The correct core drill size for a boiler flue depends on the flue system type — not just the boiler make. A standard single-pipe concentric flue, a twin-pipe system (separate air and exhaust), and a flexible flue liner all have different pipe outer diameters, and getting the hole size wrong means either a gap too large to seal cleanly or a pipe that will not pass through at all.
This guide covers all three flue types with specific bit sizes, plus advice on drill direction, clearances, and what to do when the wall material complicates the job.
Standard Single-Pipe Concentric Flue (Most Common)
The most common configuration on modern UK condensing boilers is a concentrically mounted flue — one pipe carrying exhaust gases inside a larger pipe drawing in combustion air. The outer pipe diameter on most standard concentric flues is 100mm.
Correct core bit size: 107mm
This 7mm clearance is sufficient to pass the flue assembly through and apply a weatherproof sealing collar. Manufacturers including Worcester Bosch, Vaillant, Ideal, and Baxi all specify a 100mm flue OD on their standard concentric terminal kits — verify against the installation manual for the specific model being fitted.
Twin-Pipe Flue Systems
Twin-pipe systems use separate 80mm diameter pipes — one for air inlet, one for exhaust — rather than a concentric assembly. Each pipe passes through an individual hole in the wall.
Correct core bit size for each pipe: 107mm
An 80mm pipe OD with a 107mm hole gives 27mm total clearance — more than the concentric arrangement but within standard practice for twin-pipe runs. Some installers use an 82mm bit when working in tight spaces (e.g., through a narrow wall section or close to a window frame), but 107mm is standard.
The two holes must be drilled at the correct centre-to-centre spacing specified by the boiler manufacturer — typically 150mm centre-to-centre. Mark both centres accurately before drilling either hole.
Larger Concentric Flue Systems (125mm)
Some high-output boilers and commercial condensing units use 125mm diameter concentric flue kits rather than the standard 100mm.
Correct core bit size: 127mm or 132mm
Check the flue kit specification carefully. A 125mm OD flue needs at least 127mm clearance — the standard 107mm bit leaves the pipe unable to pass through. A 132mm bit gives comfortable clearance for the sealing collar.
Flexible Flue Liner (Solid Fuel and Oil Appliances)
Flexible stainless steel flue liners used in chimney relining for solid fuel stoves or oil-fired appliances have different ODs to gas boiler flue kits. Common flexible liner diameters are 125mm, 150mm, and 175mm (for the liner plus its insulation wrap).
For flexible liner entry through a solid wall or chimney breast, the correct approach is to size the core bit to the liner OD plus a minimum of 10mm clearance for the fire-rated sealing plate.
Drill Direction for Boiler Flues
The direction of drilling matters for two reasons: wall surface finish and flue run alignment.
Drill from outside inwards where possible. This keeps any blow-out on the external wall face — easier to make good with mortar or an external sealing plate — rather than damaging internal plaster or render. It also allows you to monitor the exit point alignment from inside the property.
The flue must slope slightly downward toward the outside (minimum 1.5 degrees, typically 3 degrees) to drain condensate away from the boiler. Drilling at this slight angle means the external exit point will be slightly lower than the internal entry. On a 300mm wall with a 3-degree angle, the exit point is approximately 16mm lower than the internal mark. Account for this when marking the external position.
Wall Material Considerations
Most UK domestic boiler flue holes pass through external brick cavity walls — brick, cavity insulation, and an inner block leaf. A standard 107mm dry diamond core bit handles this without difficulty.
If the wall includes a concrete lintel, reinforced concrete, or engineering brick, a dry bit may glaze. Use a universal core bit (such as the Marcrist CCU850X) or switch to a wet-rated core bit with a water supply. For solid concrete or reinforced walls — common in commercial premises and older system-built housing — refer to the reinforced concrete drilling guide.
After Drilling: Sealing and Clearances
Building Regulations Part J requires minimum clearance distances for boiler flues from windows, doors, and other openings — typically 300mm from any opening window and 600mm from a gas meter. The Gas Safe engineer fitting the boiler is responsible for confirming compliance, but the hole position must account for these clearances before drilling.
The annular gap between the flue pipe and the hole through the wall must be sealed with fire-rated intumescent sealant or a proprietary flue sealing plate to prevent both air infiltration and the spread of combustion products into the wall cavity.
For the right bit to complete this job, see the full range in the diamond core drill bits guide, or browse recommended core drill machines for the job.
