How to Install a Boiler Flue — UK Regs and Method
Boiler flues are Gas Safe-only work but every homeowner should understand the regs — where flues can go, and how far from windows.
Step-by-step method
1. Fanned vs balanced flue
Modern condensing boilers use fanned flues (plastic 60/100mm). Older non-condensing used balanced flues (metal, larger).
2. Horizontal flue rules
Minimum 300mm from window, 300mm from opening, 500mm from adjacent boundary. Downward fall 1° back to boiler for condensate.
3. Vertical flue rules
Above roof line by 600mm, or 300mm above ridge. Requires flashing kit.
4. Twin-flue for internal boiler
Separate flue and air-in pipes for boilers positioned deep inside the house. Max horizontal run per manufacturer (10m Worcester 4000).
5. Flue guards
Required where flue exit <2m above ground or near pedestrian access. Steel mesh cage. £30.
Frequently asked questions
Can flue exit into a passageway or carport?+
Only if adequate ventilation and 300mm min from any opening. Complex — needs Gas Safe assessment.
How much does a flue extension cost?+
£40 per extra 1m section. Labour +£80–£150.