What Is an MVHR System?

 

An MVHR system is a whole-home mechanical ventilation system incorporating heat recovery.

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Stale air is normally extracted from rooms that generate moisture and odours, such as:

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• kitchens;

• bathrooms;

• shower rooms;

• utility rooms; and

• WCs.

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Fresh outside air is simultaneously drawn through the MVHR unit, filtered and supplied to rooms such as:

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• bedrooms;

• living rooms;

• dining areas;

• studies; and

• other occupied spaces.

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Inside the heat recovery unit, the two air streams pass through a heat exchanger.

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The outgoing and incoming air do not normally mix, but heat from the warm extracted air is transferred to the cooler incoming fresh air before it enters the property.

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How Does Mechanical Ventilation With Heat Recovery Work?

 

A typical mechanical ventilation heat recovery system includes:

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• a central MVHR unit;

• supply and extract fans;

• heat exchanger;

• air filters;

• ductwork;

• ceiling or wall terminals;

• external air intake;

• exhaust outlet;

• controls; and

• condensate drainage.

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The system runs continuously at relatively low speed to maintain controlled ventilation throughout the property.

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When additional ventilation is required, for example during cooking or showering, the system can operate at a higher boost rate.

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This provides a much more controlled approach than relying entirely on opening windows or uncontrolled air leakage through the building.

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What Is a Heat Recovery Ventilator?

 

The term heat recovery ventilator, or HRV, is commonly used to describe equipment that extracts stale indoor air while bringing fresh air into the property and transferring heat between the two air streams.

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In the UK, the terms HRV and MVHR are often used when discussing similar types of whole-home heat recovery ventilation.

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The important principle is that the home receives controlled fresh air without throwing away as much of the heat contained in the extracted air.

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Why Is Ventilation Important in an Energy-Efficient Home?

 

Improving insulation and reducing draughts can significantly reduce heat loss.

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However, as a property becomes more airtight, the amount of natural ventilation entering through gaps and cracks is also reduced.

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Without suitable ventilation this can contribute to:

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• condensation;

• excess humidity;

• stale air;

• cooking smells;

• mould;

• poor indoor air quality; and

• uncomfortable bedrooms and living spaces.

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A greener home therefore needs to consider ventilation and insulation together.

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Making a building airtight without providing adequate ventilation can create a different set of problems.

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What Are the Benefits of MVHR?

 

A properly designed MVHR ventilation system can provide several potential benefits.

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These may include:

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• continuous fresh air;

• reduced ventilation heat loss;

• filtered incoming air;

• improved humidity control;

• removal of stale air and odours;

• reduced condensation risk;

• improved indoor air quality;

• greater control over ventilation;

• less reliance on opening windows; and

• improved comfort in highly insulated homes.

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The greatest benefits are generally achieved where the property is sufficiently airtight for the ventilation system to operate effectively.

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MVHR and Heat Recovery

 

Heat recovery is the feature that distinguishes MVHR from simpler mechanical extract ventilation.

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Warm air leaving the property contains energy that would otherwise be lost.

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The heat exchanger inside the heat recovery unit captures part of this energy and transfers it to the incoming fresh air.

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The incoming air is therefore already partially warmed before it enters the living spaces.

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This reduces the amount of heating required compared with bringing cold outside air directly into the property.

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Does MVHR Heat the House?

 

An MVHR system is primarily a ventilation system rather than the main heating system for a conventional home.

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It recovers heat that would otherwise be lost, but it does not normally replace the need for:

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• radiators;

• underfloor heating;

• a heat pump;

• boiler heating; or

• another primary heat source.

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Its role is to reduce ventilation heat loss while providing controlled fresh air.

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MVHR can therefore work alongside a heat pump, underfloor heating or other low-energy heating strategy.

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MVHR and Air Source Heat Pumps

 

MVHR and heat pumps can complement one another.

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An air source heat pump provides heating, while MVHR manages ventilation and helps retain some of the heat that would otherwise leave the property with extracted air.

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Both technologies are particularly relevant to homes where the building fabric has been improved and heat loss has been reduced.

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However, one does not replace the other.

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A Green Home Survey can help consider insulation, heating and ventilation as parts of the same overall energy strategy.

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MVHR and Insulation

 

MVHR tends to provide the greatest benefit in properties that are already reasonably well insulated and airtight.

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If large quantities of uncontrolled air are entering through gaps in walls, windows, floors and roofs, installing mechanical ventilation alone will not solve the underlying heat-loss problem.

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This is why MVHR should usually be considered alongside:

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• insulation;

• airtightness;

• glazing;

• draught reduction; and

• the overall building fabric.

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In a major renovation, these measures can often be designed together.

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MVHR in New Builds

 

New homes are increasingly designed with much higher levels of insulation and airtightness than older properties.

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This can make controlled mechanical ventilation particularly appropriate.

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Where MVHR is incorporated from the beginning, duct routes, ceiling voids, service spaces and the location of the MVHR unit can all be planned as part of the building design.

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This generally makes installation simpler and more discreet than trying to retrofit a complete ducted system afterwards.

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MVHR in Existing Homes

 

Installing MVHR in an existing property is possible, but the practicality depends on the building.

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The main challenge is usually finding suitable routes for the ventilation ducts.

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This may be easier where the property has:

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• accessible loft space;

• suspended ceilings;

• service voids;

• planned renovation work;

• extensions;

• unused cupboards; or

• floor or ceiling areas already being opened.

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A whole-house retrofit can be more disruptive in a completely finished property, which is why alternative ventilation approaches may sometimes be more practical.

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MVHR During an Extension or Renovation

 

A major extension or refurbishment can provide an excellent opportunity to install MVHR.

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Walls and ceilings may already be open, making it much easier to conceal ductwork and provide suitable routes around the building.

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The system can also be coordinated with:

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• insulation;

• airtightness;

• heating;

• air conditioning;

• electrical systems;

• solar panels;

• heat pumps; and

• other green home improvements.

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Considering MVHR early in the design process can produce a cleaner installation and avoid difficult alterations later.

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Single Room MVHR

 

A single room MVHR system provides heat recovery ventilation to one particular room rather than using a central unit and ductwork throughout the property.

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These systems are sometimes described as decentralised heat recovery ventilators.

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They can be useful where a full whole-house MVHR installation would be difficult or unnecessarily disruptive.

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Possible applications include:

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• bedrooms;

• home offices;

• extensions;

• garden rooms;

• kitchens;

• bathrooms; and

• individual problem areas.

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Single-room systems can provide a more practical retrofit solution, although they operate differently from a fully designed whole-house MVHR system.

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Decentralised MVHR

 

Decentralised MVHR uses individual ventilation units positioned in external walls rather than one central heat recovery unit serving the whole property.

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This can reduce the amount of internal ductwork required.

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It may be particularly useful for existing buildings where installing a complete network of supply and extract ducts would be difficult.

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The number, location, airflow and control of individual units still need to be carefully considered.

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A decentralised arrangement should be designed as a ventilation system rather than simply adding isolated fans wherever space is available.

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Central MVHR Systems

 

A central MVHR system normally has one main unit connected to rooms throughout the property by ductwork.

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This provides coordinated supply and extract ventilation across the home.

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Central systems can be particularly effective where they are incorporated into:

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• new homes;

• major refurbishments;

• whole-house energy upgrades; and

• extensive extensions.

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The main MVHR unit is commonly positioned in a loft, plant room, utility area or suitable service cupboard.

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Adequate access should be provided for filter changes, maintenance and future replacement.

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Choosing an MVHR Unit

 

An MVHR unit needs to be appropriately sized for the property and the required ventilation rates.

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Factors influencing the choice of unit can include:

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• size of the home;

• number of bedrooms;

• number of wet rooms;

• required airflow;

• heat recovery efficiency;

• fan efficiency;

• noise;

• filter specification;

• controls;

• available installation space; and

• duct layout.

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Choosing an oversized unit does not automatically produce a better system.

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Correct design and commissioning are more important than simply purchasing the largest available heat recovery unit.

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MVHR Ductwork

 

Ductwork is one of the most important parts of an MVHR installation.

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Poorly designed ducts can create excessive resistance, noise and inefficient airflow.

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The design should consider:

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• duct diameter;

• route lengths;

• number of bends;

• insulation;

• airtightness;

• acoustic performance;

• access;

• supply locations; and

• extract locations.

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Ductwork passing through cold spaces may also require suitable insulation to reduce condensation and unwanted heat loss.

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Where Should the MVHR Unit Be Located?

 

The main heat recovery unit should be positioned somewhere accessible for regular maintenance.

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Possible locations can include:

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• lofts;

• utility rooms;

• plant rooms;

• service cupboards;

• garages; and

• other suitable internal spaces.

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The position should provide enough room for duct connections, filters, condensate drainage and future servicing.

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Hiding the unit somewhere that cannot easily be reached may create unnecessary maintenance problems later.

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MVHR Filters and Air Quality

 

Incoming outside air normally passes through filters before being supplied to the rooms.

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These filters can help remove airborne particles and prevent debris entering the ventilation system.

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Depending on the equipment and filter type, this may be particularly useful for households concerned about:

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• pollen;

• dust;

• external pollution; and

• general indoor air quality.

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Filters require periodic cleaning or replacement.

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A poorly maintained system can gradually lose performance, so access and ongoing maintenance need to be considered during design.

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MVHR and Condensation

 

One of the potential benefits of controlled ventilation is improved management of internal moisture.

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Cooking, bathing, drying clothes and normal occupation all release moisture into the home.

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MVHR continuously extracts air from rooms where moisture is produced and replaces it with fresh air.

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This can help reduce the conditions that contribute to condensation and mould.

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However, an MVHR system should not be treated as a substitute for repairing leaks, damp problems or other defects within the building.

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MVHR and Summer Bypass

 

Many modern MVHR units include a summer bypass function.

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During warm weather, there may be times when recovering heat from outgoing air is undesirable.

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A summer bypass allows incoming air to avoid some or all of the heat-recovery process when conditions are appropriate.

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This can help reduce unnecessary heat transfer during warmer periods.

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It is not the same as air conditioning and will not provide active cooling, but it can help the ventilation system respond more appropriately to summer conditions.

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MVHR and Air Conditioning

 

MVHR and air conditioning perform different functions.

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MVHR primarily provides ventilation and heat recovery.

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Air conditioning actively controls room temperature.

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In some highly efficient homes, the two systems may operate together.

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MVHR can provide continuous fresh air while air conditioning or an air-to-air heat pump provides heating and cooling.

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The systems should be considered together so that they complement rather than conflict with one another.

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MVHR System Cost

 

The cost of an MVHR system depends on the size and complexity of the property.

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Factors affecting cost can include:

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• MVHR unit specification;

• size of the property;

• amount of ductwork;

• number of rooms;

• ceiling and wall access;

• installation complexity;

• controls;

• acoustic requirements;

• electrical work;

• commissioning; and

• whether the system is installed during construction or retrofitted later.

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A system incorporated into a major renovation may be considerably easier to install than one added to a fully finished existing property.

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The cheapest quotation is not necessarily the best value if duct design, noise control or commissioning have been compromised.

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MVHR Running Costs

 

MVHR systems use electricity to operate the supply and extract fans.

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Running costs depend on:

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• fan efficiency;

• airflow rates;

• system resistance;

• operating mode;

• filter condition; and

• electricity prices.

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The electrical consumption needs to be balanced against the energy recovered from extracted air and the benefits of controlled ventilation.

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A well-designed system should use efficient fans and avoid unnecessary resistance within the ductwork.

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MVHR Maintenance

 

MVHR systems require ongoing maintenance.

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This will normally include:

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• checking and changing filters;

• cleaning terminals;

• inspecting external grilles;

• checking condensate drainage;

• maintaining access to the main unit; and

• periodic system servicing where appropriate.

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Filters are particularly important because blocked filters can restrict airflow and increase fan energy consumption.

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Homeowners should understand the maintenance requirements before choosing a system.

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MVHR Noise

 

A properly designed MVHR system should operate quietly.

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Noise problems can occur where:

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• units are poorly selected;

• airflow is too high;

• ducts are too small;

• duct routes create excessive resistance;

• acoustic treatment is inadequate; or

• equipment is positioned close to bedrooms without proper consideration.

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System design should therefore include acoustic performance as well as ventilation rates.

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Does MVHR Need Planning Permission?

 

Most of an MVHR system is normally installed internally, but external air intake and exhaust terminals will be required.

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For a typical house these may create relatively minor external alterations.

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Additional consideration may be needed for:

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• listed buildings;

• conservation areas;

• flats;

• leasehold properties; and

• sensitive elevations.

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Where the external alterations may affect the appearance of a protected building, planning or listed-building advice may be appropriate.

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MVHR and Building Regulations

 

Ventilation is an important part of Building Regulations when new homes, extensions and substantial alterations are being designed.

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The ventilation strategy needs to work alongside improvements to airtightness and energy efficiency.

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Where MVHR forms part of a regulated building project, the system should be properly designed, installed and commissioned to meet the relevant requirements.

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This is another reason to consider ventilation during the design stage rather than after construction has been completed.

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Is MVHR Worth It?

 

MVHR can be extremely effective in the right property, but it is not automatically the best solution for every home.

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It is particularly worth considering where:

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• the home is highly insulated;

• good airtightness is being achieved;

• major renovation work is proposed;

• indoor air quality is a priority;

• condensation or humidity needs better control;

• a new home is being designed; or

• a wider low-energy strategy is being developed.

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For a very draughty existing property requiring no other work, improving the building fabric may deserve attention before investing in a whole-house MVHR system.

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Heat Recovery as Part of a Green Home Plan

 

An energy-efficient home needs to balance heat retention with healthy ventilation.

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Insulation and airtightness help keep energy inside the property.

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MVHR provides fresh air while recovering some of the heat that would otherwise be lost.

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A heat pump can provide efficient heating.

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Solar panels can generate renewable electricity.

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Battery storage can help manage electricity use.

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Air conditioning can deal with summer overheating where required.

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The best results can come from considering these technologies together rather than installing each one independently.

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Independent MVHR and Heat Recovery Advice

 

Ventilation specialists naturally concentrate on designing and installing ventilation equipment.

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APN Green Homes takes a wider whole-home approach.

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A Green Home Survey can help you consider whether MVHR is appropriate, whether the property first needs improvements to insulation or airtightness, and how ventilation could work alongside heating, cooling, solar panels and other energy upgrades.

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The aim is to establish what the property needs before choosing the equipment.

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Start With the Whole Home

 

A good heat recovery ventilation system is not simply an MVHR unit connected to some ductwork.

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It needs to form part of the way the entire property manages heat, fresh air, humidity and energy.

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Before investing in MVHR, it makes sense to understand the building fabric, airtightness, ventilation requirements and the other energy improvements you may be planning.

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NEXT STEPS

Every home is different, so the right next step depends on your property, your budget and what you want to achieve. APN Green Homes can help you turn the information you have just read into a practical plan for your own home. Whether you are considering one improvement or looking at a complete whole-home energy upgrade, we can help you understand what is suitable, what should be prioritised and where your money is likely to make the greatest difference.

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Choose one of the options below to get the ball rolling:

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BOOK A GREEN HOME SURVEY
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BOOK A PHONE CONSULTATION
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EMAIL US
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Your greener, more efficient home starts with understanding the right improvements for your property — and putting them in the right order.