MVHR system - 12 most common installation mistakes

Heat recovery ventilation

Heat recovery ventilation or mechanical ventilation with heat recovery (MVHR) is the most efficient and sustainable type of ventilation system. It provides fresh and clean air regardless of temperature and weather conditions through controlled and energy-efficient air exchange.

Unfortunately, there are systems of different quality. Furthermore, incorrectly installed heat recovery ventilation can mean a significant decrease in the capacity of even the best heat recovery unit on the market. Installation mistakes do happen, so it is very important to know what we should pay special attention to and how to avoid them.

No professional design of mechanical ventilation

Installation of a mechanical ventilation system with heat recovery should always be preceded by a detailed design, prepared individually for each building. Suitable pipe diameters and their optimal routing can only be planned with years of experience and knowledge.
mvhr design
It is due to properly distributed exhaust and supply ducts that the required volume of air is provided to each room. A well-prepared design allows to avoid many problems that may be encountered by both the installer and user. The basic data that should be included in the design are:

Design supply and exhaust air flow rates

The air flow rates of both streams should be equal because then the system achieves the maximum heat recovery efficiency. The air volume should be designed taking into account the number of the house inhabitants and the layout of the rooms.

An air flow rate which is too low will result in insufficient ventilation manifested by too high humidity in the rooms or a high concentration of impurities. An air flow rate which is too high may cause excessive drying of the rooms during the heating season and generates higher electricity consumption for the operation of fans.

Routing of ventilation ducts

This can help avoid the crossing of ducts, which is very important when they are run in the ceiling, screed or suspended ceiling. It also reduces the risk of collision with other systems.
heat recovery ventilation ducting

Selection of endings of air intake and exhaust vents

The endings of air intake and exhaust vents should be selected in such a way so as not to cause excessive flow resistance. Often, a size larger than the diameters of the nozzles installed in the heat recovery unit is selected from the ventilation size range to reduce resistance.

Distance between air intake and exhaust vents

Placing the air intake and exhaust vents close to each other may cause potential air pollution, so a practical rule should be followed to maintain at least 1.5 m distance between the components ending the ventilation system.

Selecting the right air handling unit

The device should be selected according to the designed air flow rate and calculated system resistance. Only then is it certain that the operating point will be within the characteristics of the unit.

The heat recovery unit should be selected so that the air flow rate calculated based on the design is achieved at an air handling unit capacity of no more than 70%. This gives a certain reserve capacity for quick ventilation of the rooms (boost mode) and prevents the unit from emitting excessive noise.

Each heat recovery ventilator should also have the ability to control the air flow rate and adjust it to the user's needs, as it is supposed to provide comfort, which is individual for each of us.

Undersized heat recovery unit

If you choose a heat recovery ventilation unit with too little capacity, the unit will not perform its function. Too low capacity/compression ratio can lead to the situation in which the air valves located in the greatest distance from the air handling unit will have too little or no airflow. This means that no air exchange will occur in such a room.

That is why it is so important to select a heat recovery unit with the right parameters for your home.

Impact on the heating system

Installation of mechanical ventilation with heat recovery affects the demand for heat for heating the rooms. Taking this into account in the calculations reduces the design heat demand, so that the heat source power and the power of radiators can be decreased.

Heat recovery unit installed in an unheated room

Heat recovery ventilation units have an insulated casing, but this does not mean that a heat recovery unit can be installed in an unheated room. Most manufacturers of heat recovery ventilation air handling units precisely define the conditions that should be met by the room in which a heat recovery unit can be installed in a safe way.

Most often, the device is installed in the attic, garage or another technical room. Regardless of the location you choose, such as a non-habitable attic, it must be insulated – the device should be installed in the room where the temperature does not fall below 0oC.

No insulation or not properly insulated ventilation ducts

Air ducts are insulated for two reasons: to prevent heat loss (usually when ducts with warm air go through unheated rooms) and to prevent steam condensation on the duct walls (when ducts with cold air go through heated rooms). Inaccurate duct insulation causes unnecessary heat loss or condensation of moisture that will flood the surrounding room.
epp ducts

Air intake and exhaust ducts must be insulated. Furthermore, all ventilation ducts behind the unit that go through unheated rooms should be covered with insulation to prevent condensation of moisture.

System made of flexible ducts

Flexible ducts are flexible ventilation pipes made of steel sheet and plastic, available in two versions – insulated with mineral wool and uninsulated. HVAC installers often choose them because of their relatively low price and simple installation, especially in narrow/cramped spaces.

Flexible ducting has low mechanical strength, is fragile and prone to damage or breakage during installation and use. Another potential problem is the duct inner wall, as all kinds of dirt can accumulate in its irregularities over time. Cleaning of flexible ducts is difficult or, in fact, virtually impossible.
flexible ducts

A common mistake is to use this type of ducts on longer sections of the ventilation system. Understandably, it is much easier to run a network of flex ducts than rigid steel ducts, especially in a confined space such as under a ceiling, in a corner, or where there are also other systems. In such a situation, installers bend flexible ducts at large angles to a great extent, which has a destructive effect on the heat recovery unit's capacity. Flexible ducts, due to the lack of smooth internal structure, generate considerable pressure resistance, so we do not recommend them for installation directly at the heat recovery unit or on sections longer than 4 m.


We don't recommend the use of flex ducts in mechanical ventilation systems with heat recovery. If it is not possible to eliminate them completely, we recommend limiting their use to a minimum.

No access to the MVHR system

Access to the entire system is not required. Often it is not even possible when the ducts are hidden in the ceiling or screed. However, access to certain parts of the system such as distributors, control dampers, supply and exhaust points is required. This is necessary to adjust the system and to clean it periodically.

No HRV system commissioning

Installation of heat recovery ventilation does not end with connecting the heat recovery unit to the rest of the system. Many people think that after installing the system, it is enough to start it with default settings. In this case, however, it may cause the system to work without you seeing the benefits. Proper start-up will ensure that the heat recovery unit meets the specified airflow requirements in the most efficient way, which should reduce the noise as well as energy bills.

The first start-up of the device should be performed by a qualified employee. It is their responsibility to make the necessary adjustments, so that the air flow rates in all rooms are consistent with the design assumptions. Then, the operation of all controllers and any optional accessories should be checked. Finally, the installer should train the user in the basic operation of the device.
Every heat recovery ventilation ventilation system requires the first commissioning to be performed by a qualified technician of HVAC systems.

Improperly installed ventilation ducts

When installing mechanical ventilation, it is extremely important that all its ducts are properly laid, insulated and connected. In any other case, the system capacity will decrease significantly, reducing the heat recovery rate and the device efficiency.

Connecting a garage/boiler room to the system

A garage and boiler room should not be connected to the mechanical supply and exhaust ventilation system distributed in the rest of the house. In these rooms, gravitational ventilation will perform its function well. Additionally, it is a good idea to install tight doors there to prevent air from these rooms from entering the house (as it may contain flue gas or carbon monoxide).

Improperly installed air intake and exhaust vents

Improperly installed air intake and exhaust vents can cause precipitation water to get inside the ventilation ducts. Another mistake is not using a special drip cap when installing the grille on the facade, which prevents the formation of water stains on the wall of the building.

Improperly installed pre-heater

A duct filter should be installed in front of the electric pre-heater to protect the device from particulate matter, dust and dirt, as there is a risk of overheating or even, in theory, catching fire.
preheater for hrv
When installing a water air heater as the pre-heater, an antifreeze system equipped with a thermostat should be provided. Otherwise, there is a risk that the heating medium will freeze in the heater in the rooms where the temperature is below 0˚C.

No duct silencers

The "disadvantages" of heat recovery ventilation presented by its opponents often include, among other things, the allegation that heat recovery ventilation is noisy. However, a noisy mechanical ventilation system with heat recovery is not caused by the general idea of heat recovery ventilation, but by a poorly selected heat recovery unit or, most often, errors in the design of the system and its installation.

The cause of noisy ventilation system may be, for example, an incorrect selection of the diameters of ventilation ducts. If the duct diameter has been selected too small, it will result in excessive speed in the system and, consequently, noise.

Another mistake, or rather an action not in accordance with good practice, is the lack of duct silencers in the heat recovery ventilation system. Although nowadays manufacturers of heat recovery ventilators use all available solutions (insulation of the casing, ultra-quiet fans, etc.) to make the device silent, every heat recovery unit will generate noise towards the ventilation ducts.

Therefore, to increase the acoustic comfort of the house inhabitants, we recommend installing a silencer downstream of the heat recovery unit.

*Window joinery

In the case of mechanical ventilation with heat recovery systems, window ventilators should not be installed. Windows should be tight and preferably installed using the "warm window sill installation" technique. This improves the airtightness of the building and prevents the formation of thermal bridging at the frame-wall contact point.

No doors with undercuts

If the system is designed in a typical way, i.e. the air intakes are in living rooms and bedrooms, whereas the exhausts are in wet rooms (bathroom, toilet, kitchen), the doors should be equipped with ventilation slots, allowing air circulation from the intake to the exhaust.

The slot net area for bedroom and living room doors should be at least 80 cm2, while for bathroom and kitchen doors – at least 200 cm2. If it is not possible to use doors with ventilation undercuts, you can apply transfer grilles or install the supply and exhaust in each room.

Would you like to know more? Read other articles on heat recovery ventilation:

  1. How can You install heat recovery unit in the ready-built house?  - the article about decentralized heat recovery systems.
  2. What is an Electrostatic Precipitator and how does it work? - the article about the highest filtration efficiency solutions.
  3. 14 Things You Need to Know about PremAIR - pretty self-explanatory title about the best HRU for clients looking for "premium" solutions. 
  4. Air distribution system for the MVHR - overview of FLX-REKU system: radial, semi-rigid ducting, plenum and distribution boxes, etc.
Alnor Systemy Wentylacji
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tel. +48 22 737 40 00