Specifications

41
5. Determine available draft
D
a
= -D
t
+ P
s
= -0.091 + 0.039
= -0.052”WC
The available draft is negative so the system should
work.
If the draft is positive, adjustments can be made to
the system to make it negative. There are two ways
to solve this problem: increase the chimney height or
increase the chimney diameter.
Analyzing other factors
If the replace and chimney seem to be sized
correctly, but there is a smoke problem, other
information may be needed.
A depressurized building will negatively affect the
draft situation. If the negative pressure is found to
be 0.1”WC, the chimney would have to be able to
overcome this as well and the draft calculation would
look like this:
D
a
= -0.052”WC + 0.1”WC = +0.048”WC
In other words, the negative building pressure easily
overcomes the draft and would make the system
back draft.