This is right from Field Controls. http://www.fieldcontrols.com/draftcontrol.php
Read about 3/4 of the way down.
Draft Control (Installation Options)
"Recommended Locations for Field Draft Controls
For gas-fired equipment, the preferred location of the control is on the bull head tee. This location provides maximum relief of downdrafts with minimum positive pressure. (See Figure 1, Diagram A-C)With oil or solid fuels, the bull head tee is not recommended
, so locate the control as shown. (See Figure 1, Diagram D-J) These locations are acceptable for gas units as well. Except on forced draft systems, locate the control as close as possible to the furnace or boiler, at least 12" beyond a stack switch on oil-fired units, and at least 18" from a combustible ceiling or wall."