Mechanical Ventilation with Heat Recovery (MVHR) systems are designed to improve air quality, reduce energy consumption, and maintain a comfortable indoor environment by recovering heat from the outgoing air. However, for your MVHR system to operate at peak efficiency, occasional maintenance and troubleshooting are essential. Here’s an in-depth guide covering the most common MVHR issues and the steps to resolve them.


1. Check the Power Supply

If your MVHR system suddenly stops working, a power issue may be to blame.

  • Solution: Verify that the system is plugged in, and check that the circuit breaker hasn’t tripped or that a fuse hasn’t blown. If you discover an issue with the power supply, reset the breaker or replace the fuse as necessary. Persistent power issues should be assessed by a qualified electrician.

2. Inspect the Condensation Drain

The condensation drain, located at the bottom of the unit, is essential to prevent moisture build-up in your MVHR system. On new installations, confirm that it’s properly connected.

  • Solution: Check the drain regularly for blockages. If you notice water pooling around the unit or detect excess humidity, the drain could be clogged or disconnected. Ensure it’s routed to a suitable drainpipe or vented outside to avoid moisture backflow that could harm your system.
condennsation drain example

Mid-installation image:

Condensation drain to be connected


3. Examine the Filters

Filters play a critical role in keeping your MVHR system clean and functional. Dirty or clogged filters restrict airflow, which can raise energy use and potentially damage the unit.

  • Solution: Inspect the filters every three months and clean or replace them per the manufacturer’s instructions (typically every 6-12 months). Regular filter maintenance not only optimises air quality but also extends the life of the system’s internal components.

A very dirty air filter

(should be snow-white)


4. Check for Moisture in the Ducting

Moisture build-up in ducting often occurs if the ducts are not adequately insulated. When warm, moist air from showers or cooking enters cold, uninsulated ducting (typically in areas like attics), it condenses and eventually pools, potentially leading to blockages.

  • Solution: Insulate any exposed ductwork to prevent condensation. If you hear gurgling sounds from your extract ducts—especially in bathrooms—it’s likely a sign of water pooling. Proper insulation can prevent condensation and avoid costly water blockages.

Mid-installation image:

Ducting insulation to be completed


5. Look Out for Air Leakage

Leaks in ductwork or the system housing can reduce efficiency, forcing the unit to work harder, potentially leading to overheating.

  • Solution: Inspect the ductwork for visible cracks, loose connections, or other signs of air leakage. Use duct tape or sealant for minor repairs, but for significant leaks, contact a professional. Airtight ducts ensure that airflow is balanced, optimising performance and efficiency.
ducting issue - disconnected duct
ducting issue – disconnected duct

6. Test Fan Operation

Fans are the workhorses of an MVHR system, drawing air through the ducts and heat exchanger. Malfunctioning fans may cause a total loss of airflow or reduced performance in specific zones.

  • Solution: Listen for unusual noises or vibrations, which could indicate fan issues. If a fan isn’t working, it may need cleaning, lubrication, or replacement. Always turn off the system before attempting fan maintenance and refer to the user manual for specific instructions.
  • Inactivity Issues – Sometimes, fans nearing the end of their service life fail to restart after the unit has been powered down for a period. This can be caused by a range of issues: mould build-up creating resistance, bearings seizing due to wear, or an increased start-up energy demand that the ageing motor can no longer meet. High-quality systems, such as those from Zehnder, are designed for easy motor replacement, allowing for a quick fix without disrupting the entire unit. However, not all systems offer this convenience; in some cases, a complete unit replacement may be the most effective solution if individual fan parts cannot be accessed or serviced separately.

7. Inspect the Heat Exchanger

The heat exchanger transfers warmth from outgoing to incoming air, enabling energy-efficient heating. A dirty or damaged heat exchanger will reduce system efficiency.

  • Solution: Clean the heat exchanger periodically according to manufacturer guidelines, checking for any dirt buildup or physical damage. If damaged, it’s best to seek a replacement or professional repair.

8. Check the Humidity Sensor

The humidity sensor helps regulate indoor moisture levels. A faulty sensor can lead to high humidity or overly dry air, impacting comfort and possibly damaging furniture.

  • Solution: Check the sensor’s functionality using built-in diagnostics if available. Ensure the sensor is dust-free and calibrated. If it’s misreading humidity, recalibration or replacement may be necessary.

9. Examine Ducting Quality and Installation

Low-grade or improperly installed ducting can create airflow issues. Inferior ducting may crush easily, kink on tight bends, or disconnect at joins, leading to airflow imbalances or blockages.

  • Solution: Use high-quality, double-walled, semi-rigid circular ducting with no joins. At Smart-Ventilation, we recommend using durable ducting for reduced airflow resistance and easier cleaning. High-quality ducting also simplifies maintenance, allowing a clear path from each valve to the manifold.
ducting issue - disconnected duct
ducting issue – disconnected duct

10. Inspect for On-Site Deviations from MVHR Design

Installers sometimes modify the MVHR layout during installation, which can lead to problems. Changes to duct routes, improperly sealed ducts, or crushed sections can cause air leaks and disrupt proper airflow.

  • Solution: Ensure the system’s installation follows the original design to prevent airflow problems. At Smart-Ventilation, we perform comprehensive quality checks to confirm that every system matches the design specifications and optimally distributes air throughout the home.

11. Evaluate the Quality of the Initial MVHR Design

If the MVHR system was designed without precise airflow calculations, airflow imbalances are likely. Shorter runs may receive too much air, while longer runs receive too little, resulting in uneven distribution.

  • Solution: Ensure your MVHR design includes accurate airflow calculations tailored to your home. Our design team at Smart-Ventilation customises each system for balanced, effective ventilation, avoiding the pitfalls of poorly calculated designs. Avoid manual adjustments without an anemometer and proper calibration data, as this can worsen the imbalance.

12. Verify Proper Commissioning

Each room must be individually balanced to ensure proper airflow, which is essential for consistent indoor air quality and system efficiency. If commissioning isn’t performed, some rooms may receive inadequate ventilation.

  • Solution: Commissioning should be completed by a qualified professional who can balance each room’s airflow. At Smart-Ventilation, we only finalise installations once we’ve verified that every room meets the designed airflow rates.

13: Check the Controller Board

The controller board is the “brain” of your MVHR system, managing airflow settings, fan speeds, and other vital functions. If the controller board fails, it can cause the entire system to stop functioning or lead to erratic behaviour, such as fans not responding to settings, incorrect airflow, or system shutdowns.

  • Solution: Inspect the controller board for any visible signs of damage, such as burned components or loose connections. If you suspect the controller board is malfunctioning, it’s best to contact a professional for a proper diagnosis and replacement, as these components are complex and require specific expertise to replace. For peace of mind, systems from reputable manufacturers often come with readily available replacement parts, so ensure you have the model details handy when contacting support.

14: Regular Duct Cleaning

Over time, dust, debris, and other contaminants can accumulate in the ductwork, restricting airflow, reducing system efficiency, and affecting indoor air quality. Additionally, dirty ducts can harbour mould and allergens, which may circulate throughout your home.

  • Solution: Schedule regular duct cleaning to keep your system running efficiently and maintain optimal air quality. This is especially important in homes with pets, high traffic, or nearby construction, where dust levels may be higher. Professional duct cleaning services can access and clean each run thoroughly, ensuring no build-up obstructs airflow. Additionally, clean ducts reduce wear on the MVHR system’s fans and motors, prolonging the life of the unit.

Additional Tips for MVHR System Maintenance

  • Prevent Mould Build-up on Motors: Mould can grow on motor components, leading to imbalances and motor burnout. Routine maintenance prevents mould and extends motor life.
  • Regular Filter Changes: Clean or replace filters as per the manufacturer’s instructions. Clogged filters reduce efficiency and increase energy usage.
  • Keep Detailed Records: Maintain a log of any repairs or adjustments. This helps in troubleshooting and is useful for future service visits.

Final Thoughts

By following these troubleshooting steps and performing regular maintenance, you’ll extend the life of your MVHR system and maintain peak performance. MVHR systems can drastically improve your home’s indoor air quality while lowering energy costs. With the right upkeep, you’ll enjoy years of efficient, balanced, and fresh air throughout your home.

If you’re experiencing issues with your MVHR system—whether it’s one of the challenges listed here or something more unique—don’t hesitate to reach out to us at Smart Ventilation. Our experienced team is here to help you diagnose, maintain, and optimise your MVHR system for long-term efficiency and comfort. Get in touch today, and we’ll ensure your home’s ventilation is in top working order!