Filter replacement or cleaning should occur every 3 – 6 months depending on conditions (dustiness of the environment, presence of pets, building surroundings). More on this topic can be found in the article Maintenance and cleaning of heat recovery units – how often and how to do it properly.

Noise

Noise is a decisive factor for many customers, especially for bedrooms. In centralized systems, noise in the rooms is mainly determined by the noise of the air stream at the outlets (aerodynamic noise) and possible transmission of vibrations through the ducts. The unit itself in the technical room typically produces 35 – 50 dB(A), which is acceptable.

With decentralized units, the fan is directly in the room, which deters many. However, modern units are very quiet at low speeds (25 – 32 dB(A) in night mode), comparable to the distant hum of an air conditioner. Noise issues and their solutions are thoroughly discussed in the article Noise from heat recovery units – causes and solutions in the Knowledge Center.

Control and automation

Modern systems offer various levels of automation. The basic version has only manual switching between a few performance levels. A reasonable minimum today is intelligent control with a CO₂ sensor or humidity sensor – the unit automatically increases performance when poor air quality is detected and reduces it when the air is fine. This saves energy and noise.

Premium systems are integrable into home automation (KNX, Modbus, WiFi). For simpler homes, a simple controller with a weekly schedule is sufficient. A very practical function is cooking boost – a temporary increase in performance for 30 – 60 minutes during cooking, controllable by a simple button in the kitchen.

System economics – investment, operation, and return on investment

Investment costs vary significantly depending on the chosen system and the size of the house. Approximate values for a 150 m² family house:

Centralized system (new build): Equipment 1,200 – 4,500 € depending on performance and brand, installation and ducting 2,000 – 5,000 € (depending on layout and installation conditions). Total 3,500 – 10,000 €.
Centralized system (renovation): Installation is 30 – 50 % more expensive due to more complicated duct routing in an existing building. Real cost 5,000 – 14,000 €.
Decentralized system (4 – 6 rooms): Equipment 300 – 700 € per unit, installation 200 – 400 € per unit. For 6 units total approximately 3,000 – 6,500 €.

Operational consumption of a centralized system for a 150 m² house is typically 80 – 150 W of fan power, which amounts to 700 – 1 300 kWh of electrical energy per year (approximately 140 – 260 € at a price of 0.20 €/kWh). In contrast, the system saves 500 – 1 500 kWh per year on heating (depending on the climatic zone and house parameters). The net energy balance is usually positive depending on the specific case, and the return on the energy savings alone is around 8 – 15 years. However, the real benefit of heat recovery is primarily the quality of the indoor environment and the prevention of moisture-related damage, which is difficult to quantify.

Step by step – how to proceed with the selection

Based on experience from ten times as many similar projects, we recommend the following procedure:

Determine the type of house and project phase: New construction → centralized system with ductwork in the rough-in stage. Renovation → consider decentralized units or a small centralized system with flexible hoses in the ceiling voids.
Calculate the required air flow according to the number of people and the volume of the house. Add a 20 – 30 % reserve.
Choose the type of heat exchanger: For new constructions with temperatures below –10 °C (mountainous areas, northern Slovakia) – counterflow plate with preheating or electric heat exchanger. For typical lowland areas – counterflow without special requirements.
Select the duct layout and have a ventilation design prepared by a specialist. Do not buy a unit without a design!
Check the noise level – ask for dB(A) values at 1st and 2nd speed, not at maximum.
Verify service and spare parts availability – a heat recovery unit has a lifespan of 15 – 25 years, but filters, belts, capacitors and electronics will need replacement. The seller should be able to guarantee the availability of parts for at least 10 years.

A detailed guide for the installation of decentralized units can be found in the article Installation of a decentralized ventilation unit HRC step by step. The article How to properly design a ventilation system with heat recovery discusses the correct design of the entire distribution system.

Most common mistakes in selection and installation

From practice we know that there are several typical mistakes that repeat in various projects:

Underestimating the capacity: The customer buys a cheaper, smaller unit and finds out that the house is not sufficiently ventilated. Later, they have to pay for reconstruction or add additional equipment.
Ignoring frost protection: A unit without anti-frost protection freezes and is damaged during the first strong winter. The cost of repair often exceeds the cost of a properly designed unit from the start.
Poor duct design: Too small diameters, too many elbows, long runs without boosters. Result: uneven ventilation, noise, unnecessarily high consumption.
Unbalanced system: The volume of supplied and extracted air must be balanced (maximum deviation ±10 %). An unbalanced system causes negative or positive pressure in the house, which makes opening doors difficult and can cause mold growth.
Neglected maintenance: Clogged filters, uncleaned ceramic inserts or neglected condensate drains lead to malfunctions and reduced air quality worse than if the system was not installed at all.
Incorrect location of air outlets: Supply air is brought in where exhaust should be and vice versa (bathroom). As a result, humidity and odors circulate throughout the house.

Frequently asked questions (FAQ)

Can I install a heat recovery ventilation system in an older house from the 80s without tearing up the ceilings?

Yes, but you should realistically assess the possibilities. For houses with ceiling voids, it is possible to run flexible ducts (diameter 63 – 90 mm) without major construction work. An alternative is decentralized wall units – it is sufficient to drill one hole of diameter 160 – 200 mm per room. A full-featured centralized system with rigid ducts in an existing building is technically more complicated and significantly more expensive, but feasible. In any case, we recommend having a survey and consultation with a ventilation designer before purchasing a unit.

What is the actual lifespan of a heat recovery unit?

Quality centralized units from reputable manufacturers have a lifespan of 15 – 25 years with regular maintenance. Fans typically last 10 – 15 years and are in most cases replaceable without replacing the entire unit. Ceramic storage inserts in decentralized units have a lifespan of 8 – 15 years depending on the quality of water and air in the region. Filters have the shortest lifespan (3 – 12 months depending on the load) and electronic control boards (8 – 15 years). Regular maintenance has a decisive impact on lifespan – neglected service halves it.

Heat recovery in summer – what happens when it is warmer outside than inside?

Most modern units have a summer bypass function: in warm weather, the heat recovery core is bypassed and air passes through the unit without heat exchange. The system thus functions as forced fresh ventilation without heat recovery in summer, which is correct – you do not want the unit to heat your interior with outside heat. Some premium systems can also handle so-called night cooling – at night, when it is cooler outside, they increase performance and pre-cool the house without air conditioning.

Do I need a building permit for heat recovery?

In most cases, no – installation of a ventilation system without changes to the building envelope (except for small penetrations) does not require a building permit, but only a minor construction notice or no permit at all, depending on the local building office. For new constructions, ventilation is part of the required project documentation. For installation in an apartment block or adjacent walls, the situation is more complicated. Always check the current situation at your local building office or consult with a designer.

What is the aERO test (building envelope pressure test) and is it related to heat recovery?

The Blower Door test (airtightness pressure test) measures how much air leaks through the building envelope. For passive houses, the value is n50 ≤ 0.6 1/h, for low-energy houses ≤ 1.5 1/h. The tighter the envelope, the more necessary it is to have forced ventilation with heat recovery – natural airflow is simply insufficient. If your house has passed the Blower Door test and achieved n50 < 2.0 1/h, heat recovery is not only suitable – it is essential for a healthy indoor environment.

Can I combine heat recovery ventilation with underfloor heating or a heat pump?

You can not only – it is now a common and recommended combination. Heat recovery ventilation with a heat pump forms an energy-efficient whole. Some systems (so-called compact units) integrate a heat pump air-to-air directly into the heat recovery unit and allow heating or cooling of the supplied air with COP 3 – 4. However, heat recovery is not a replacement for heating – in severe cold and a well-insulated house, it can cover 20 – 40 % of heat losses, the rest must be provided by a full heating system.

Conclusion – what to take away from this article

Selecting a mechanical ventilation system with heat recovery is not an impulsive decision, but an investment for a generation. The key is proper dimensioning based on the actual needs of the house and its occupants, not based on the lowest price of the equipment. The type of system – centralized or decentralized – must be chosen according to the project phase and construction conditions, not according to the marketing preferences of one seller.

Pay attention to the quality of components throughout the entire chain: from the unit itself, through the ducts and seals, to the external outlets. Every weak link reduces the performance of the entire system. And above all – do not neglect regular maintenance. A system that you take care of will serve you reliably and efficiently for twenty or more years. A system that you neglect will cause problems from the first winter on.

If you are unsure about the selection or want to be sure that the proposed system meets your specific conditions, take a look at the entire mechanical ventilation with heat recovery category and other articles in the Knowledge Centre – especially Common questions about mechanical ventilation with heat recovery and Common faults of heat recovery units and how to eliminate them.

Do you have a question about this topic?

Having trouble making a decision or dealing with a specific situation in your home? Write to us – we will be happy to help.