How Does MVHR Help With Condensation and Moisture?
How Does MVHR Help With Condensation and Moisture?
MVHR can help reduce condensation by continuously removing humid, stale air from moisture-producing rooms and replacing it with fresh filtered air. By preventing excess moisture from building up inside the property, a correctly designed MVHR system can help control indoor humidity and reduce the conditions that lead to condensation and mould.
Mechanical Ventilation with Heat Recovery, or MVHR, also recovers heat from the outgoing air before it leaves the building. This allows a property to receive continuous fresh-air ventilation while reducing the heat loss associated with simply opening windows or extracting heated indoor air directly outside.
For homes experiencing condensation, however, it is important to understand that MVHR is a ventilation system rather than a dehumidifier. Correct system design, installation and commissioning all play an important role in effective moisture control.
Why Does Condensation Form in a Home?
Everyday activities continuously add moisture to indoor air.
Common sources include:
- Showering and bathing
- Cooking
- Drying clothes indoors
- Washing
- Breathing and normal occupancy
Warm indoor air can hold water vapour. When that humid air comes into contact with a sufficiently cold surface, such as a window, external wall or other cold part of the building fabric, it can cool to the point where water vapour condenses into visible droplets.
This is why condensation commonly appears on windows and other cold surfaces during colder weather.
Reducing the amount of excess moisture allowed to accumulate in the indoor air is therefore an important part of condensation control.
How Does MVHR Help Control Moisture?
A whole-house MVHR system provides controlled ventilation throughout the property rather than relying solely on windows or intermittent extractor fans.
The system normally extracts air continuously from wet rooms such as:
- Bathrooms
- Kitchens
- Ensuites
- Utility rooms
- WCs
Fresh filtered air is then supplied to habitable rooms such as bedrooms and living areas.
This continuous movement of air helps prevent water vapour generated in wet rooms from remaining in the property and spreading throughout the building.
UK ventilation guidance follows the same general principle: extract ventilation should remove water vapour where it is generated, while whole-dwelling ventilation helps dilute, disperse and remove water vapour and other pollutants.
Does MVHR Reduce Humidity?
A correctly operating MVHR system can help control excessive indoor humidity by continually removing moisture-laden extract air from the property.
Rather than waiting until a bathroom mirror or bedroom window is covered in condensation before ventilation is introduced, MVHR is generally designed to operate continuously at a background ventilation rate.
Higher ventilation rates or boost modes can then be used when greater levels of moisture are being produced.
The aim is not necessarily to make the indoor air as dry as possible. Instead, the ventilation system should help maintain a comfortable indoor environment while preventing excessive moisture from accumulating.
How Does Heat Recovery Help?
Ventilation is necessary for controlling moisture, but simply replacing warm indoor air with cold outdoor air can result in substantial ventilation heat loss.
An MVHR system addresses this using a heat exchanger.
Warm extracted air passes through the MVHR unit at the same time as fresh outdoor air is brought into the property. The two air streams remain separate, but heat from the outgoing air is transferred to the incoming fresh air.
This means fresh air can be introduced without losing as much of the heat that would otherwise leave the building.
The combination of continuous ventilation and heat recovery makes MVHR particularly suited to modern, well-insulated and airtight homes where uncontrolled air leakage is much lower.
What Is an Enthalpy Heat Exchanger?
Some MVHR systems use an enthalpy heat exchanger, which can transfer both heat and a proportion of moisture between the outgoing and incoming air streams.
This is slightly different from a conventional heat exchanger that primarily transfers heat.
An enthalpy exchanger does not mean that the system simply sends all unwanted moisture back into the property. The MVHR system continues extracting stale, moisture-laden air from wet rooms, while the exchanger transfers a controlled proportion of moisture between the air streams.
This can help prevent indoor air becoming excessively dry under certain conditions while still providing continuous ventilation.
Can MVHR Completely Stop Condensation?
MVHR can significantly help with condensation caused by excess indoor humidity and inadequate ventilation, but it cannot guarantee that condensation will never occur.
Condensation can also be influenced by:
- Poor insulation
- Thermal bridging
- Very cold internal surfaces
- Insufficient heating
- Water ingress or building defects
- Incorrectly sized ventilation equipment
- Poorly designed ductwork
- Incorrect system balancing
- Blocked or dirty filters
- Ventilation rates that are too low
If moisture is entering the building because of a leak, rising damp or another building defect, increasing ventilation alone will not correct the underlying cause.
Similarly, even a high-quality MVHR unit may not perform correctly if the system has not been properly designed and commissioned.
Government guidance also notes a high risk of mould in poorly ventilated homes, reinforcing the importance of providing adequate ventilation rather than allowing moisture and pollutants to accumulate indoors.
Why MVHR Design Matters for Condensation Control
Choosing an MVHR unit is only one part of creating an effective ventilation system.
The complete design should consider:
- Required room-by-room airflow rates
- Property size and layout
- Extract and supply valve positions
- Ductwork size and resistance
- Distribution-box locations
- External intake and exhaust positions
- MVHR unit capacity
- System balancing and commissioning
For example, fitting a powerful MVHR unit does not automatically solve moisture problems if insufficient air is actually being extracted from the bathroom or other wet rooms.
The ventilation system should be designed as a complete system rather than selecting components individually.
Get Help Designing Your MVHR System
If you are planning a new build or renovation, Quiet-Vent's design team can help develop an MVHR system around the requirements of your project.
The Quiet-Vent Design Request service provides support with MVHR system selection, ductwork design, layout and product compatibility. Quiet-Vent's design team can review project plans and help develop an appropriate ventilation solution.