The H Cowl is probably the ultimate static anti downdraught chimney cowl, as it completely isolates the combustion gases from the prevailing winds. Manufactured using stainless steel grade 430.
These H Cowls have mesh inserts, and are suitable for use with appliances fired by oil or gas.
Suitable for use with 175mm internal diameter pipes.
The H cowl has a shaft which is just slightly under the diameter of the cowl, so for example, a 125mm H Cowl will have a shaft with a diameter of around 120mm. It then slides inside the chimney or flue system.
Fitting to a prefabricated chimney system
Slide the cowl inside the chimney system. Secure in place with stainless steel self tapping screws and seal with a high temperature silicone or sealant.
Fitting to a masonry chimney stack or pot
Slide inside the chimney or pot and mortar in place.
Guaranteed Solution at a Price The H cowl is going to prevent any downdraught, no question, as the combustion gasses and prevailing wind are completely isolated from each other. The downsides are that it's physically large and, ioho, ugly. There is no fixing method as such, it needs to be cemented in place. Does the job though.
fluesystems.com - High Wycombe