MVHR Systems: Design, Installation and Maintenance

What This Requirement Covers

Mechanical Ventilation with Heat Recovery (MVHR) is increasingly used in UK homes, particularly those built to high airtightness standards. An MVHR system extracts stale air from wet rooms (kitchens, bathrooms, WCs) and supplies fresh filtered air to habitable rooms (bedrooms, living rooms), recovering heat from the exhaust air to warm the incoming fresh air.

Key Requirements

When MVHR Is Appropriate

1. MVHR is most effective in buildings with air permeability of 3 m³/(h.m²) at 50 Pa or better
2. In leakier buildings, the heat recovered may not offset the energy used by the fans
3. MVHR is the standard ventilation solution for Passivhaus and near-Passivhaus buildings
4. Under the Future Homes Standard, MVHR is expected to become the default ventilation system for new dwellings

Performance Standards

1. Heat recovery efficiency: Minimum 73% (Approved Document F); typical good-quality units achieve 85-95%
2. Specific fan power (SFP): Maximum 0.5 W/(l/s) for balanced MVHR systems
3. Flow rates must match the Approved Document F requirements:

- Extract from kitchen: 13 l/s continuous, 30 l/s boost - Extract from bathroom: 8 l/s continuous, boost mode available - Supply to habitable rooms: As required to balance the extract

Ductwork Design

1. Use rigid ductwork wherever possible (smooth-bore metal or plastic); flexible duct has much higher resistance
2. Keep duct runs as short and straight as possible; each bend increases resistance
3. Duct diameter must be sized for the design flow rate at acceptable velocities (typically 3-4 m/s in branches)
4. Insulate ductwork in unheated spaces (loft, garage) to prevent condensation
5. Use acoustic attenuators if duct noise is a concern

Commissioning

1. The MVHR system must be commissioned by a competent person
2. Commissioning includes measuring the flow rate at every supply and extract terminal
3. The system must be balanced so that the total supply and extract flow rates are approximately equal
4. A commissioning certificate must be provided to Building Control

Practical Compliance Tips

1. Plan the MVHR unit location early; it needs to be accessible for filter changes (every 3-6 months) and must have condensate drainage
2. Route ductwork during the first fix, before plasterboard installation
3. Use the manufacturer's duct sizing software to ensure adequate flow rates with acceptable pressure drops
4. Avoid routing ducts through uninsulated spaces where possible; if unavoidable, insulate the ducts thoroughly
5. Ensure the airtightness of the building is sufficient for MVHR to work effectively; a leaky building undermines the system
6. Provide homeowner instructions on filter maintenance, boost control, and summer bypass operation
7. The summer bypass function allows the MVHR to supply fresh air without heat recovery during warm weather