Chimney OH Cowl
The OH Cowl is suitable for use on most standard conventional clay chimney pots with an 8 inch internal diameter.
Designed to promote positive updraughts and prevent foreign bodies from entering chimney the OH Cowl also prevent rain ingress.
What is the Difference Between the H Cowl and the OH Cowl?
In normal or still air conditions the hot gases and smoke pass up the chimney and exit out of the top of both of the vertical tubes. (See diagram below on left)
If wind drives downwards in turbulent conditions the air moving down the tubes induces suction as it passes the horizontal tube and the hot gases and the smoke exits out of the bottom of the vertical tubes. (See diagram above on right)
OH Cowl
In normal or still air conditions the hot gases and smoke pass up the chimney and exit out of the top of both of the vertical tubes. (see diagram below on left)
If wind drives downwards in turbulent conditions, the air moving down the tubes increase in velocity because it is restricted as it is forced through a narrow gap as it passes over and around each end of the horizontal tubular section projecting half way into the vertical tubes.
The increase in velocity (basically a venturi effect) induces suction as it passes the horizontal tube and the hot gases and smoke exit out of the vertical tubes. Any upward gusts of air will also act in a similar manner so this design of OH cowl becomes very effective in windy conditions where the chimney is subject to downdraught. (see diagram above on right)
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