That’s not a balanced flue. It’s a draught stabiliser as above. A balanced flue has its air inlet for the fire in a tube concentric to the flue outlet so that wind effects on the terminal are balanced across the air inlet and flue outlet.A balanced flue, needed for things like old agas and Rayburns. Oil burning things like this need a specific (ish) amount of draught/pressure differential to operate efficiently.
May be used elsewhere but that's my experience of them
A draught stabiliser regulates the draught. Responding to changes in pressure in the flue. If there is too much draw it will be pulled open allowing the flue to draw air in from the room rather than pulling more air through the firebox. As said above some appliances are really sensitive to draught so need to be protected from fluctuations. It needs to be set using a draught gauge in the flue below the stabiliser and set according to the appliance manufacturers instructions. The weight at the bottom is used to adjust the sensitivity.I'm assuming this is one?
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What does it do and how do you set it?
Mine is currently ticking away opening and closing , but what is the function?
Is it so the wind dosnt blow the fumes back?