Quick Answer

For HVAC ventilation openings, match the core bit to the duct outer diameter plus 10-20mm for the sleeve: 150mm for 125mm ducts, 200mm for 160mm ducts, and 250mm for 200mm ducts. Use a wet core bit on reinforced concrete walls and a dry bit on block and brick. A clean round hole lets you fit the sleeve and seal without patching.

Common HVAC Duct Sizes and Core Bits

Duct Diameter	Recommended Core Bit	Material Note
100mm	125mm core bit	Sleeve + seal clearance
125mm	150mm core bit	Standard extractor fan
160mm	200mm core bit	Kitchen extract, larger fans
200mm	250mm core bit	Main ventilation ducts

How to Drill Clean Duct Openings

• Confirm the duct route and the hole centre on both sides of the wall.
• Use a core drill rig for holes over 100mm — handheld is not safe at 200mm+.
• Drill from the outside in, or use a pilot through both leaves of a cavity wall.
• Keep the machine level so the duct sleeve sits true.
• Remove the core, then test-fit the sleeve before sealing.

Wet or Dry for HVAC Work?

• Wet — exterior walls, reinforced concrete, dense precast. Cleanest finish.
• Dry — interior block and brick, where water cleanup is a problem; use a dust collector.

Safety Tips

• Check for rebar, cables and other ducts before drilling.
• Slope the sleeve slightly outward for extractor ducts to avoid rain ingress.
• Seal the gap with foam or mastic rated for the application; use core drill accessories for sleeves and adapters.

Frequently Asked Questions

Q: What if my duct is bigger than 200mm?

A: Core bits go up to 350mm+ — match the sleeve size. JOY TECH supplies large-diameter bits for industrial ventilation.

Q: Can I drill an HVAC hole with a holesaw?

A: Not in concrete — use a diamond core bit. For masonry see the diamond core drill bits range.

Q: Do I need to core both leaves of a cavity wall?

A:Yes — drill the first leaf, then drill through the second leaf aligned with a pilot or guide. A single long bit can also pass both leaves.

Conclusion

Size the core bit to the duct plus sleeve clearance, drill square with a rig and seal properly. JOY TECH supplies HVAC-ready core bits in 100-350mm diameters; contact us for project quotes.