Am installing new PB 105 and the instructions say not to install the venting in a chimney over a certain length according to a table in the manual. It looks like that if I vent the boiler and connect its 4" vent to my 8" masonry chimney (lined with a 6" liner), the vent will exceed the recommended overall length. My masonry flue is constructed as a 28' vertical connected to an 8" diameter horizontal 18" tile. My install manual says that's too long. Anyone know why? Is it because the boiler has a positive exhaust pressure? Also, the manual suggests that the draft in a tall chimney may be too high. It should not be over 0.85" on the water column. What is the device that this is measured with and how can it be done?