How much ducting can I attach to my fan
A common question, and the guidance below is more relevant to domestic type installations.
Domestic Axial fans
An axial fan has a fan blade like a ship propeller. These are low pressure fans and a general rule of thumb for these is to use no more than 1.5 metres of ducting. On a 100mm fan/ducting system, a 90 degree bend is equivalent to 1 metre, preferably rigid duct. For longer runs, a centrifugal type fan should be considered.
Domestic Centrifugal fans
A Centrifugal fan has a fan blade like a hamster wheel. The question of duct run length becomes more difficult here. Centrifugal fans are higher pressure fans and so cope better with ducted runs or where installations are in exposed positions such as an external wall prone to strong gusty winds. To know how far a Centrifugal fan is going to push the air means knowing the overall system resistance/pressure - Anything in the path of the airflow causes a resistance - this could be ducting straight lengths, bends, filters, grilles etc..... In a domestic wet room, you need to be considering a Centrifugal fan once you are over 1.5 meters of ducting, but you must still ensure that the chosen fan will achieve the required airflow for the room being ventilated with the system resistance taken into account.
Can I put more than one fan into a common duct
You should not put more than one fan into a common duct.
Each fan should have its own duct run to atmosphere. If you try to combine them into a common duct it will bring with it a number of problems, including an unbalanced system with positive/negative air pressures which play havoc with the fan motors and could cause motor failure. It can also cause cross-contamination issues.
Also not to forget that incorrect installation of the fan could void the product warranty.
In residential and some commercial applications, an alternative to multiple fans into common ducts could be a Central Extract System Fan such as our Lo-Carbon Multivent MVDC-MS or Lo-Carbon Sentinel Multivent