Bellow is an introduction to MVHR  copied from

The international Passivhaus Standard does not explicitly require mechanical ventilation. And yet almost every certified Passivhaus building includes a mechanical ventilation system with heat recovery.

What is mechanical ventilation with heat recovery (MVHR) and why is it needed in Passivhaus buildings?

Mechanical ventilation is familiar to most people. This leads to the assumption that a Passivhaus ventilation system is the same as other mechanical ventilation systems. It is not.

There are things that MVHR does not do. It is important to know what these are in order to understand the difference between Passivhaus MVHR and other types of ventilation.

And there are things that Passivhaus MVHR does do that other ventilation systems don’t, including ‘natural ventilation’. It is important to know what MVHR does do, as MVHR is vital to the consistent success of the passivhaus standard.

Mechanical ventilation with heat recovery (MVHR) is key to delivering the benefits the Passivhaus Standard promises – radical energy efficiency and exceptional comfort.

Mechanical ventilation is simply a duct coming into the building with a fan blowing fresh air in and a duct going out of the building with a fan blowing stale air out. Since the fans are powered and controlled, the ventilation is considered “mechanical” as opposed to a ventilation system with no power and no control such as ‘natural ventilation’. There is probably mechanical ventilation in some form in the building you are in right now – such as mechanical extract ventilation from a bathroom or kitchen.

Heat recovery is using a heat exchanger that takes the heat from warm air and gives it to cold air. The air inside a building is typically warm because it has been heated in some way so that the rooms are comfortable to inhabit. On the other hand, outside air is typically colder than indoor air for a large part of the year. Often, outdoor air is colder overnight all year round. This is the case even in climates we generally consider to be warm. By using a heat exchanger, fresh cold outside air can be warmed up to a comfortable temperature by using the heat of the warm stale indoor air as it is extracted.

The two air paths never come into contact with each other, so stale air is not mixed with fresh air. No contamination or pollution from the extract air can get into the fresh supply air. Only heat is transferred in the heat exchanger. (Moisture can also be transferred in an energy or enthalpy exchanger, but that isn’t the subject of this post.)

Passivhaus mechanical ventilation with heat recovery is balanced whole house (or building) ventilation.

The amount of air that goes into the building through the MVHR matches the amount of air that comes out of the building through the MVHR. This gives control over the air movement and means that the maximum amount of heat can be recovered from the outgoing air. This is one of the reasons a Passivhaus building must be airtight. You can still open windows and doors whenever you like, though. Opening a window is a choice you make and in your control. You have no choice or control over infiltration and air leakage through a building envelope that is not airtight!

MVHR is also known as heat recovery ventilation (HRV) and mechanical heat recovery ventilation (MHRV). In the context of a Passivhaus building, these terms all mean the same thing.

Mechanical ventilation with heat recovery in a Passivhaus building typically looks like the following:

  • On the external façade, there are two grilles. One is for incoming fresh air and one is for exhaust stale air.
  • Behind each of the grilles is a short length of duct going directly to the MVHR unit. These ducts both have cold air inside. One duct has cold fresh air from the outside being blown into the building. The other duct has cold stale air being blown out of the building. Even though the latter has air from inside the building where it is warm, the heat has already been exchanged at this point so the air is cold. These two (cold) ducts need to be insulated from the (warm) inside of the building; they are effectively part of the thermal envelope. The insulation needs to be vapour impermeable to prevent condensation forming on these ducts. And, as part of the thermal envelope, these ducts also need to be airtight, sealed to the airtight barrier of the external fabric they pass through and sealed to the MVHR unit.
  • These two ducts connect to the MVHR unit, where the air paths are also insulated.
  • Inside the MVHR unit, the fresh incoming air passes through a filter to remove pollen, debris and pollution. This protects the heat exchanger from blocking and also ensures the indoor air is clean and healthy.
  • Both supply and exhaust air streams pass through a counterflow heat exchanger. A counterflow heat exchanger can recover between 75 and 95% of the heat compared to a non-counterflow heat exchanger, which can only recover a maximum of 50% of the heat. Many MVHR units also have a by-pass function so that in summer when the outdoor air is warm, heat doesn’t need to be recovered. The “summer by-pass” function reduces any risk of overheating and is recommended!
  • Both air streams have a small fan in them to blow the air in the right direction.
  • Two ducts connect to the other side of the MVHR unit. One of these ducts goes to all the habitable rooms of the building to deliver fresh air. In a house, these are living rooms, dining rooms, bedrooms etc.. The other duct comes back to the MVHR from all the rooms where air is extracted. In a house, these are kitchens, bathrooms, toilets etc.. Both ducts may run from room to room, or branch out several times to reach all the necessary rooms.
  • Extract air from the rooms also passes through a filter to remove any airborne dust and debris before entering the heat exchanger to preventing it from blocking.
  • Both ducts have silencers on them close to the MVHR unit so that the sound of the fans, although very quiet, does not travel along the ducts and into the rooms. The international Passivhaus Standard requires that MVHR units are for all intents and purposes completely silent in all habitable rooms. The humming sound of a fan, even if it is very quiet, is uncomfortable and undesirable.
  • Where a duct passes between different rooms, the duct has a “cross-talk” silencer to ensure that sounds don’t transfer between the rooms. If either duct passes through a fire rated part of the building, a fire damper is fitted.
  • Where the ducts supply or extract air from rooms, there is a terminal on the wall or ceiling. The terminal can be a grille or another suitable kind of fitting.

Early in the development of the international Passivhaus Standard, there were several examples of buildings that also incorporated a “ground heat exchanger” (or “earth tubes”). Is this system, the fresh air passes through an underground duct to be pre-warmed by the ground, which is a stable temperature all year round, before it gets to the MVHR. This type of system is not necessary or recommended in Passivhaus buildings now.

There are several excellent examples of Passivhaus buildings in the book ”Passive House Design – Planning and design of energy-efficient buildings”. (Read my review here.) In each case, the MVHR system is clearly described and illustrated in drawings and diagrams. If you want to learn more about this subject, I highly recommend this book.