MVHR Commissioning & Balancing: The Critical Last Step

An MVHR system can be designed perfectly on paper and installed to a high standard on site — and still fail building regulations sign-off because the commissioning was done incorrectly, done in the wrong sequence, or simply not done at all.

Commissioning is where the designed performance meets reality. It's the process of measuring, adjusting and verifying that every terminal in the system is delivering the right airflow rate in the right direction. For housing developers and M&E contractors working on new build residential in Scotland, understanding what commissioning involves is essential to avoiding delays at practical completion.

Why Commissioning Matters for Building Standards

Scottish Building Standards Section 6 (Energy) requires that all ventilation systems are commissioned and a commissioning certificate is provided to Building Control before occupancy is permitted. For MVHR, this is not a formality — the calculated energy performance of the building (SAP or RdSAP for dwellings, SBEM for non-domestic buildings) assumes specific airflow rates. If the system isn't balanced to those rates, the as-built energy performance deviates from the compliance calculation.

In practice: an unbalanced MVHR system wastes energy, creates noise complaints, and fails the competent person's sign-off. The developer cannot certify completion to Building Control without a signed commissioning certificate from a qualified commissioning engineer.

What Commissioning Involves: Step by Step

1. Pre-Commissioning Checks

Before airflow measurements can begin, the commissioning engineer checks:

• All ductwork is installed, connected and free of obstructions (no packing materials in ducts)
• All terminals (supply diffusers and extract grilles) are installed and accessible
• The MVHR unit is powered and in normal run mode (not boost)
• Filters are clean (new installation should have new filters)
• All manual dampers are set to fully open as the starting position
• External intake and exhaust connections are complete and free of obstruction

2. Measuring Actual Airflows

The commissioning engineer uses a calibrated air flow meter (typically a rotating vane anemometer or a Shortridge FlowHood / equivalent) to measure the actual airflow volume at each supply and extract terminal. Readings are taken in litres per second (l/s) and compared to the design value for each terminal.

For a typical 3-bedroom house, there might be:

• 4 supply terminals (living room, bedroom 1, bedroom 2, bedroom 3)
• 4 extract terminals (kitchen, bathroom, en-suite, utility)

Each terminal has a design flow rate from the ventilation design. The initial measured rates will almost certainly differ from design — the purpose of commissioning is to bring them into tolerance.

3. Balancing the System

Balancing is the iterative process of adjusting dampers at each terminal until the measured rates match the design values within an acceptable tolerance. The tolerance specified in CIBSE AM10 and the NHBC Standards is typically ±10% of the design value.

Additionally, the total supply and total extract must be balanced against each other. MVHR systems should be slightly extract-dominant (5–10% more air extracted than supplied) to maintain a slight negative pressure in the dwelling, which helps prevent moisture accumulation in the building fabric.

4. Setting Boost Mode

MVHR units have a boost mode activated when cooking or showering. The commissioning engineer sets the boost airflow rates and confirms the unit responds correctly when the boost signal is given (via wall switch, PIR sensor or humidistat). Boost rates must comply with the minimum values in Scottish Building Standards Section 3 for the specific room type.

5. Documenting the Results

All measured airflow rates — at normal speed and at boost — must be documented in the commissioning certificate. The certificate must include:

• Site address and unit number (for multi-unit developments)
• MVHR unit make and model
• Date of commissioning
• Name and competency evidence of the commissioning engineer
• Design flow rate and measured flow rate for every terminal
• Unit fan speed settings
• Boost settings and confirmation of operation
• Filter type installed and confirmation of cleanliness
• Any remedial actions taken during commissioning

Common Commissioning Failures

Problem	Cause	Consequence
Airflows outside ±10% tolerance	Incorrectly set dampers; duct obstruction; undersized ductwork	Commissioning certificate cannot be issued
Total supply ≠ total extract	Damper settings not balanced; unit setting error	Positive pressure buildup; condensation risk
Boost mode not working	Wiring fault; wrong sensor type; unit configuration error	Building Standards non-compliance
Noise at terminals	Ductwork velocity too high; terminal incorrectly positioned	Occupant complaints; potential re-design required
Filter blocked at commissioning	Construction dust entered system during installation	Flow rates reduced; unit running at high static pressure

Who Should Commission MVHR?

Commissioning must be carried out by a competent person. In practice this means someone who:

• Holds a recognised commissioning qualification (CIBSE commissioning codes; BPEC ventilation commissioning; manufacturer-specific training)
• Has calibrated airflow measurement equipment
• Understands the specific MVHR unit installed and can access its configuration menu
• Can produce a commissioning report that meets Building Control's requirements

On small residential projects, the MVHR installation subcontractor often provides commissioning as part of their package. On larger commercial or multi-unit residential projects, commissioning is frequently carried out by a specialist commissioning company working under the M&E engineer's instruction.

Commissioning and the Homeowner

Once the commissioning certificate is issued and the building is occupied, the homeowner (or facilities manager for rental stock) needs a basic introduction to the MVHR system. Key information to hand over includes:

• How to operate boost mode
• How often filters should be changed (typically every 6–12 months depending on the unit)
• Where the filter access panel is
• What the normal running sound should be (so abnormal noise can be identified)
• Who to contact if the system stops working

This handover information should be in the O&M manual and ideally provided as a one-page quick-start card in the plant cupboard or airing cupboard near the unit.