Ventilation & Part F Under the Future Homes Standard

Ventilation is one of the most significant practical changes under the Future Homes Standard. The combination of dramatically tighter airtightness targets and the shift to low-carbon heating makes mechanical ventilation with heat recovery (MVHR) the de facto standard ventilation strategy for new homes. Meanwhile, the Home Energy Model (HEM) introduces a far more sophisticated ventilation calculation that rewards well-designed systems.

Why Ventilation Matters More Under the FHS

In a conventionally built home (airtightness of 5–8 m³/(h·m²) at 50 Pa), a significant proportion of ventilation happens unintentionally — through gaps around windows, at junctions, and through the building fabric. This uncontrolled infiltration wastes energy but does at least provide some fresh air.

FHS homes target an airtightness of 3 m³/(h·m²) at 50 Pa — tight enough that uncontrolled infiltration is minimal. This is excellent for energy efficiency, but it means controlled mechanical ventilation must take over the job of providing fresh air and removing moisture, odours, and pollutants. Without it, occupants would face poor indoor air quality, condensation, and potentially mould growth.

This is where Part F (Means of Ventilation) of the Building Regulations comes in. Part F sets the minimum ventilation rates that must be achieved, and it is being updated alongside Part L under the FHS to reflect the new building envelope standards.

Mechanical Ventilation with Heat Recovery (MVHR)

MVHR systems continuously extract stale air from wet rooms (kitchens, bathrooms, utility rooms) and supply fresh filtered air to habitable rooms (bedrooms, living rooms). The key innovation is the heat exchanger at the heart of the system, which transfers heat from the outgoing stale air to the incoming fresh air — typically recovering 85–95% of the heat that would otherwise be lost.

How MVHR Works

1. Extract: Stale, moist air is continuously drawn from kitchens, bathrooms, en-suites, and utility rooms through ceiling or wall-mounted valves
2. Heat exchange: The warm extract air passes through a heat exchanger, transferring its heat to the incoming fresh air without the two airstreams mixing
3. Supply: Pre-warmed, filtered fresh air is delivered to bedrooms, living rooms, and other habitable spaces
4. Exhaust: The cooled, stale air is expelled to the outside

Design Considerations for MVHR

MVHR must be designed into the building from the outset — retrofitting is difficult, expensive, and often compromises performance. Key design considerations include:

• Duct routing: Short, straight duct runs minimise pressure drop and fan energy. Rigid or semi-rigid ducting is preferred over flexible for performance and hygiene
• Unit location: Central placement reduces duct lengths. Common locations include utility rooms, cupboards, or loft spaces (provided the unit and ducts are within the insulated envelope)
• Acoustic performance: Units must be acoustically isolated from habitable rooms. Duct attenuators may be needed on supply branches to bedrooms
• Commissioning: Correct airflow balancing at each valve is essential. Poorly commissioned MVHR systems are a common source of complaints
• Maintenance access: Filters need replacing every 6–12 months. The unit and primary filters must be accessible to occupants

MVHR vs Other Ventilation Strategies

How HEM Models Ventilation

One of HEM's most significant technical advances over SAP is its ventilation calculation. SAP used simplified shelter factors and wind correction tables to estimate infiltration and ventilation rates. HEM replaces this with a detailed pressure-driven model based on BS EN 16798-7:2017.

The Pressure-Driven Approach

At each half-hourly timestep, HEM calculates air movement through every opening in the building envelope based on:

• Wind speed and direction from hourly weather data for the specific location
• Stack effect — the pressure difference caused by indoor/outdoor temperature differences (warm air rises)
• Mechanical system pressures — the extract and supply rates of the MVHR or MEV system
• Opening characteristics — the size, location, and flow characteristics of every ventilation opening, including trickle vents, extract valves, and adventitious openings

This means HEM can properly model the interaction between wind conditions, the mechanical ventilation system, and the building envelope — a level of detail that SAP's simplified approach simply could not capture.

What This Means in Practice

The more sophisticated ventilation model rewards:

• Good airtightness — reducing uncontrolled infiltration saves energy, and HEM properly credits this
• Efficient MVHR systems — higher specific fan power (SFP) systems are penalised more accurately
• Correct system sizing — oversized or undersized ventilation systems show their true energy impact
• Site-specific wind exposure — sheltered sites genuinely perform differently from exposed ones

Part F Requirements

Part F of the Building Regulations sets minimum ventilation rates to ensure adequate indoor air quality. The key requirements include:

Whole-Dwelling Ventilation Rate

The minimum whole-dwelling ventilation rate is typically calculated based on the number of bedrooms and the floor area, with a minimum of 13 l/s for a one-bedroom dwelling rising to 25 l/s for a five-bedroom dwelling. MVHR systems must be designed to deliver at least this continuous background rate.

Extract Ventilation Rates

Boost rates must be achievable for cooking, bathing, and other moisture-generating activities. Most MVHR systems include boost control via humidity sensors, occupancy sensors, or manual switches.

Purge Ventilation

All habitable rooms must have purge ventilation capability — typically through opening windows — to rapidly clear the room in case of accidental pollutant release. This requirement remains regardless of the mechanical ventilation strategy and overlaps with Part O's summer ventilation requirements.

Airtightness and Ventilation — The Critical Relationship

Airtightness and ventilation are two sides of the same coin. The principle is simple: build tight, ventilate right. An airtight building envelope eliminates uncontrolled heat loss, while the mechanical ventilation system provides controlled fresh air with heat recovery.

The FHS target of 3 m³/(h·m²) sits firmly in the range where MVHR is not just beneficial but essential. Below about 5 m³/(h·m²), natural infiltration is insufficient to maintain indoor air quality, and any ventilation strategy without heat recovery wastes the energy saved by improved airtightness.

Practical Guidance for Designers

• Design airtightness and ventilation together — they are interdependent. Specify the air barrier strategy and MVHR layout simultaneously, not sequentially
• Allow space for duct runs — MVHR requires dedicated duct routes from the central unit to every habitable room and wet room. A typical 3-bed home needs 80–120 m of ducting
• Specify the MVHR unit early — HEM requires specific make/model data from the PCDB, and the unit dimensions affect the building design
• Consider acoustic impact — avoid locating the MVHR unit adjacent to bedrooms. Specify duct attenuators on bedroom supply branches as standard
• Plan for commissioning — allow for airflow measurement at every valve during commissioning. Record measured flow rates for handover documentation
• Educate occupants — many FHS homeowners will be unfamiliar with MVHR. Include clear instructions for filter replacement, boost control, and summer bypass operation

Frequently Asked Questions

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