H1: MVHR System Design & Sizing: The Complete Guide

Mechanical Ventilation with Heat Recovery (MVHR) systems are essential for modern airtight homes, especially across the United Kingdom where energy efficiency standards are increasingly strict. However, even the best MVHR unit will fail to perform efficiently if the system is poorly designed or incorrectly sized.

This guide walks you through everything you need to know about MVHR system design and sizing—from airflow calculations to duct layout best practices.

H2: What is an MVHR System?

An MVHR system extracts stale air from wet rooms (kitchens, bathrooms) and supplies fresh filtered air to living spaces while recovering heat from outgoing air.

Key Benefits:

• Reduced heat loss
• Improved indoor air quality
• Lower energy bills
• Continuous ventilation

H2: Why Proper MVHR System Design & Sizing Matters

Incorrect sizing leads to:

• Poor airflow and ventilation imbalance
• Noise issues due to high air velocity
• Increased energy consumption
• Failure to meet building regulations

In the UK, MVHR systems must comply with Building Regulations Part F, making correct design not just important—but mandatory.

H2: MVHR System Design Basics

H3: 1. Understanding Airflow Requirements

Airflow is measured in litres per second (L/s). Typical minimum extract rates:

• Kitchen: 13–30 L/s
• Bathroom: 8–15 L/s
• Utility room: 8–10 L/s

Whole dwelling ventilation is calculated based on floor area and occupancy.

H3: 2. Supply vs Extract Balance

An MVHR system must be balanced:

• Equal supply and extract airflow
• Prevents pressure imbalance
• Ensures optimal heat recovery efficiency

H2: MVHR System Sizing Calculations

H3: Step 1: Calculate Total Ventilation Rate

Use:
Whole House Ventilation Rate = Floor Area × 0.3 L/s per m²

Example:

• 120 m² house → 36 L/s total airflow

H3: Step 2: Room-by-Room Airflow Design

Allocate airflow:

• Supply to bedrooms & living rooms
• Extract from kitchen & bathrooms

H3: Step 3: Select MVHR Unit Capacity

Choose a unit that:

• Meets peak airflow demand
• Operates efficiently at ~70–80% capacity

Popular MVHR brands include:

• Vent-Axia
• Zehnder
• Nuaire

H3: Step 4: Duct Sizing & Layout

Proper duct design is critical:

Key Principles:

• Keep duct runs short and straight
• Use smooth, rigid ducts where possible
• Minimize bends and resistance
• Size ducts to maintain low air velocity (<3 m/s)

H2: MVHR Duct Design Best Practices

H3: Radial vs Branch Systems

• Radial systems: Easier balancing, quieter operation
• Branch systems: Lower cost but harder to balance

H3: Noise Reduction Tips

• Use silencers/attenuators
• Avoid sharp bends
• Install unit away from bedrooms

H2: Common MVHR Sizing Mistakes

❌ Oversizing the Unit

Leads to inefficiency and higher costs.

❌ Undersizing

Results in poor ventilation and condensation issues.

❌ Poor Duct Design

Even a correctly sized unit will fail with bad ductwork.

❌ Ignoring Commissioning

Proper setup and testing are essential.

H2: MVHR Commissioning & Testing

Once installed:

• Measure airflow rates
• Adjust valves for balance
• Verify compliance with regulations

Professional commissioning ensures optimal performance.

H2: MVHR System Design for Different Property Types

H3: New Builds

• Easier integration
• Optimized duct routing
• Higher efficiency potential

H3: Retrofit Projects

• Space constraints
• Creative duct routing needed
• May require compact units

H2: Energy Efficiency & Cost Savings

A well-designed MVHR system can:

• Recover up to 90% of heat
• Reduce heating demand
• Improve EPC ratings

This is particularly important in energy-conscious regions like the United Kingdom.

H2: Tools for MVHR Design & Sizing

• Manufacturer sizing software
• SAP calculations (UK compliance)
• Airflow calculators
• CAD duct design tools

H2: Future Trends in MVHR Systems (2026)

• Smart MVHR with sensors
• AI-driven airflow optimization
• Integration with smart home systems
• Ultra-low energy passive house designs

H2: Final Thoughts

MVHR system design and sizing is both a science and an art. It requires careful planning, accurate calculations, and attention to detail. When done correctly, it delivers exceptional comfort, energy efficiency, and air quality.

Whether you're a homeowner, builder, or HVAC professional, investing time in proper MVHR design will pay off for years to come.