The Last Mile of MVHR: Why Commissioning is the Most Important Step

The Commissioning Gap

MVHR design is a precision exercise. The designer calculates supply and extract airflow for each room based on occupancy, floor area, and pollutant load. Duct diameters are sized to the calculated index circuit pressure drop. The unit is selected to provide the design flow rate at the total system resistance. The result on paper is a system in balance.

In practice, the commissioning engineer measures something different. Supply airflows are typically 10–30% below design in the first commissioning attempt. Extract flows show even greater variation. The reasons are well understood: additional fittings added during installation, duct bends tighter than drawn, flexible duct kinked behind joists, valve positions set to factory defaults rather than calculated positions.

This gap between design and as-built performance is where Part F Ventilation Certificates are failed, where occupants experience noise problems, and where condensation risk in bathrooms persists despite an MVHR system being installed.

Specific Fan Power: The Performance Metric That Matters

The regulatory benchmark for MVHR performance is Specific Fan Power (SFP), defined as the electrical power consumed by the fan per unit of airflow — expressed in W/(litre/second) or W/(m³/h). SAP 10.2 sets a maximum SFP of 1.5 W/(l/s) for a certified system at design airflow. Lower is better: a well-commissioned system should achieve 0.9–1.1 W/(l/s).

SFP increases when duct resistance is higher than designed. Every additional 10 Pa of resistance pushes the fan motor to work harder to maintain airflow — consuming more power and generating more noise. In practice, the difference between a well-installed system and a poorly installed one can be 0.4–0.6 W/(l/s) of SFP — equivalent to running the ventilation fan at 30–40% higher power consumption continuously for 25 years.

The Index Run Calculation: Why 77.6 Pa Matters

The index circuit of an MVHR duct layout is the path with the highest resistance — the circuit that will be the hardest to push air through. The index run determines the duty point of the fan selection: the total external static pressure (TESP) the unit must overcome.

In a typical two-storey house with a central MVHR unit in the loft, the index circuit is often the ground floor extract from the kitchen — the longest run, with the most bends and fittings. For a 175 m² house, this circuit commonly has a calculated resistance of 65–90 Pa. Adding the unit internal resistance (typically 10–18 Pa at design flow), the total fan duty is 75–108 Pa.

The Integravolt MVHR calculator outputs the index run resistance as part of the duct design. A calculated index pressure of 77.6 Pa, for example, maps directly to the fan duty curve of the selected unit, confirming that at design flow rate the unit operates within its certified performance band — neither under-powered (insufficient airflow) nor over-powered (excessive noise and energy consumption).

If the unit is selected without reference to the index circuit resistance, it is impossible to know whether the fan will operate at its optimal efficiency point.

The Part F Ventilation Certificate

Approved Document F (2021 edition) requires a commissioning record for all MVHR systems installed in new dwellings. The Ventilation Certificate must include:

1. The dwelling address and installer details
2. The MVHR unit make, model, and certification reference
3. Design supply and extract flow rates for each room (l/s)
4. Measured supply and extract flow rates for each room after commissioning (l/s)
5. Measured Specific Fan Power (W/(l/s)) at the commissioning flow rate
6. Date of commissioning and commissioning engineer signature

The tolerances allowed between design and measured flows are defined in BSRIA BG 2/2010: plus or minus 10% on individual room flows, with the whole-house extract total within +10%/−5% of design. Exceeding these tolerances means the certificate cannot be signed, the Building Control completion certificate may be withheld, and — in the case of an SAP-assessed dwelling — the energy performance certificate may not be issuable.

Why Balancing Takes Longer Than Installation

A skilled MVHR commissioning engineer will typically spend 2–4 hours on site balancing a 4-bedroom house system. This involves iterative adjustment of valve positions — starting from the index circuit and working back to the unit — re-measuring after each adjustment set. Attempting to commission in under an hour is a red flag; it means the engineer is recording design values rather than measured ones.

The Integravolt Engineering PDF includes an Installer Technicals section on Page 2 that outputs the room-by-room design flow table, the index circuit resistance, and the commissioning target tolerances. This document is intended to be handed to the commissioning engineer on the day, ensuring the measured data collected against the design basis — not an approximation of it.