Common MVHR installation mistakes often stem from poor planning, shortcuts in ductwork, improper sealing or skipping key steps like commissioning. These errors can lead to reduced airflow, excessive noise, poor heat recovery efficiency, condensation issues, higher energy bills, stale air or even system damage. Many problems arise in new builds or retrofits where installers deviate from best practices.
Here are some of the most frequently reported mistakes:
- No professional or inadequate system design. Skipping a detailed, property-specific design (including airflow calculations, pressure drop assessments, and duct routing) is one of the top issues. This results in uneven ventilation, insufficient fresh air in some rooms, excessive noise, or the unit working harder than necessary. Always insist on a bespoke design compliant with building regulations (eg Part F in the UK).
- Excessive or improper use of flexible ducting. Over-relying on flexible (flexi) ducting instead of rigid or semi-rigid smooth-bore ducts creates high resistance to airflow, leading to poor performance, noise, and reduced efficiency. Flexi duct is acceptable only for short connections; avoid long runs or sagging/kinked sections.
- Inadequate duct sealing and support. Failing to properly seal all joints (using appropriate seals, non-hardening sealant, or proprietary fittings) allows air leaks, which drop system efficiency and cause imbalances. Ducts also need secure support to prevent sagging, crushing, or disconnection over time.
- Poor or missing insulation on ducts. External/supply ducts (especially those in unheated spaces like lofts, attics or outside) must be insulated to prevent condensation buildup, heat loss, and blockages. Uninsulated ducts cause moisture to form inside, potentially leading to mould, water damage, or reduced heat recovery.
- Incorrect placement or positioning of the MVHR unit. Installing the unit in a cold/unheated space (eg uninsulated loft) reduces efficiency despite insulated casings. Avoid placing it above bedrooms (noise transmission) or in areas that block access for maintenance.
- Improper condensate trap installation. Fitting the trap on the wrong side (eg intake instead of exhaust), using a wet trap instead of a dry trap (or forgetting it entirely) or poor routing leads to leaks, water in the unit, odours or blockages. The trap prevents backflow of air/gases and ensures proper drainage.
- Skipping or poor commissioning and balancing. Not properly balancing airflow (adjusting dampers/valves so each room gets the correct supply/extract rate) causes over-ventilation in some areas and under-ventilation in others, noise from high fan speeds or inefficiency. Commissioning should include airflow measurements.
- Wrong duct type, size or fittings. Using undersized ducts, incorrect terminals (eg, non-compliant roof cowls), sharp bends, or non-compliant fittings increases resistance and reduces performance. External terminals need proper drip caps to avoid water ingress/stains.
- Missing components or rushed installation. Omitting silencers/attenuators (leading to noise), filters, or separation baffles in grilles (causing cross-contamination of air), or crushing ducts during installation are common in hurried jobs.
- Deviations from the design on site. Changing duct routes, shortening/lengthening runs or poor routing without recalculating pressure drops disrupts the system's balance and efficiency.
To avoid these pitfalls, it is essential to use qualified installers experienced with MVHR (ideally certified), follow manufacturer guidelines strictly, and ensure the system is fully commissioned and tested post-installation. Regular maintenance (eg filter changes) also prevents many post-install issues.
If you'd like to talk to the experts, please contact our UK-based team on 01892 600121 or email mvhr@epicair.co.uk and we will be delighted to help you!
