FHS Construction Changes — A Builder's Guide to New Builds

Building a home to the Future Homes Standard (FHS) is fundamentally different from building to current Part L 2021 rules. Every major element of on-site construction changes: gas boilers are replaced by heat pumps, trickle vents give way to MVHR systems with full duct distribution, airtightness targets tighten from 8 to 3 m³/(h·m²), double glazing becomes triple glazing, and rooftop solar PV is mandatory. This guide walks through each change in practical detail, covering what builders need to do differently and what new skills are needed.

Heat Pump Installation

Every FHS home will be heated by a heat pump — typically an air source heat pump (ASHP). For builders accustomed to gas boiler installation, this is a significant change in both the equipment and the construction process.

External Unit

• Concrete plinth or mounting frame: The external unit needs a level, stable base. A concrete plinth (typically 900 mm × 600 mm × 100 mm) is standard, with anti-vibration mounts
• Clearance: Allow at least 1 metre around the unit for airflow. Do not locate in enclosed corners, against fences, or where airflow is restricted
• Acoustic considerations: Avoid locating directly below bedroom windows or adjacent to neighbouring properties. Check local planning noise conditions
• Pipework penetrations: Refrigerant pipework and electrical cabling route between external unit and internal components, typically through the external wall at low level

Internal Components

• Hot water cylinder: 150–250 litres, typically located in a utility room or airing cupboard. Heat pumps cannot provide instant hot water like combi boilers
• Buffer vessel (if required): 50–100 litres, prevents short-cycling on smaller systems. Located adjacent to cylinder
• Controls: Heat pump controller, room thermostat, and potentially zone valves for multi-zone systems

Heat Distribution

Heat pumps operate most efficiently at low flow temperatures (35–45°C), compared to the 70–80°C typical of gas boilers. This requires differently sized emitters:

Commissioning

Heat pump commissioning is more involved than gas boiler commissioning and typically requires a specialist:

• F-gas certification: Required for anyone handling refrigerant — checking charge, topping up, or decommissioning
• Flow rate verification: Water flow through the heating circuit must match the heat pump's requirements
• Controller programming: Weather compensation curves, hot water schedules, and defrost settings must be correctly configured
• Performance recording: Commissioning data must be recorded for handover documentation and Golden Thread records

Airtightness — Building to 3 m³/(h·m²)

Achieving the FHS airtightness target requires every trade on site to understand and respect the air barrier. It is not solely the responsibility of the drylining team or the membrane installer — any trade that penetrates the air barrier must seal their work.

Identifying the Air Barrier

The air barrier must be a single, continuous layer throughout the building envelope, clearly identified on all construction drawings. Common approaches include:

Sealing Service Penetrations

Every pipe, cable, and duct that passes through the air barrier must be individually sealed. Best practice includes:

• Proprietary grommets for individual cables and small pipes — far more reliable than site-applied mastic
• Top-hat seals or flexible collars for larger pipes and ducts
• Service zones on the warm side of the air barrier to minimise the number of penetrations — run electrical and plumbing distribution within the service void
• Pre-formed penetration sleeves for MVHR supply and exhaust ducts through the external wall

Mid-Build Testing

MVHR Installation

MVHR installation is a first-fix activity that must be coordinated with the structural, electrical, and plumbing programmes. Poor installation is the most common cause of MVHR performance problems.

Duct Routes and Installation

• Rigid or semi-rigid ducting: Preferred over flexible for lower pressure drop, better hygiene, and longer service life
• Short, straight runs: Minimise bends — each 90° bend increases pressure drop and fan energy consumption
• Route through floor voids and ceiling spaces: Plan routes to avoid clashes with joists, steelwork, and other services
• Protect during construction: Cap open duct ends to prevent debris ingress during the build. Clean ducts before commissioning
• Acoustic attenuation: Install duct attenuators on supply branches to bedrooms to prevent noise transfer from the central unit

Commissioning

MVHR commissioning requires measured airflow at every supply and extract valve, adjusted to match the design specification. Common issues include:

• Unbalanced airflow between supply and extract (causes pressurisation or depressurisation of the dwelling)
• Incorrect boost rates in kitchens and bathrooms
• Blocked or kinked duct runs reducing airflow to individual rooms
• Filters not installed or installed incorrectly

Enhanced Fabric and Insulation

The FHS requires improved fabric performance across all elements of the building envelope:

Triple Glazing

Triple-glazed units are heavier than double-glazed (typically 30–40 kg/m² versus 20–25 kg/m²). Builders should plan for:

• Stronger window fixings and wider frame profiles to carry the additional weight
• Two-person handling for larger units — manual handling assessments should be updated
• Deeper reveals to accommodate the thicker glazing units (typically 44–52 mm versus 24–28 mm for double)
• Careful air barrier detailing around window frames — the window-to-wall junction is a critical airtightness and thermal bridge location

Solar PV Installation

Solar PV must be installed on most FHS homes. Key considerations for builders:

• Structural loading: Panels add approximately 12–15 kg/m² to the roof. Verify roof structure is designed to carry the additional load
• Mounting system: Rails or integrated mounting brackets fixed to rafters through the roof covering. Weathertight sealing at every fixing point
• DC cabling: Route from panels to inverter location (typically near the consumer unit). DC cabling carries fire risk — route through fireproof conduit where required
• Inverter location: Typically internal, near the consumer unit. Allow space for the inverter and ensure adequate ventilation for heat dissipation
• Coordination with roofing: Ideally install PV mounting rails during the roofing programme to avoid re-accessing the roof. Some integrated PV-tile systems replace conventional roof tiles entirely

Digital Records and the Golden Thread

The Building Safety Act introduces the concept of a Golden Thread — a comprehensive digital record of building information that must be created during construction and maintained throughout the building's life. For FHS homes, this includes:

• As-built drawings showing actual construction details, not just design intent
• Product data sheets for every installed component (heat pump, MVHR unit, windows, insulation, etc.)
• Commissioning records for heat pump, MVHR, solar PV, and electrical installation
• Airtightness test certificates (both mid-build if conducted, and final)
• Photographic evidence of key construction stages, particularly air barrier continuity, insulation installation, and service penetration sealing
• Homeowner handover documentation covering operation of heat pump, MVHR (filter replacement, boost controls), and solar PV monitoring

Skills and Training

Builder Preparation Checklist

Frequently Asked Questions

Related Pages