To begin with, let's conduct a test that will show whether we need a fan in the kitchen or bathroom in general. Bring a match or lighter to the vent. If the flame goes out or deviates in the direction of the hole, then there is no point in installing the fan, the ventilation system is working. If the flame does not react, then a fan must be installed.
You can sometimes hear advice devoid of reason, saw off the bottom of the door so that a gap forms between the floor and the door, I wonder if the advisers themselves have already done this with their $ 300-500 door, which assumes a tight closing?
In our XXI century, the century of perfect technologies, there are fans that will not only not spoil the external, aesthetic appearance of your kitchen or bathroom, but also, on the contrary, will organically merge with the interior. If it is a fan for a bathroom, it must be protected from steam and splashes, its electrical safety rating must be at least IP 45, the capacity must correspond to an indicator of 100 m3 / hour.
The kitchen is more demanding on the circulation of constant fresh air in it, which is why a more powerful fan should be connected in the kitchen, its capacity should be 200-300 m3 / h. Depending on the room, the connection diagram for the exhaust fan should also be selected. Where it does not have any fundamental requirements, you should use the "classic" connection scheme, for example, in the toilet, this is done by parallel connection with the lamp. It is very convenient, the fan turns on and off at the same time as turning on / off the light in the toilet.
It is important with such a connection to take into account that if the voltage of the lamp is 12 volts, then it is necessary to connect the fan to the step-down transformer. In the case of a kitchen, this method would be inappropriate. Since the fan must also work during daylight hours, when we do not use light, for these purposes there are quite convenient fans with a built-in timer, which is set on and off times, or simply turn on, and it will work as long as you do not turn it off yourself.
An exhaust fan with a sensor that reads the level of humidity in the room is ideal for a bathroom, they are more expensive than ordinary ones, but they will work better, it will automatically turn on when the humidity values are exceeded.