Mechanical Ventilation Heat Recovery

How does an MVHR system work?

MVHR systems consistently renew the air supply within a building by extracting any poor, moist, or stale air that is trapped within the wet rooms of the property, usually found in kitchens and bathrooms. It also recovers any heat produced inside a property from wet rooms, lighting, utilities, and domestic appliances. Instead of wasting the energy and releasing it through outgoing air, it is first passed through a heat exchange cell which retains that heat to be re-used by heating the incoming fresh air. This warm fresh air can then be distributed to habitable areas from living rooms to offices, increasing the efficiency of your heating whilst ensuring a more comfortable environment.

The Asthma and Allergy Association recommends that homes are aired out for between five and ten minutes, three times a day, with a thorough draught, all year round to expel damp and harmful particles. It is key to remember that the efficiency and airtightness of a property are measured on a scale. At some points on the scale, the MVHR system will perform at the highest efficiencies and at some, it will be less effective. If a building has an airtightness of 5 m³.hr/m² @50Pa (5 air changes per hour when tested at 50 Pascal) or lower, then an MVHR system will work efficiently. Anything over that and the system will have depreciating benefits as the natural draughts in the property will give the required air change, albeit with no heat recovery.

Stages of an MVHR Cycle

• The MVHR unit extracts any moist, stale, or polluted air within the property. This can be found in rooms that produce high amounts of moisture and heat like bathrooms, utility rooms, and kitchens.
• The extracted air is then passed over a heat exchange cell. This cell then recovers and retains the heat from the outgoing air that would’ve previously been expelled outside.
• This recovered heat is then fed into the incoming fresh, tempered, and filtered air that is distributed through vents into the habitable spaces.
• This cycle is repeated ensuring there is a constant fresh air supply with minimal heat loss, creating a more comfortable environment.

Technologies that enhance MVHR units 

MVHR is a sustainable energy saving technology and can be paired with renewable heating systems like air source or ground source heat pumps. Installing a heat pump would help the property become even more sustainable as the heat that is being recovered has been created through renewable energy. Pairing both MVHR units and heat pumps will help the temperature of the house stay constant for longer, further increasing the heat pump efficiency.

There are MVHR units that include their own integrated heat pump. The unit also provides low level of cooling by utilizing its reverse cycle function, ensuring consistent temperatures all year round no matter the weather. It also can provide support to prevent overheating issues which are common in apartments. As with any sustainable technology, there is an increase in electricity demand to power the system which can be as little as 20p per day. Installing solar panels can partially offset this increase and combine with the MVHR unit to create savings on heating and electricity bills.

Retrofit or Newbuild? 

It is much easier to install an MVHR system during the building phase of a property. The various ducting that is required for the system to work can sometimes influence the layout of a building or the heating system design. For example, heating may not be required throughout the whole of the building, especially those which have high-efficiency standards. Instead, heating would only be required in rooms that are higher priority, e.g., habitable rooms.

However, retrofit installations are possible but require more consideration and planning for where and how to install the ducting. If enough voids have been left behind wardrobes, cabinets, or floor and ceiling joists then the installation can be done with some disruption.

Cooling

MVHR systems with integrated heat pumps like the Nilan Combi 302 Polar can cool a property due to the reversible cooling circuit. Summer heatwaves provide discomfort in buildings throughout the UK, with an integrated cooling circuit the air supply can be cooled up to 10ºC below ambient when required. However, the unit should not be compared to an air conditioning unit in terms of cooling power. The added benefit of a cooling MVHR unit over air conditioning is the reduction of indoor humidity as the ventilation still extracts the moist indoor air, providing a less humid and more comfortable environment.

Costs/savings 

Not only do these units reduce a property’s carbon footprint but they also provide significant financial benefits. The Carbon Trust estimates that a heat recovery system operating, in a typical office environment, at an efficiency of 70% could save a business or organisation up to 38% on its gas bill. For an integrated heat pump heat recovery system, the return on investment can be seen in as little as five years.

Installations in domestic properties can save between 20-50% on energy bills due to the reduced heating demand. These potential savings could pay for the initial investment within five years. All the above figures are relative to various factors including the energy efficiency of the building (insulation, double glazing, etc) and the quality of the installation and components used. In winter periods the savings will be highest as the heated air will not escape out the window but instead be recycled back into the property with fresh air.

With current trends and rising prices for heating bills, the savings from installing a heat recovery system in either domestic or commercial properties will most likely scale over the upcoming years. Future-proofing the property for any further increases in these volatile prices.

Building types 

An MVHR system can be installed in a variety of buildings. All buildings can benefit from an MVHR system, with a few elements that will prevent efficiency. According to research by the Passivhaus Trust, homes that are less airtight and slightly draughty will still benefit from lower carbon emissions rates than natural ventilation. However, at a certain point, the MVHR will lose its efficiency.

Passive Housing 

Passive houses focus on containing the most up-to-date materials to ensure an ecological, comfortable, and efficient home. This means that these passive homes use up to 90% less energy than a typical house. However, the downside is the lack of fresh air being supplied and moved throughout the day. Installing an MVHR unit would create a constant recycling movement of air within the home, decreasing the chances for any mold or damage to occur inside where the damp air would be trapped.

With the movement towards higher insulation standards, MVHR units will become essential to ensure that these passive homes stay ventilated, whilst ensuring high levels of comfort.

Large or Commercial Properties 

Commercial properties, especially offices, will benefit in many areas such as improvements in ventilation for employees’ health, reduced impact on the environment, and significant savings over time. Larger MVHR units can be installed to handle the higher amount of heat generated and to distribute it evenly throughout the building.

Flats

Buildings with multiple flats/housing will result in a higher heating load as people have various preferred indoor environments. Installing an MVHR system would reduce the amount of heat escaping and instead recycle air throughout the flats. This especially helps with ventilation from wet rooms and kitchen areas as flats have fewer access points to outdoor air.

Maintenance 

Like heat pumps, you are required to have your MVHR units checked annually. Any damage or problems that arise need to be dealt with immediately. Purchasing a service plan will ensure your heat recovery system is operating at maximum efficiency all year round, so you can enjoy your comfy renewable home.

Sizes/space required

Just like heat pumps, MVHR units also come in different sizes to handle different properties. The first stage of installing an MVHR system is to check airflow throughout the house and how well warmth is retained inside a property. From this, the results will dictate what size of MVHR system is required to suit the property and whether it is worth installing an MVHR system with an integrated heat pump.

If you would like to find out if an MVHR system is right for your project, get in touch or call us on 01293 821 345.

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