Core Drilling for HVAC Installations: Hole Sizes and Bit Selection

HVAC contractors face the most diverse range of core drilling requirements of any UK trade. A domestic kitchen extractor fan needs a 107mm hole through a standard cavity brick wall. An MVHR unit needs a 133mm or 160mm duct entry through the same wall type. A commercial chiller installation may require multiple 200mm+ penetrations through reinforced concrete. Each requires different equipment, bit specification, and pre-drill assessment.

Core Drill Sizes for HVAC Applications

Extract Fans — 107mm

The standard 100mm spigot extractor fan (kitchen, bathroom, utility room) requires a 107mm core hole through the external wall. This is the most common HVAC core drilling size on domestic sites. A dry diamond core bit on a dedicated core drill machine handles standard brick or block cavity walls at 300–500 RPM. See: extractor fan core drill size guide.

125mm Spigot Fans — 133mm

Higher-airflow extract fans and some MVHR intake/exhaust units use a 125mm spigot and require a 133mm core hole. This diameter exceeds the reliable limit of SDS adaptors — a dedicated core drill machine is required. For brick and block external walls, a dry bit at 200–400 RPM. For concrete or harder masonry external walls, switch to a wet bit and reduce RPM to the lower end of the range.

MVHR Systems — 133mm to 160mm

Mechanical Ventilation with Heat Recovery (MVHR) units require separate supply and exhaust duct penetrations. Most residential MVHR systems use 160mm or 180mm diameter ductwork — requiring matching core holes through the external wall. Where the unit uses a coaxial (concentric) through-wall connection, a single larger hole (typically 200mm) may be used. Confirm the unit specification datasheet before drilling — duct diameter and spigot dimensions vary between manufacturers.

VRF and Split System Refrigerant Pipe Entries — 65–80mm

VRF and split system air conditioning installations require a lineset entry hole for the refrigerant pipes, electrical cables, and condensate drain. The combined diameter of a standard residential split system lineset plus condensate drain typically requires a 65–80mm core hole. Through concrete walls or floors, use a wet-rated bit on a dedicated machine. The hole must be sealed with a proprietary pipe sleeve or foam fill to prevent vapour transfer and draughts.

Large Supply and Return Air Ducts — 150mm to 300mm+

Commercial HVAC installations for AHUs, FCUs, and rooftop plant require large-diameter core holes for supply and return air ducting. Holes above 150mm in hard masonry or concrete require a dedicated core drill machine with a rig stand or vacuum base — torque reaction at these diameters makes handheld operation unsafe. For reinforced concrete, use a rebar-rated wet diamond core bit and reduce RPM to 100–200 RPM at 200mm+. For holes above 250mm, stitch drilling (multiple overlapping cores) may be more practical than a single large core.

Material Considerations for HVAC Drilling

External Cavity Walls (Domestic)

Standard domestic cavity wall construction is the most straightforward material for HVAC drilling. A dry diamond core bit at the appropriate RPM for the diameter handles brick outer leaf, cavity, and block inner leaf in a single pass. For 133mm and above, ensure the machine is properly braced against torque — a side handle and braced body position at a minimum; a machine rig for 160mm and above.

External Concrete Walls (Commercial and Industrial)

Commercial buildings with precast concrete panel or in-situ reinforced concrete external walls require wet coring. Confirm the concrete specification and scan for reinforcement before drilling. Rebar-rated wet diamond core bits with a soft bond matrix are correct. Machine power of 1,500W minimum is recommended for diameters above 107mm in concrete. See: core drilling through concrete: UK contractor's guide.

Roof and Floor Slabs for Plant Connections

Penetrations through concrete floor slabs for condensate drainage, refrigerant pipe drops, or vertical duct runs require wet coring. Where the slab may contain post-tensioned tendons — common in commercial buildings constructed from the 1960s onwards — a GPR scan by a specialist is mandatory before drilling. Cutting a post-tensioned tendon can cause localised structural failure. See: GPR scanning before core drilling.

Fire Dampers and Compartmentation

Where HVAC ductwork penetrates a fire compartment wall or floor, a fire damper or intumescent fire sleeve must be installed to maintain compartment integrity. The correct product depends on the duct material, duct size, and wall or floor construction:

• Circular steel ducts — fire rated duct lining or an approved fire sleeve installed at the compartment boundary. The annular gap between the duct and the wall opening must be fire stopped with mineral wool and intumescent sealant.
• Flexible ducts — generally not acceptable through compartment barriers without fire dampers. Check the approved document B guidance for the building type.
• Intumescent grilles — for terminal applications (supply/extract grilles) through compartment walls where there is no ductwork continuation beyond the wall.

HVAC contractors are responsible for ensuring ductwork penetrations through compartment boundaries comply with Building Regulations Part B. Core drilling contractors who create the holes are responsible for leaving the penetration in a condition that permits correct fire stopping installation. See: core drilling permits and regulations.

Core Drilling for HVAC: Common Questions