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."
The baro I installed was left over from the oil burner that was installed here in 1989. The installer determined it wasn't needed and left it behind. The instructions in the box showed the bullhead installation acceptable for solid fuel appliances. The current
installation instructions indicate that this position should not be used for the damper location.
It sometimes can be used in a bullhead TEE if you have a good draft as many members here run that install. It will however alter the plate's curve in relation to the draft.
My chimney drafts like a vacuum cleaner, I opened the ash door last year and held the baro closed and the draft reached -.13" WC and was still climbing when I let the baro go and closed the ash door. The baro seems to work well in this location. A manometer is a must when setting the draft as the baro is neither horizontally or vertically installed, but rather a combination of both. The markings on the baro are meaningless. I have seen the baro flutter the past few days with the windy conditions, and the stove is cold as a well digger's shovel right now.
One advantage of this location is it makes it easy to check the horizontal pipe for fly ash accumulation.