Summary

Many AC installers and building owners assume that fitting an air conditioning system deals with ventilation. It does not. Part F:2021 is clear: recirculating systems — including split units, cassettes, and multi-splits — recirculate room air through a refrigerant-cooled coil. They do not introduce fresh outside air. A room relying solely on a split AC unit for "ventilation" will accumulate CO2, moisture, and pollutants from occupants, cooking, and materials off-gassing. Part F requires separate provision for fresh air, regardless of whether cooling is present.

The 2021 revision of Part F was a significant update. Volume 1 covers dwellings; Volume 2 covers buildings other than dwellings. Both introduced stricter requirements around whole-dwelling ventilation, purge ventilation, and — critically for AC installers — the interaction between mechanical cooling and mechanical ventilation systems. Where MVHR (Mechanical Ventilation with Heat Recovery) is installed alongside or instead of AC, the commissioning and balancing of both systems must be coordinated.

CIBSE TM65 is increasingly relevant as Local Authorities and developers require embodied carbon assessments as part of planning applications. TM65 provides a standardised methodology for calculating the embodied carbon of mechanical, electrical, and public health equipment — including all HVAC plant. Installers working on new builds, large commercial fit-outs, or publicly funded projects should be familiar with what information TM65 requires and how to provide it.

Key Facts

  • Part F:2021 applies to — new dwellings, material changes of use, and certain extensions in England. Wales, Scotland, and Northern Ireland have separate but broadly similar regimes.
  • Recirculating AC does not count as ventilation — split units, cassettes, and ducted systems without fresh air intake are not ventilation under Part F.
  • Minimum whole-dwelling ventilation rate (dwellings) — 0.3 l/s per m² of floor area (background ventilation), plus extract rates from wet rooms.
  • Whole-building ventilation rate (non-dwellings) — typically 10 l/s per person (sedentary office) per CIBSE Guide A, referenced by Part F Volume 2.
  • Purge ventilation — all habitable rooms must be capable of rapid ventilation (typically opening windows equivalent to 1/20th of floor area, or a mechanical purge system).
  • MVHR systems — must achieve a heat recovery efficiency of at least 70% (measured at balanced flow), and specific fan power (SFP) not exceeding 1.5 W/(l/s) per SAP 10.2.
  • Part L:2021 minimum SEER — new AC systems in buildings subject to Part L must achieve minimum SEER ratings; for split systems cooling-only, this is typically SEER ≥ 3.2 for systems up to 12 kW.
  • SAP calculations — AC systems are an energy input in SAP 10.2 (the Standard Assessment Procedure for dwellings); any fixed AC system must be declared with its efficiency.
  • TM65 data requirement — manufacturers are expected to provide embodied carbon data using TM65 Category A (product stage) at minimum; installers may be asked to compile this data for project submissions.
  • Noise from ventilation — MVHR systems must not generate more than 30 dB(A) in bedrooms or 35 dB(A) in living rooms per Part F Approved Document guidance.
  • Air permeability testing — new dwellings must achieve ≤ 8 m³/(h·m²) at 50 Pa (Part L); tighter envelopes make mechanical ventilation more critical.
  • CO2 monitoring — Part F Volume 2 recommends demand-controlled ventilation (DCV) using CO2 sensors in offices and classrooms; fixed-rate systems may over- or under-ventilate.
  • Filter maintenance — MVHR units must be accessible for filter cleaning; Part F requires a commissioning notice and user manual handed to building owner.
  • Interlock requirement — where both AC and MVHR are present, control systems should prevent simultaneous cooling and heat recovery bypass where this would be inefficient.
  • Scotland — Scottish Building Standards Section 3.14 covers ventilation; broadly aligned with England but with some different performance thresholds.

Quick Reference Table

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Ventilation Type Counts as Part F Ventilation? Typical Application
Split AC (recirculating) No Cooling only
Cassette AC (recirculating) No Cooling only
Ducted AC with fresh air intake Yes (if properly sized) Commercial HVAC
MVHR unit Yes Dwellings, offices
Positive Input Ventilation (PIV) Yes (dwellings) Retrofits
Continuous Mechanical Extract Yes Dwellings
Trickle vents + intermittent extract Yes (background) Most dwellings
Portable AC (exhaust hose) Partial (creates negative pressure) Not a Part F solution
Part F Requirement Dwelling (Vol 1) Non-Dwelling (Vol 2)
Background ventilation 0.3 l/s per m² Variable by occupancy
Extract rate — kitchen 13 l/s (continuous) or 30 l/s (intermittent) Per CIBSE Guide B
Extract rate — bathroom 8 l/s (continuous) or 15 l/s (intermittent) Per CIBSE Guide B
Purge ventilation 1/20th floor area equivalent Manual or mechanical
Commissioning certificate Required Required
Handover information Required Required

Detailed Guidance

What Part F:2021 Changed

The 2021 update (effective June 2022 for new buildings, June 2023 for material changes of use) introduced several changes relevant to AC installers. The previous single-document Approved Document F was split into Volume 1 (dwellings) and Volume 2 (non-dwellings). This matters because the standards for offices, shops, and commercial spaces are now more explicitly documented and cross-referenced to CIBSE guides.

The update also placed greater emphasis on indoor air quality (IAQ) monitoring and commissioning. For systems serving non-dwellings, there is now a stronger steer towards CO2 demand-controlled ventilation where occupancy varies — relevant in meeting rooms or spaces that might also have AC. An AC system that cools a space but allows CO2 to build up because the ventilation system is under-specified is now more clearly non-compliant with Part F intent.

Installers should note that where a building notice or full plans application is submitted for work that triggers Part F, the Building Control Body (BCB) may ask for ventilation calculations. Having the AC specification does not satisfy this — a separate ventilation strategy is needed.

MVHR Interaction with AC Systems

MVHR and AC can coexist, but they need to be designed together rather than added independently. The typical problem occurs in new-build homes where an MVHR unit is installed by the main contractor and a split AC system is added by a specialist later. If the MVHR is running in heat recovery mode and the AC is simultaneously cooling the room, the MVHR is recovering heat from the extract air and adding it back to the incoming supply — working directly against the AC. Energy is wasted and occupant comfort suffers.

Best practice is to interlock the systems so that when AC is in active cooling mode, the MVHR switches to summer bypass (where outside air bypasses the heat exchanger and is supplied directly). Most modern MVHR units have a summer bypass damper. The trigger for bypass should be based on supply air temperature, not ambient temperature alone.

Commissioning of both systems together is essential. Airflow rates on the MVHR must be balanced (supply and extract within 10% of each other per BESA TR19 and manufacturer guidance) before AC performance testing begins. An unbalanced MVHR can create positive or negative pressure in the dwelling, affecting AC performance and creating condensation risks.

For the commissioning record required by Part F, both the MVHR commissioning data and any AC commissioning data should be included in the handover file.

SAP Calculations and Fixed AC Systems

SAP 10.2 is the government-approved methodology for calculating the energy performance of dwellings. It feeds into the Standard Assessment Procedure used to produce Energy Performance Certificates (EPCs) and for Part L compliance in new dwellings.

Where a split AC system is permanently fixed (wall-mounted, ceiling cassette, or ducted), it is classified as a fixed cooling system and must be declared in the SAP calculation. The SAP assessor needs the SEER (Seasonal Energy Efficiency Ratio) of the unit. For most modern inverter split systems this will be SEER 5–7+, well above the minimum, but the assessor needs the certified value from the manufacturer's documentation or the Eurovent database.

Portable AC units are not declared in SAP because they are not fixed installations. However, if a developer is knowingly leaving a space to be cooled by portable units, Building Control may query whether the dwelling meets the overheating risk assessment under Part O:2021, which is a separate but related issue.

Part L Energy Efficiency Requirements for New AC

Part L:2021 (Conservation of Fuel and Power) sets minimum energy efficiency requirements for new and replacement fixed building services, including air conditioning. For systems installed as part of a new build or as a replacement in an existing building undergoing notifiable work, the system must meet minimum seasonal performance figures.

For split and multi-split systems up to 12 kW cooling capacity, the minimum is SEER ≥ 3.2 (Eurovent-certified or equivalent). For larger systems, the Non-Domestic Building Services Compliance Guide (published by MHCLG alongside Part L Volume 2) specifies minimum COP and SEER values by system type and capacity band.

Part L also requires that new AC systems be commissioned and have their energy consumption capable of being metered separately from other services. In commercial buildings, sub-metering of HVAC is a firm requirement under the Non-Domestic Compliance Guide where the system exceeds certain thresholds.

CIBSE TM65: Embodied Carbon Assessment for HVAC

TM65 (published by CIBSE in 2021, updated guidance 2023) provides a standardised methodology for calculating the embodied carbon of building services equipment. It covers Modules A1–A3 (product stage: raw materials, transport, manufacturing) and optionally A4–A5 (construction stage) and C1–C4 (end of life).

For an AC installer, TM65 is most likely to appear on commercial, public sector, or developer-led projects where a whole-building embodied carbon assessment is required — increasingly common under Local Authority planning conditions and RIBA 2030 Climate Challenge commitments. The Structural Engineer or MEP consultant will typically coordinate the TM65 assessment, but they need data from equipment manufacturers and sometimes from installers.

TM65 assigns equipment to categories (A, B, or C) based on data quality. Category A uses manufacturer-provided product-specific Environmental Product Declarations (EPDs); Category B uses manufacturer data with TM65 calculations; Category C uses default values from the TM65 database. Most HVAC manufacturers now publish TM65-compatible data for their main product ranges.

As an installer, if asked to contribute to a TM65 assessment, you will typically need to provide: equipment model numbers and quantities, refrigerant type and charge weight (relevant for GWP calculations), any ancillary items (pipework, brackets, insulation) not covered by the equipment EPDs.

Frequently Asked Questions

Does fitting a split AC unit mean I've dealt with ventilation?

No. A split AC unit recirculates room air — it does not introduce fresh air from outside. Building Regulations Part F requires separate ventilation provision regardless of whether cooling is present. In a domestic installation, this means trickle vents plus intermittent extract fans at minimum, or a whole-house MVHR system.

Does Part F apply to a like-for-like AC replacement?

Generally no — like-for-like replacement of an existing AC unit in a dwelling is unlikely to trigger Part F. However, if you are replacing a system and the building has no adequate ventilation, you are not required by Part F to rectify this (though you should advise the customer). Where a replacement is part of a broader refurbishment that constitutes a material change of use or major renovation, Part F may apply to the whole project.

What is the difference between Part F and Part L in relation to AC?

Part F covers ventilation and indoor air quality — how much fresh air gets into the building. Part L covers energy efficiency — how efficiently the AC system uses electricity. Both can be triggered by the same installation, but they require different documentation: Part F needs a ventilation strategy and commissioning record; Part L needs SEER/COP data and metering provisions.

When does TM65 apply to my installation?

TM65 is not a Building Regulation — it is a CIBSE technical memorandum used voluntarily or as specified in a project's sustainability brief. You are likely to encounter it on commercial, public sector, or developer-led projects where a whole-building carbon assessment is required. If your client or the MEP consultant asks for TM65 data, they typically need equipment model numbers, refrigerant type and charge, and ideally EPD references from the manufacturer.

What SFP limit applies to MVHR fans?

Under SAP 10.2 and Part F, MVHR systems should have a specific fan power (SFP) of no more than 1.5 W/(l/s). Most modern MVHR units meet this, but it is worth checking the data sheet. Higher SFP means the fan motor is consuming more electricity relative to the airflow it moves, which degrades the overall energy performance of the dwelling.

Regulations & Standards