RAIS Palo Instructions For Installation, Use And Maintenance Manual page 9

If it is found that there is excessive draught in the chimney then either an adjustable
flue damper or alternatively a draught stabiliser should be fitted. The adjustable flue
damper should not close off the flue entirely, but should in its closed position leave a
minimum continuous opening free area of at least 20 % of the total cross sectional
area of the flue or fluepipe.
Adequate provision e.g. easily accessible soot door or doors must be provided for
sweeping the chimney and connecting fluepipe.
You should also familiarise yourself with the draught conditions for chimneys with 2
flues.
For strong draughts, the chimney or flue should be fitted with a draught stabiliser. In
which case, it is important to ensure that there is a free flow-through area of minimum
2
20 cm when the regulating gate is shut. Otherwise, the fuel energy may not be used
optimally.
If, at any time, you are unsure about the condition of the chimney, you should contact
a chimney sweep.
Remember that access to the soot door should be kept clear.