Introduction
Choosing an MVHR unit is not as simple as picking the biggest one you can afford. If you choose a unit that is too large, take up space and cost more and may impact on installation due to larger ductwork being required.
Too small and it will struggle to maintain adequate air quality and will run at high speed, wearing itself out prematurely and will likely be noisy.
The right choice depends on your house's floor area, airtightness level, the number of rooms and the unit's heat recovery efficiency and that it can meet regulatory requirements. This guide takes you through each factor in plain language.
What size MVHR unit do I need?
MVHR units are rated by their maximum airflow, typically expressed in litres per second (l/s) or cubic metres per hour (m³/h). The required airflow for your home is calculated on the number and type of rooms, extract rooms such as bathrooms and kitchens and supply rooms such as bedrooms and living spaces, floor area and number of occupants.
Approved Document F (Part F) of the Building Regulations sets minimum extract and supply rates per room type. A proper MVHR design calculates the total required airflow and selects a unit that can deliver this at low to medium speed (not at maximum). Running a unit at or near maximum speed makes it noisier and less efficient.
Rule of thumb
For a typical three-bedroom new build home of around 100–120m², a unit capable of 250–350m³/h (approximately 70–100 l/s) at medium speed is usually appropriate. Always have a full design calculation carried out rather than guessing.
Heat recovery efficiency - what does it actually mean?
Heat recovery efficiency (or thermal efficiency) is the percentage of heat from the outgoing stale air that is transferred to the incoming fresh air. A unit rated at 90% efficiency is capable of recovering 90% of the heat that would otherwise be lost through extract only ventilation.
Look for units certified to SAP Appendix Q, which is the UK Government's approved method for assessing MVHR efficiency. Units on the SAP Q list can be used in SAP calculations for Building Regulations compliance.
Noise levels (SFP and decibels)
Noise is one of the more common faults of a badly installed MVHR system, rather than the unit itself. That said, choosing a quiet unit helps.
Specific Fan Power (SFP) measures electrical efficiency and is expressed in W/(l/s) — the lower the number, the better. Part F requires an SFP of no more than 2.0 W/(l/s) for new dwellings. Good units achieve 1.0–1.5 W/(l/s) or lower.
Sound power is measured in decibels (dB). For any installation, you want a unit that produces less than 35 dB(A) at normal running speeds and <30 dBa in areas of higher sensitivity.
Key questions to ask before buying
- Do I need to ensure compliance with Building Regulations or am I installing this as a refurbishment to improve the air quality in my home?
- What filter grades are used, and are genuine replacement filters readily available?
- What is the warranty period, and who provides after-sales support in the UK?
- Does the unit have a summer bypass, and how does it work?
- Can the unit communicate with smart home systems or controls?
- Where will the unit be installed, and does it need to be particularly quiet?
