Should I Leave My MVHR System On All the Time?

A Mechanical Ventilation with Heat Recovery (MVHR) system is designed to keep your home supplied with fresh, filtered air while recovering heat from the outgoing air. Many homeowners across Hampshire, Berkshire, Surrey, and Oxfordshire wonder whether they should switch it off when it’s not in use or leave it running continuously.

The short answer is that MVHR systems should run all the time. They are built to operate continuously at low speed, maintaining balanced airflow and stable humidity levels. Switching them off can quickly lead to condensation, stale air, and reduced system efficiency.

Why MVHR Systems Should Run Continuously

The main purpose of an MVHR system is to ensure consistent air exchange while saving energy. It removes stale air from kitchens and bathrooms and replaces it with filtered fresh air in living rooms and bedrooms.

If you turn the system off, several issues can arise:

• Moisture builds up inside the property, increasing the risk of damp or mould.
• Air quality drops as pollutants, allergens, and CO₂ accumulate.
• The heat exchanger can lose efficiency when restarted after long idle periods.

By running constantly, the system maintains equilibrium between temperature, humidity, and airflow.

How MVHR Systems Are Designed to Operate

MVHR units use two low-energy fans that run 24 hours a day, usually at variable speeds. During normal use, they operate at a background speed that uses minimal electricity.

When you cook or shower, the system temporarily increases airflow to remove moisture and odours faster, then returns to normal operation.

Modern systems are energy-efficient, using around 20–40 watts per hour, which costs only a few pence daily.

Benefits of Continuous Operation

Leaving your MVHR system running provides measurable benefits:

• Consistent indoor air quality throughout the home
• Reduced condensation in bathrooms, kitchens, and utility rooms
• Balanced humidity levels, protecting walls and furnishings
• Steady indoor temperatures, reducing strain on heating systems
• Longer equipment lifespan due to stable operation

In Basingstoke, one homeowner noticed reduced condensation on bedroom windows after switching to constant operation. In Guildford, another client found that leaving their MVHR on low speed improved the warmth of their house during winter.

Energy and Cost Considerations

A common concern is that running 24/7 might increase electricity use. In reality, the opposite is true. MVHR saves more energy than it consumes by recycling up to 90% of the heat from outgoing air.

For most systems, the total annual running cost is around £40 to £60. That’s less than the cost of running a standard extractor fan for the same period.

When It’s Acceptable to Turn It Off

You can switch off your MVHR in certain rare situations:

• During maintenance or filter replacement
• If the property is vacant for an extended time and completely unheated
• When instructed by your installer during servicing or adjustments

Otherwise, turning it off regularly can cause more harm than good.

Seasonal Use and Settings

In winter, the system works harder to recover heat from extracted air. In summer, many units have a bypass mode, which allows cooler night air to enter without heat exchange.

This ensures comfortable indoor temperatures all year round without unnecessary energy use.

For example:

• In Reading, a family used bypass mode during summer nights to cool upstairs bedrooms.
• In Oxford, a homeowner kept their MVHR in standard mode through winter, reducing heating costs by about 20%.

Electrical and Maintenance Requirements

As with all systems, the power supply to the MVHR unit must be installed and certificated by a qualified electrician.

Maintenance involves:

• Cleaning or replacing filters every six months
• Checking exterior vents for blockages
• Annual servicing to clean the heat exchanger and confirm airflow balance

Consistent operation keeps the filters active, protecting both air quality and the internal components.

Why Installers Recommend Continuous Use

Professional installers across the South East recommend keeping MVHR running 24 hours a day because:

• It maintains building health by avoiding damp and condensation
• It provides stable indoor comfort
• It preserves the system’s performance and compliance with Building Regulations Part F

At ClimateWorks, we install and maintain MVHR systems across Hampshire, Berkshire, Surrey, and Oxfordshire. All our installations are designed for quiet, energy-efficient continuous operation.

References

• UK Government – Building Regulations Part F
• Energy Saving Trust – Home Ventilation
• CIBSE – Guidance on Mechanical Ventilation

Author Bio

Dr Julian Carter is Technical and Compliance Director at ClimateWorks. With over 25 years of experience in air conditioning, refrigeration, and ventilation, Julian has managed projects across Hampshire, Berkshire, Surrey, and Oxfordshire. He also lectures at a UK university, specialising in low-energy building design and ventilation compliance. His work combines academic knowledge with hands-on expertise to deliver healthy, efficient indoor environments for clients.

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