Intergas
- Horizontal or vertical: no white showing. No boxing allowed.
- All components must be accessible.
Worcester Bosch
- Horizontal or vertical flues: white is allowed to show as long as the joints are siliconed.
Vaillant
- Horizontal flue: white exposed is ok.
- Vertical: no white showing.
- A weather-tight box with an inspection hatch is possibly ok.
Viessmann
- The white part of the flue is ok to expose because it is UV resistant.
- 300mm away from the wall for a vertical terminal.
- On a vertical flue, no white showing.
- 1m extension on a vertical flue.
Guidance only. Check the current installation manual, and have a Gas Safe registered engineer fit the flue.