I am currently working on a bathroom remodel and installing an exhaust fan. The bathroom never had an exhaust fan before, so I am trying to decide the best way to complete the installation. There is an adjacent bathroom with an existing fan and 6" duct work vented outside. Is it acceptable and to code to tie into the existing duct work with a connector for the new fan? If not, what is the most efficient and cost effective way to install the new fan?
I am unaware of the building codes in your local area, but in general I like to see each bathroom within a home have its own independent exhaust fan, duct work, and exterior vent. It is also crucial that a bathroom's mechanical exhaust system does not recirculate unwanted moisture back into the house by way of the soffit, crawl space, attic or a shared duct.
If possible, you should install an exhaust fan in your new bathroom and run the exhaust duct within the space between the overhead ceiling/floor joists to a self sealing louvered vent located on an exterior wall of your home. Most bathroom exhaust fans rely on a short run of 4-inch or 6-inch ducting to move moisture and smells out of a home's bathroom, but the acceptable length of the duct work should be determined by the size of the unit's exhaust motor, and installation guidelines are generally included with the fan when purchased.
If you do want to install independent exhaust fans for each of your home's bathrooms, then you may be able to utilize a whole-house exhaust system. A whole house exhaust system would rely on one large in-line fan to pull the stale and moist air out of several locations within your home and expel the unwanted air through a single exterior vent. Keep in mind that depending on the needs of your specific house, this type of system may require an eight-inch (or larger) roof penetration to accommodate the necessary exhaust duct and roof vent, or might necessitate adding a make-up air unit or a heat recovery ventilation system. If you decide that you want a whole house exhaust system, then you should probably hire an HVAC contractor to design and install the system for you.