The Ultimate Guide to Decentralised Heat Recovery Ventilation

Single room heat recovery units

Guide: Decentralised ventilation systems with heat recovery

Should I get a single room heat recovery unit for my house? This is a question commonly asked by our customers when they have to decide about the mechanical ventilation system with heat recovery (MVHR): centralized vs decentralised. It's a pretty understandable, the MVHR system costs are rather high (when compared to gravity ventilation) and the choice we make is made for many years. 

That is why when we're asked Should I get a single room HRU? not everyone gets an affirmative response "YES". Why? Because 1) it depends on various aspects and 2) there is no one perfect solution to solve all problems and suit all clients.

We've decided to collect frequently asked questions from customers in this complete guide book about decentralized heat recovery ventilation. We hope that after reading it, you will make the right decision Yourself and have no further doubts if through-wall heat recovery unit is the solution You are looking for.

How does Decentralized Heat Recovery Unit work? 

When browsing the Internet in the search for information and opinions about decentralised heat recovery unit, you can find different names that are used to describe decentralized heat recovery units. For example, we refer to them as single-room heat recovery unit or push-pull HRUs, other manufacturers use terms as decentralised mechanical ventilation unit, decentralised heat recovery system for single rooms, compact ventilation system with heat recovery, compact wall units, through-wall ventilation units, ductless heat recovery ventilation (HRV) system, etc. Our "favourite one" is a term used by adversaries of decentralized HRV - "hole in the wall" heat recovery unit. For sure, we cannot deny that wall opening is crucial to install the decentralised heat recovery unit. But the unit operation principle is much more complex...

The main principle of the decentralised MVHR system is the same as in the case of a standard centralised MVHR system, namely to supply fresh air and extract stale "dirty" air from the room. The difference is that the decentralised heat recovery systems are ductless, so the process of supplying and extracting air is conducted without ducting. While in the standard whole house HRV system we've got separate air distribution ducts: supply air and extract air ducting to distribute. 

Both airflows pass through a heat exchanger (frequently counterflow heat exchanger) where heat transfer between them take place. In the case of  "push-pull" type, wall-mounted decentralised heat recovery unit, it is equipped with regenerative heat exchanger. Air extracted from the room has its thermal energy. During the extract air mode, this energy is transferred to the mass of heat exchanger and it accumulates there. Normally one cycle lasts for 70 sec. Then fan stops, changes rotation direction and supplies fresh air. Because outside air is cold, heat exchanger heats it up with accumulated energy.

Does air flow mixing occur? There is no air flows mixing because cycles change one by one in time intervals, so the fan supplies air then extracts it with the pauses between cycles. In our single room heat recovery unit, there is only one fan that is why air supply and extract modes aren't performed simultaneously. There are on the market, however, solutions where wall-mounted HRUs have two fans and both airflows are distributed at the same time.

How does decentralised heat recovery ventilation works

Decentralised single room heat recovery units need to be located in every room.

We highly recommend the installation of decentralised heat recovery units in pairs, in two rooms or on the opposite wall of a larger living room area, to synchronize mvhr units. The synchronization of decentralised heat recovery units means that if one unit works in supply mode, the second one operates in extract mode and we achieve balanced air flow in the building. After the cycle ends each unit changes its operating mode from supply to extract/extract to supply.

In this way, we can achieve efficient and balanced ventilation in the total building space. On the other hand, if you have a limited budget and want to simply improve your comfort or humidity level in the room, a single, decentralised unit will be enough too. In theory, using a single unit that operates in changeover mode, supply and extract air stream can mix in very little proportion. That is why we recommend installing single room heat recovery units in pairs, but if this is impossible due to small space, only one external wall, a single unit will do its job anyway.

How to Install Single Room Hea Recovery unit? Check a step-by-step instruction manual. 

What is the difference between decentralized and centralized heat recovery ventilation?

The simplest explanation is that the centralized heat recovery ventilation is the ventilation system where one air handling unit (a heat recovery unit) ventilates whole building. Fresh air from the outside is distributed to each room and stale air is extracted through a network of ventilation ducts, air valves, external intake/outlet and other air distribution components. 

In the case of decentralized heat recovery ventilation, there is no ductwork installation and other elements of the air distribution system. The decentralized heat recovery system is limited only to one or a few ductless heat recovery units. Considering decentralized heat recovery ventilation it is necessary to install recovery unit in each room because there is no ventilation ductwork installation through which fresh air is transported to the rooms and stale air is extracted to the outside.

Air filtration

Decentralized heat recovery units cannot compete with a standard mechanical ventilation system with heat recovery for higher air filtration efficiency. Why? Decentralised heat recovery units usually have small air flow capacity and very limited static pressure. High filtration class filters, like HEPA filters, cannot be mounted there due to the unit's construction limitation. Our decentralized heat recovery unit utilizes basic coarse filter which stops only big dust fractions.

Air filters for decentralised heat recovery unit HRU-WALL Air filters for heat recovery unit HRU-PremAIRAir filters for heat recovery unit HRU-MinistAIR
Anti-dust filters for decentralised heat recovery unit - HRU-WALL series.  Air filters for the heat recovery unit
Air filters for the heat recovery unit 

You could ask why can't we just simply replace the coarse filter with a fine filter? As a rule, a single room heat recovery unit is designed to have extremely low power consumption (e.g. HRU-WALL model - 3-5 W) and has to be ultra silent because you install it in the bedroom, living room and another household's occupied spaces.

Replacing the anti-dust filter with high-efficiency fine filter, e.g. HEPA air filter in the decentralised hrv unit would involve changing the construction of the whole device, the installation of a more powerful fan which at the same time generates more noise. If the high air quality in your house is the main or extremely important role of Your MVHR system, then a decentralized heat recovery unit is not the best choice for you. 

Looking for high-efficiency air filters for the MVHR system?  What is an Electrostatic Precipitator and how does it work? or Smog filter box MOCarz

Heat recovery unit location

If you decided to install centralized heat recovery ventilation you have to set the air handling unit in a separate room. You have to locate it as far as possible from bedrooms that the noise from the unit does not disturb the sleep of householders. On the other hand, the location should allow easy and convenient access to the device for filters replacement and service or maintenance works. Typically, air handling unit is mounted on the attic, in the garage, laundry or storage room. 

Single room heat recovery units assembly Single room heat recovery units installation

Decentralised, ductless heat recovery units are installed inside the wall.

In case of wall-mounted single room heat recovery unit you just need a little space on an external wall, while most of the hrv unit is hidden inside the wall. It is worth to remember that with centralized HRV you need only one device and with decentralized HRV good practice is to install them in pairs. This means mounting two or more devices depending on house area and occupants quantity. As the minimal requirements, you should consider installing a wall-mounted decentralised heat recovery unit in each bedroom.


Comparing controls of centralized and decentralized HRV systems both systems allow the possibility to select the fan speed. The most popular solution is with 3 fan speed modes. In centralized HRV you have much more options variety for regulating the airflow volume. On the other hand, commissioning centralized heat recovery ventilation and adjustment of the airflow in ductwork is much more complicated and time-consuming since we have to regulate the ventilation system for the whole building.

With decentralized heat recovery unit, you just consider the unit's capacity for an individual room so it is easier to calculate airflow of the fresh air.

HRV controller Heat recovery unit controller Remote controller for HRU-WALL-RC
Dedicated 3-button controller (surface-mounted) HRU-WALL-CONTR-I Dedicated 3-button controller (flush-mounted) HRU-WALL-CONTR Remote controller for 

The general rule for how both systems work is the same. We can control fan speed and add external sensors. The main difference is in the complexity of the airflow regulation process which defines the control level we have over AHU operation.

What about CO2 and RH sensors connection? There's no doubt that wall-mounted decentralised heat recovery unit is a simple device, you install it in the single room it works only locally and that is why the sensor installed in this room works only with this individual unit. Regarding centralized HRV system, as an example, if we utilize sensors in three different rooms when one of the sensors reads control value which exceeds the limit threshold, the AHU will increase or decrease airflow for the whole building. In this case, one-room influence the operation of the MVHR system, and this could be energy inefficient.

How to control heat recovery ventilation? Check the best solution for HRV control. 

Centralized vs decentralized heat recovery ventilation

Let assume a hypothetical situation in which a customer asks us about a complete mechanical ventilation system with heat recovery. At the moment he is at the designing stage of the house and has no budget limitation. Which system we will recommend to him as a manufacturer of both - centralized and decentralized MVHR system? In this case, we will suggest a centralized heat recovery ventilation system. Why? 

Comfort and IAQ

Standard centralized ventilation systems with heat recovery assure higher comfort level and better indoor air quality. For example, in the MVHR system, we can additionally protect the heat exchanger from freezing and avoid moist condensation by simply insulating ventilation ducting. What is more, the standard heat recovery unit has higher heat recovery efficiency thanks to the counterflow heat exchanger.

Mechanical ventilation system with heat recovery mvhr

A complete mechanical ventilation system with heat recovery consists of  1) a heat recovery unit; 2) semi-rigid non-metallic ducting;
3-4) plenum boxes; 5) distribution boxes; 6) sound attenuators; 7) rigid, metallic ducting and duct fittings 8) bends 9) reducers
10) couplers; 11) suspension rings; 12) air vents; 13-14) air valves 15) mounting band; 16) sealing tape 17) flexible ducting,
18) female couplers; 19) end caps for non-metallic ducting; 20) end caps for metallic ducting. 

In addition to above, if you require even higher comfort, it is possible to install in the ventilation system a preheater which will heat up cold outside air in the winter before the heat exchanger or/and add an extra secondary heater to heat up supply air to improve the comfort level. With decentralized heat recovery ventilation, you do not have additional options, you cannot connect any air heater. Because ceramic, regenerative heat exchanger has lower heat recovery efficiency, the comfort level could be slightly lower.

To sum it up, if you've just started to plan your new house we recommend doing proper plans of centralized mechanical ventilation system because this system gives you more possibilities and has higher efficiency. Any time, you will be able to add extra sensors in each room and stepless control the HRU. That is true that investment costs are higher, but also the benefits are considerably higher: higher comfort and general system efficiency, higher indoor air quality and a healthy environment.

New building or retrofitting

We also observe the same market trend, that standard centralized heat recovery ventilation is more popular in new buildings. While, decentralized HRV systems excel in refurbishment, retrofitting projects where natural ventilation is inefficient. Wall-mounted HRUs are mainly used to solve problems with moist, the major cause of mould and mildew. That is why decentralized heat recovery ventilation is so popular in older/existing buildings, old houses and tenement houses where natural ventilation does not work or works incorrectly.

When are decentralised heat recovery units recommended?

It truly depends on the individual case. Our experience shows that the main reasons for choosing decentralised heat recovery units are: lack of space for supply/extract ductwork installation, faulty natural ventilation (mould and mildew) and simple installation. 

Let’s take a closer look at the issue of space for HRU. What do we mean by that?  Lack of space for large device and complete centralized ventilation ducts network is a common problem for already built and habited houses where such installation was not considered. Another case concerns projects, where the decision to install the MHRV system was made during the construction works e.g. after floor screeds. This makes it impossible to hide the ducts under the floor.

Dimentions of heat recovery unti HRU-PremAIR Dimensions for decentralised heat recovery unit HRU-WALL
Dimensions of heat recovery units
Dimensions of a decentralised heat recovery unit  

In the advanced stage of the construction works or in the already inhabited house it will be a great challenge for installer and householders to mount device size of washing machine and route ductwork installation without complete demolition inside the house. That is why it is so important to implement MHRV installation in the house project design stage. At that moment you can easily decide where to locate the unit and how simply and cost-efficiently route the ductwork network.

The cost of a heat recovery system is the next reason why customers sometimes decide to choose decentralized HRV. Comparing only materials and device costs without labour costs, a set of few wall-mounted single room heat recovery units will be less expensive than complete centralized MHRV system.

Lastly, the customer decides to install a decentralized system, because it's a compact and simple solution both in the aspect of installation as well as operation. To mount a decentralised heat recovery unit you just need to prepare an opening in an external wall and connect the power supply to each unit.

How many decentralised heat recovery units to select?

Single room heat recovery unit's sizing is made similarly to a standard centralized HRV system. There are different methods to select the HRV system, our way is to consider the occupant's number and room volume. If we already know how much fresh air we need e.g. 20, 30, 50 m3/h, just select MVHR unit which will manage this capacity. We suggest selecting HRU for capacity at second speed and to use max. speed for extra ventilation (boost). The same principles are used for centralized HRU which should be calculated for an HRV run at 70% of nominal capacity.

As an example let’s take a model HRU-WALL-150-60 decentralised heat recovery unit with its nominal capacity 60 m3/h at max. speed, and 45 m3/h at second speed and that should be our reference airflow to select this unit. Considering selection for the living room with two occupants, the capacity of 40m3/h is enough for acceptable comfort and air quality. Of course, it would be ideal to select the quantity of HRUs according to room volume and occupants number, but the single unit will also slightly improve indoor air quality and room comfort if you have a limited budget or wall space.

If You're looking for more tips and instructions about the MVHR (mechanical ventilation with heat recovery), check also the following articles:

  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 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 Ventilation Systems
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05-552 Wola Mrokowska

tel. +48 22 737 40 00