What Is MVHR? Heat Recovery Ventilation Systems Explained
What is MVHR?
MVHR stands for Mechanical Ventilation with Heat Recovery. It is a whole-house ventilation system that continuously removes stale, humid air from a property and replaces it with fresh, filtered air from outside.
An MVHR system is different from a standard extractor fan because it is designed to ventilate the whole home rather than one room at a time. It extracts air from areas where moisture, odours and pollutants are commonly produced, such as kitchens and bathrooms, while supplying fresh air to bedrooms, living rooms and other regularly occupied spaces.
The heat recovery part of the system helps reduce the amount of warmth lost through ventilation. Before stale air is discharged outside, it passes through a heat exchanger inside the MVHR unit. Heat from the outgoing air is transferred to the cooler incoming air without the two air streams mixing.
This allows a heat recovery ventilation system to provide a steady supply of fresh air while retaining more of the heat that has already been generated inside the home.
What Does MVHR Mean?
MVHR stands for Mechanical Ventilation with Heat Recovery.
Each part of the name describes an important function:
- Mechanical ventilation means that fans are used to move air through the property in a controlled way.
- Heat recovery means that heat from the outgoing stale air is transferred to the incoming fresh air.
- Whole-house system means that the ventilation is designed to serve multiple rooms throughout the property.
You may also see MVHR described as a heat recovery system, heat recovery ventilation system, whole-house heat recovery system or mechanical ventilation system with heat recovery.
How Does an MVHR System Work?
An MVHR system uses a central heat recovery unit connected to a network of ducts installed throughout the property.
The system normally has four main air paths:
- Fresh outdoor air is drawn into the MVHR unit.
- The incoming air passes through a filter.
- Stale air is extracted from inside the home and brought back to the unit.
- The stale air is discharged outside after heat has been recovered from it.
Inside the MVHR unit, the incoming and outgoing air streams pass through a heat exchanger. The air streams do not mix, but heat energy moves from the warmer air to the cooler air.
During colder weather, warm air extracted from a bathroom, kitchen or utility room can be used to raise the temperature of the fresh air entering the home. This reduces the amount of heat that would otherwise be lost when ventilating the property.
An MVHR system does not replace a boiler, heat pump or other heating system. It does not create heat by itself. Instead, it helps the home retain more of the heat that has already been produced.
Which Rooms Does an MVHR System Supply and Extract From?
A properly designed MVHR system moves air through the home in a controlled direction.
Fresh, filtered air is normally supplied to habitable rooms, such as:
- Bedrooms
- Living rooms
- Dining rooms
- Home offices
- Other regularly occupied spaces
Stale air is normally extracted from wet rooms, such as:
- Kitchens
- Bathrooms
- En-suites
- Utility rooms
- WCs and sanitary accommodation
The fresh air supplied to bedrooms and living areas moves through the property before reaching the extract rooms. This helps remove moisture, cooking odours and indoor pollutants close to where they are produced.
Air usually moves between rooms through gaps beneath internal doors or other suitable transfer paths. The exact airflow required in each room should be calculated for the individual property rather than estimated from the number of rooms alone.
What Are the Main Benefits of MVHR?
Continuous Fresh Air
An MVHR system provides a regular supply of fresh air without relying entirely on open windows, trickle vents or uncontrolled draughts.
This is especially useful in modern homes that have been designed or upgraded to reduce unwanted air leakage.
Reduced Ventilation Heat Loss
Traditional ventilation methods allow warm indoor air to leave the property without recovering its heat.
A heat recovery ventilation system transfers a proportion of the heat from the outgoing stale air to the incoming fresh air. This can reduce ventilation-related heat loss, particularly during colder weather.
Improved Indoor Air Quality
Everyday activities such as showering, cooking, cleaning and drying clothes indoors can add moisture and pollutants to the air.
By continuously removing stale air and supplying filtered outdoor air, an MVHR system can help create a fresher and more comfortable indoor environment.
Moisture and Condensation Control
MVHR extracts humid air from bathrooms, kitchens and other wet rooms before it has as much opportunity to spread through the home.
This can help reduce condensation caused by excess indoor humidity. However, a heat recovery system will not fix moisture problems caused by leaks, rising damp, water ingress or defects in the building fabric.
Filtered Incoming Air
The filters inside an MVHR unit help reduce the amount of dust, pollen and airborne debris entering the home through the ventilation system.
The level of filtration depends on the unit and filter type selected. Filters must be checked and replaced when required to remain effective.
Less Reliance on Open Windows
A heat recovery ventilation system can provide background ventilation while windows remain closed.
This may be useful for homes near busy roads, noisy areas or locations where occupants prefer not to leave windows open, windows can still be opened whenever additional ventilation is wanted.
What Are the Limitations of MVHR?
MVHR can provide effective ventilation, but it is not suitable for every property or every problem.
MVHR Requires Space
A whole-house heat recovery system needs space for the central MVHR unit, ductwork, air valves, external terminals and maintenance access.
This is normally easier to plan in a new build or major renovation, installing MVHR in an existing home may be more challenging where ceilings, walls and floors are already finished.
Poor Design Can Reduce Performance
An incorrectly sized MVHR unit, restrictive ductwork, excessive bends or poorly positioned air valves can reduce airflow and increase noise.
The system must be designed, installed and commissioned correctly to achieve the intended ventilation rates.
Maintenance Is Required
MVHR is not a fit-and-forget product, filters collect dust and debris over time and must be checked, cleaned or replaced according to the manufacturer’s instructions.
Ignoring maintenance can reduce airflow, increase fan energy use and affect the quality of the air supplied to the home.
Is MVHR Suitable for Every Home?
MVHR is most commonly installed in new-build homes, self-build projects and energy-efficient properties with relatively low levels of uncontrolled air leakage.
As a home becomes more airtight, it becomes increasingly important to provide a planned and reliable source of ventilation. A well-designed MVHR system can provide fresh air without depending on gaps, draughts or windows being left open.
A heat recovery ventilation system can also be installed in an existing home, but the property must be assessed carefully. There must be enough space for the MVHR unit and ductwork, and the disruption involved in creating duct routes should be considered before installation begins.
In an older or very leaky property, outdoor air may continue to enter through uncontrolled gaps in the building. This can reduce the benefit of recovering heat through the mechanical ventilation system.
Before choosing MVHR, consider:
- The airtightness of the property
- Whether suitable duct routes are available
- The location of the MVHR unit
- Access for filter changes and future servicing
- The amount of installation work required
- The airflow required in each room
- Noise levels in bedrooms and living spaces
- The building regulations that apply to the project
Ventilation requirements differ across England, Wales, Scotland, Northern Ireland and the Republic of Ireland. Always check the guidance that applies to the location of the property.
Choosing an MVHR Unit
Choosing an MVHR unit involves more than comparing prices or maximum airflow rates.
The selected unit must be suitable for the property’s required airflow, available installation space, duct layout and maintenance needs. You should also consider efficiency, noise levels, controls, filter availability and the position of the duct connections.
A complete MVHR system may require more than the heat recovery unit itself. Depending on the installation, additional products can include:
- MVHR ducting
- Manifolds and distribution boxes
- Air valves and diffusers
- External grilles and terminals
- Silencers
- Insulation
- Controllers
- Replacement filters
- Fixings, seals and accessories
Ventilation Superstore supplies a range of MVHR units, heat recovery ventilation products, ducting and accessories for domestic ventilation projects.
Browse our MVHR range online to compare heat recovery units and find the components required for your system.
