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The Beginners Guide To Mechanical Ventilation with Heat Recovery

Heat recovery units

Heat recovery unit is an appliance in a mechanical ventilation system. It recovers energy from extracted air. Thanks to this we minimize the heat loss and costs of heating. However, the main and the most important function of a mechanical ventilation system with heat recovery is not only the energy recovery (or savings it gives) but the correct and effective air exchange in the building. What is more, HRU also filters the fresh air. Filter class used in HRU determine the efficiency of the filtration process. So the heat recovery unit is good for the household not only to minimize the heating system expenses but also for health reason. What else you should know?

What is heat recovery ventilation?

Heat recovery ventilation (HRV/HRS) is also known as mechanical ventilation with heat recovery (MVHR). The ventilation system is used to extract stale air from the rooms and replace it with fresh and filtered outdoor air while a significant part of the energy consumed to heat the house is recovered and transferred again to the premises. 

The whole process is similar to natural ventilation where ventilation system exhausts air from the so-called wet rooms like kitchen, bathroom and supply fresh air into all habitable rooms and bedrooms with one difference. The proper operation of natural ventilation depends on weather conditions and house airtightness.

In details, during spring-summer season, when the temperature difference between indoor and outdoor air is rather small the air flow is reduced or even stops completely. In autumn-winter season natural ventilation should operate properly if we assure the inflow of outdoor air to the premises, but the consequence will be a heat loss.

Mechanical ventilation with heat recovery (MVHR) still on many markets is only an alternative for natural ventilation as a primary solution, however, it becomes more and more popular every year. Why is it so? Properly designed and correctly mounted heat recovery ventilation system guarantees effective air exchange in the house all year round. In addition to the above, outside air is filtered before it is introduced to the buildings. Air filters are standard equipment of heat recovery unit. Moreover, if the outside air quality is poor, an anti-smog duct air filter can be additionally mounted before heat recovery ventilator.

Besides that, HRU has the heat exchanger, which enables recovering heat that normally would be exhausted from the building - the best units are able to recover up to 90% of heat. How does it work? In the MVHR system, we have two airflows (supply & exhaust) which pass one another parallelly within the heat-recovery ventilator without mixing physically. In the unit's heat exchanger heat from "stale" extracted air from the house is transferred to the "fresh" air brought from outside in order to warm air (if necessary).

The heat recovery system is widely recommended by HVAC associations and also more and more by EU legislators, as it perfectly suits the energy efficiency trend. If you are planning to build an energy-efficient or a passive house, mechanical ventilation with heat recovery is a mandatory choice. 

Heat Recovery Ventilation types

The typical mechanical ventilation system is built of heat recovery unit, ventilation ducts to distribute fresh, filtered air and exhaust stale air, air valves to supply and extract air in the rooms and intake and exhaust louvre in a wall or at a roof. However, there are other types of mechanical ventilation systems. We can divide mechanical ventilation systems with heat recovery into two groups depending on the type of heat recovery unit and solution of air distribution ductwork. 

  • Centralized heat recovery unit
  • Decentralized heat recovery unit

Centralized heat recovery unit is used to ventilate whole building (house), flat or group of flats. Such HRU is usually located connected to supply and extract ductwork system which distributes fresh air from outside and extract stale air from the premises recovering heat as well.

The second group represent decentralized ductless heat recovery units to ventilate the single room or small flat. Such appliances have a compact design, low airflow capacity and no need to install supply and extract air ductwork.

Additional, in ductless heat recovery units we can distinguish two solutions according to the type of heat exchanger installed: with a regenerative heat exchanger (so-called "push-pull" unit) and plate heat exchanger.

The "push-pull" units are based on the changeover cycle between supplying and extracting air at the same spot (ex. 70 seconds cycles). This kind of appliances can be connected in bigger systems to synchronize the operation (one unit works in supply mode, second in extract mode).

There are some issues to consider choosing this solution. First, there is a risk that supply and extract airflows might mix up. Another thing is that due to the unit's simple and compact design, the efficiency of heat recovery is smaller than standard centralized heat recovery unit, but on the other hand compact design is a great advantage in retrofit projects in old houses.

Decentralized heat recovery units with counterflow heat exchanger are very similar do centralized units. Of course, they are smaller and there is no possibility to connect ducts (there are some exclusions). Air is supplied and extracted within a single unit mounted on the external wall - extract air below supply air. 

How does heat recovery unit work?

Heat recovery unit is nothing else than an air handling unit equipped with heat recovery exchanger. It is designed for efficient and effective air exchange in the premises, in other words, to extract stale and supply fresh air. In the same time, it recovers heat from extracted air. 

  • Fresh air is drawn through the intake louvre from the outdoor. 
  • Then, via ductwork, the air is directed to the air handling unit. 
  • Within the unit, it is purified on filters.
  • Next, it heats up in the heat exchanger during the winter or cools down in the summer. 
  • After that, it is distributed all over the building through the ductwork system.
  • HRU also extracts air. 
  • In the rooms, the air is extracted by air valves or grills. 
  • Next, it is transported via dedicated extract air ducts system to the air handling unit.
  • Before air gets to HRU, air filters purify it. 
  • In the heat exchanger, heating or cooling energy is transferred from extract air to supply air. 
  • At the end, stale air is extracted to the outdoor through exhaust louvre. 

Heat recovery unit equipment

The efficiency of the heat recovery unit mainly depends on the quality and type of components it has been made of. When selecting the heat recovery system you should be aware of this. That is why it is good to know what function each of them handles and how they affect the efficiency of heat recovery system. 

Fans

Single HRU has two separate fans. One fan supplies air and the second one extracts it. They drive the air to move along through the ductwork system. The operation of fans has to be automated. What is more, they determine how much noise will the HRU generate and how long it will operate reliably. 

Fans are the most important components of every single air handling unit. Energy consumption, airflow capacity/available pressure and noise emission of HRU depends on them. The major of domestic HRU's are offered with radial fans, backward curved impellers and EC motors. In comparison to AC or DC motors, EC motors have lower energy consumption rate and wider speed regulation range.

In older units, AC or DC fans and forward-curved impellers can be found. Decentralized HRU's can be also equipped with axial fans.

Heat exchangers

A heat exchanger is the main element of the heat recovery units because it makes the functionality. It is responsible for extract air heat recovery. We can distinguish a few types of heat exchangers. The most popular is a crossflow heat exchanger, which suits well in small heat recovery systems for dwellings and offices. The efficiency of this heat exchanger is around 70%.

The rotary heat exchanger is a more advanced and more expensive solution. Its efficiency is also higher and might reach 90%. However, it generates more noise at operation. The highest efficiency is reached with counterflow heat exchangers. On the other hand, it requires installing more sophisticated control system. 

The majority of HRU's on the market are equipped with counterflow plate heat exchangers. Counterflow heat exchangers reach efficiency even over 90% but the production cost, when compared to the simple design of crossflow heat exchangers, is much higher. 

Control system

Control system is the heart of HRU. With a little interference of user, it will operate the unit at maximal efficiency and will regulate parameters automatically. Control system allows the temperature and airflow control as well as fans operation. HRU can be equipped with special probes and sensors to initiate fans start and stop sequence at an appropriate time. 

Casing

Simple and convenient maintenance of the unit is up to the casing. The clue feature is a good thermal and acoustic casing insulation. This allows attenuate unit operation and minimize noise level. Additionally, heat losses through the unit will be reduced. 

So, casing task is to keep all the components in the right place and guarantee appropriate thermal and acoustic insulation. Casing design will also determine duct connection spigots location (on the top/sides) and type of unit mounting (floor/stand, wall, under ceiling).

Another important thing is easy access to filters which need to be periodically replaced and to heat exchanger for cleaning. Quality installer or technician also will keep in mind convenient access to other components like fans and temperature probes, which might be necessary to replace in event of a malfunction.

There are different casing constructions, but the most popular are:

  • prefabricated sandwich panels “sheet steel-insulation-sheet steel” (insulation with mineral wool, PIR or PUR foam)
  • casing made of metal sheet steel (often painted) with internal construction elements made of EPS, XPS or EPP
  • self-supporting casing design made of EPP
  • casing made of metal sheet steel with expanded rubber insulation

Filters

Filters class determine how well air will be purified from dust and other impurities. Filters task is to stop all unhealthy substances and do not let them reach occupied rooms. In heat recovery unit separate filters are used for supply and extract airflow. It is also possible to mount additional filters for higher air filtration efficiency. 

The same principle applies to supply-extract air handling units. Filters are utilized for two reasons - the first one is to ensure good indoor air quality and the second is to protect internal AHU's components (fans, heat exchanger).

Accessory equipment 

Every unit can be always modified for better and more reliable operation. To do this you can choose extra accessories. HRU can be equipped with an additional filter (mentioned above) and pressure switch to control its dirtiness.

As an option preheater can be mounted to heat up cold outdoor air before it enters the heat exchanger or secondary heater to raise the temperature of supply air which enters the rooms. 

Last but not least, special humidity (RH level) or CO2 (carbon dioxide level) sensors will allow demand-controlled ventilation (DCV), regulation to actual needs. 

Important parameters when buying HRU

If you've already decided that You want to install a heat recovery system, before You'll select the heat recovery unit, you should know two basic parameters to make the right choice. What HRU airflow capacity do you need? What is HRU available pressure and why it is so important? 

Airflow capacity

HRU capacity will determine the optimal air exchange rate in the building. The appropriate airflow level has to be computed for supply and extract air. Normally air exchange rate in the house should be around 0.7 times per hour. How to calculate this? Multiply usable floor area by average room height. Of course that is a big simplification and detailed calculation should be performed by the architect based on room types, room volumes and occupants quantity. Above information is in accordance with EN 16798-1:2019 standard.

HRU available pressure or external static pressure

Many of HRU's buyers do not pay a lot of attention to unit available pressure. It's a mistake as this parameter plays an essential role in the unit as well as hear recovery system efficiency. You can imagine it as a force, which HRU generates to push the air along the ductwork system and draw air from outdoor. If the external static pressure is too low, air will not be able to reach the last air valves in the ductwork system.

As an example, on the market, there are offered HRUs with external static pressure less than 100 Pa but at very attractive price level. They have a common fault like a thermal overload of the fans, fans have to be replaced then, generating extra costs. What is more, such an appliance is characterized by very low energy efficiency and effectiveness, thus not fulfilling its function. 

Effects of Oversizing or Undersizing the HRU?

Let’s start from that a heat recovery unit should be oversized a little. The rule of thumb is to select HRU in such a way, that it would operate on its 70-75% of maximal capacity. This guarantee the backup airflow to use if needed, ex. when we have more guests or during party.

But the consequences of undersized in other words too small AHU are critical, simply our ventilation system will not fulfil its function. Last air registers in the ductwork system will not reach designed airflow, if the HRU will not have enough capacity or available pressure and that can lead to zero flow in the premise. You will have no air exchange in such a room.

Where to install the Heat Recovery Unit at home?

In the beginning, you should check manufacturer instructions about the location of the unit. Usually, they are similar for different models and manufacturers. That's because there are general rules about unit operation environment. Thus, the heat recovery unit should be mounted indoor, in space where the temperature is above zero degrees all year round and the air should be moderately dry, unless the manufacturer allows other conditions. A heat recovery unit should be installed in such a way that the ducts carrying the outside and expelled air are as short as possible. It will decrease the airflow resistance. This way you will achieve the best performance with low energy consumption.

That is why commonly heat recovery unit is located in the garage or other utility room. Before you decide about the location, check the air humidity there.  You should choose easily accessible space in event of periodic maintenance, repairs or just to verify the correct operation of components. Also, the HRU location should allow the later unit disassemble or replacement. 

People who live in a two-story house do not have to worry about giving up a room. Two-story houses usually have an attic. For the most part, it is not used for anything particular other than storage; thus, chances are that there is enough space for ventilation ducts.

When considering floor or stand type unit remember that it requires a solid floor. Smaller units can be installed on the wall even if it is a floor unit. You have to remember that it requires special mounting brackets for safe securing the unit to the wall. 

Remember that floor unit has to be mounted on an appropriate base with minimal hight to guarantee condensate drain. Drainage piping has to be installed with minimal descend towards the sewage system. 

How to install Heat Recovery Unit?

When installing a heat recovery unit, consider using as short as possible duct routes to the outdoor. You will also need to wire the control panel or connect the unit to LAN router/switch, unless you have chosen the unit with wireless communication. Always mount the appliance according to manufacturer instructions. You can choose from different types of mountings and position the unit in different directions. Using optional mounting brackets available for your unit you can mount it in alternative ways to suit the room. For example, you can suspend the unit under the ceiling.

Location of the air intake should ensure the coldest possible air temperature in the summer. Choose north faced and mount it at least 2 m over ground level. Exhaust air discharge has to be at minimum distance 1,5 m from air intake, best on another side of the building. The most popular method to plan ducting in house is the cascade approach. 

If you decide to install MVHR in a new house, you should consider it in the project at the very beginning. In retrofit projects, planning a heat recovery system is more complicated and requires greater construction works. Good idea then is to combine the installation of mechanical ventilation with heat recovery with other construction works or plumbing and HVAC systems modernization. 

How often replace filters in HRU?

There are few recommendations about periodic filters replacement. Some recommend replacing filters every 6 months, others every 3-4 months. But in general, this is a very individual issue and filters have to be replaced always when they get dirty. On the other hand, this requires regular periodic checks. So, how often check the condition of the filter...

It would be best to check filters every 2-3months. If they definitely look dirty, you have to replace them. Otherwise, the airflow will be not sufficient and filtration performance is reduced (dirty air gets to the premises). This means again, that HRU does not fulfil its functionality, its performance and effectiveness get worse significantly. 

To sum it up, are heat recovery systems worth it? For sure, the installation of mechanical ventilation with heat recovery in the house is a more expensive investment when compared to gravity ventilation. Yet, it guarantees heat recovery and continuous supply of filtered, fresh air all year round. This is the best solution for people who want to breath healthy air and ensure their family the best living comfort, as well as for people who like to reduce monthly bills for heating system and raise their comfort. 

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