Breckwell P24FS - Manual (Page 9)

C. VERTICALLY INTO EXISTING CHIMNEY SYSTEM
Adapters are available to adapt from 3” L-Vent to 6” or 8” Class-A
chimney. (Figure 10a)
As an alternative, 3” or 4” L-Vent can be run inside existing chimney
to termination. (Figure 10b)
This is the preferred method.
Follow guidelines for equivalent vent length.
INSTALLATION 9
FIGURE 10a
FIGURE 10b