Specifier’s Guide

As new-build dwellings have become increasingly air-tight in order to reduce energy use, MVHR is now a critical element in a low-energy, high-performance Specification; it is the most effective ventilation method allowed by Building Regulations Part F and is fundamental to the Passivhaus Standard.

With such a widespread swing towards centralised MVHR Systems, Architects and Specifiers now have to embrace this ventilation strategy, but it is essential to ensure that MVHR is correctly specified, under suitable conditions and with the technical requirements fully understood from the outset.

This page summarises the Specification considerations, but we urge you to contact an independent MVHR expert as early as possible, to ensure the right outcome.

1. Air-tightness:

In theory, any dwelling could be suitable for an MVHR System, but knowing the expected air-tightness of the envelope is vital in planning the correct ventilation strategy.

If your project is governed by Building Regulations, the maximum Design Air Permeability value appropriate for MVHR is 5.0m3/hr/m2 @50Pa and the lower this figure, the better; a value of 3.0 is considered good and 1.0, excellent.

If your project is to be Passivhaus Certified, the building physics will already have been calculated and the design air-tightness value is guaranteed to be suitable.

The air-tightness level will be tested at completion and the as-built figure must not exceed the design value.

If the design air-tightness level is anticipated to be higher, an alternative ventilation strategy should be considered.

2. System performance:

The performance requirement of an MVHR System must be calculated first, as it directly influences the materials & equipment to be installed and the construction methods most suited to accommodate them. The construction and ventilation system designs must be synchronised and the involvement of an expert at this early stage is vital.

For compliance with Building Regulations Part F, the minimum continuous ventilation rate will be 0.3l/s/m2 gross internal floor area, balanced by the sum total of room-specific extract rates, with an additional, higher rate for boost airflow.

For Passivhaus Certified projects, the minimum hygienic ventilation rate is 30m3/hr per occupant, balanced by the sum total of room-specific extract rates, with additional minimum and maximum rates to cater for lower and higher-occupancy circumstances respectively.

Once the ventilation performance requirement is established, suitable equipment and materials can be selected and it is at this point that the design must also be considered.

3. Equipment selection:

The MVHR Unit(s) should be selected to provide the design airflow in a low-resistance manner, generally at no more than 50-60% of Unit capacity and within the manufacturer’s optimum resistance range – normally 150Pa.  The Unit manufacturer’s performance data should be consulted for this purpose and some also provide online product selection tools, for further assistance.

Units for residential new-build must be listed in the Product Characteristics Database in order to achieve the maximum benefit in SAP Calculations and, for Passivhaus Certified projects, they must be listed in PHPP.

The choice of Unit(s) will dictate the primary duct size for intake, exhaust, supply and extract and the secondary supply and extract duct sizes must be calculated to deliver the required airflow to receiving rooms, whilst minimising resistance and sound levels throughout.

4. Design:

Ventilation ductwork is usually the largest service to be installed in a new-build or refurbishment, but often the last to be considered and this can cause significant problems if the project is allowed to proceed without agreed designs.

A viable design is critical and duct sizes must be known from the outset to prevent being compromised by site conditions at a later stage; this will help ensure that suitable construction materials are selected, such as open web joists rather than traditional timber and that suspended ceiling voids are sufficient to accommodate co-ordinated services.

A suitably knowledgeable system designer will set out the equipment in appropriate locations (with duct routes to atmosphere and the plumbing requirements in mind), with the distribution of correctly-sized ductwork to deliver low-resistance, inaudible, balanced airflow throughout.

Intake and exhaust ducts will be insulated to the required standard – normally ≥25mm foil-backed mineral wool for Part F and ≥25mm Armaflex for Passivhaus projects – and any other ductwork installed in cold (uninsulated) voids will also be fully insulated.

Ductwork will be rigid (ideally galvanised steel, but PVC could also be considered) and the use of flexible ducting must be limited to short, taut final connections.

Semi-rigid or “radial” polyethylene ducting has become fashionable during recent years, as it allows longer, continuous runs and can be easier to route through awkward spaces; due to its small diameter, however, its capability is limited and it must not be installed in lengthy runs.

Finally, airflow must be delivered into every room in a measurable and adjustable manner – this is a mandatory Commissioning requirement of both Building Regulations Part F and the Passivhaus Standard; the use of certain products, such as linear diffusers, is not generally acceptable.

5. Implementation:

Mechanical ventilation is a strictly-regulated specialist service and must be treated as such; the completion of a new-build dwelling is conditional on its successful Commissioning & Certification and it is therefore essential to involve suitably-qualified and experienced personnel – packaging mechanical ventilation alongside other services may appear convenient, but it is often not the correct approach.

A suitable independent MVHR specialist must be registered under a National Competent Person Scheme (BPEC or NICEIC in the UK) and should be able to provide the following services:

  • System performance and compliance calculations
  • Off-plan CAD design – the dwg file will be required for this function
  • Design survey to verify the accuracy of the design & revise as necessary
  • Equipment & materials procurement
  • Turnkey Installation or Technical Installer-Support, with Commissioning to Building Regulations Part F and the Passivhaus Standard
  • Post-completion filter maintenance & servicing, as required

Whether the project contract is to be formally tendered or not, it should be appropriate to nominate a specialist contractor who is able to ensure that these vital services are rendered; ideally, this specialist will have been involved with the project from a very early stage and will have a thorough knowledge of the requirements.

In summary, these steps should help ensure a successful outcome, but leave nothing to guesswork or to the inexperienced; consult with an independent expert as early as possible and ensure they remain involved from that point onward.