This can vary due to your boiler but for most purposes 75mm above the boiler flue. Protects your boiler from damaging rain ingress in heavy rain and wind. It was also protect your boiler in the event of guttering failure.
eBay
Non-Removable Wall Faucet Hole Cover Telescopic Decorative Cover Heighten Larger Faucet Shower Pipe Plug Wall Faucet Hole Cover
Aliexpress UK
They can be used for flue gas, ventilation ducts, etc.
eBay
Ironing Board Covers 150 x 50cm Sleeve Collars Ironing Board Cover Extra Wide Anti-Scalding Cover
Aliexpress UK
Stainless Steel Flue Liner Cowl.
eBay
Outdoor Swing Cover Oxford Cloth Waterproof Garden Swing Chair Cover Black Furniture Dust Cover 3 Seater A Frame Swing Protector
Aliexpress UK
Made with high-quality materials, it is built to last and withstands the rigours of daily use. Blank plates available. Four 5mm screw holes in each corner of the plate. 300mm x 300mm plate.
eBay
Dust-proof Cover Courtyard Furniture Cover, Rectangular Outdoor Table Cover, Suitable for Terrace, Balcony, Garden Protection
Aliexpress UK
STAINLESS STEEL FLUE ELBOW 45-90 drg and adjustable elbows. LENGTH OF ELBOW approx - 30 CM. LENGTH OF PIPE - 100 CM or 50cm. MATERIAL - STAINLESS STEEL. WALL BRACKET. WALL PLATES.
eBay
Swing Canopy Replacement Cover 2 Or 3 Seater Garden Swing Chair Cover Waterproof Gazebo Canopy Replacement Sunshade Cover
Aliexpress UK
Simply place it over your chimney flue and enjoy knowing your chimney is protected. Stay worry-free and keep your chimney in top condition with this chimney cap. 1x Fireplace Chimney Cap. Greenhouse/Shed/Raised Bed.
eBay
Garden Furniture Cover, Outdoor Furniture Cover, Heavy-Duty Oxford Cloth Garden Table Cover, Suitable for Outdoor Tables, Chairs
Aliexpress UK
Stainless Steel Flue Liner Rain Cap.
eBay
4pcs Radiator Pipe Covers 15*200mm 300mm 500mm Radiator Pipe Covers Adjustable Radiator Pipe Decorative Cover Wall Decoration
Aliexpress UK
【Application】: Rainproof cap is great for roof vents, pipe, chimney tubes, flue pipe, etc. It easily prevents the accumulation of leaves, rain, and snow, as well as the hazards of wind and sparks, providing year round protection for your chimney's safe.
eBay
Funshot Sun Lounger Cover Waterproof 210x75x40x80CM 210D Oxford Sunlounger Cover Windproof Anti-UV Outdoor Patio Sunbed Cover
Aliexpress UK