Here are some things you'll want to consider when you're ready to replace your bathroom exhaust fan:
There is nothing worse than a fan that roars each time you turn it on. The sound fans make is rated in sones. A typical economy fan will be rated approximately 2 or 3sones or more, and a higher quality, quieter fan will be rated 1 sone or less. I recommend you don't have anything installed that is More than 1 Sone - if you're paying to have a fan installed it is much better economy to pay the extra for a quality fan rated less than 1 sone.
How much Air Moves
Fans that don't actually change the air in the bathroom quickly are also a pain, and not good for your health or the longevity of your home. You'll need to calculate how big your fan needs to be for the space it's ventilating.
Most bathroom fans are rated 80, 90, or 100 cfm, (cubic feet of air moved per minute) but can be as high as 380 cfm. Generally they should be sized to replace the air in the room 8 times in an hour. For most bathrooms this works out to about 1 cfm per square foot of the bathroom. So, if your bathroom is 8 feet by 10 feet, you'll require an 80 cfm fan.
People are often tempted to just put in the largest fan possible; however, the larger the fan, the more noise it makes and the more energy it will require.
The size of the duct is also important - the larger the duct, the quieter the fan will be. A 3" duct will be quite noisy, where most 4" or 5" ducts are fine. Most building codes now require a 5" diameter duct.
An inline fan is also an excellent option as the noise is much lower, although the fans cost more to buy and install. In these units the fan is farther up the duct and less noisy, and there is more flexibility in where they can be installed. Many bathrooms with skylights use these as they can be mounted quite high in the bathroom and at an angle if necessary.
A better quality fan will require less energy to run, so it's important to consider this factor when you're choosing a brand and quality. The size of the fan factors in here too, the larger the cfm rating, the more energy it takes.
There are two-stage fans available as well, which are slightly more energy efficient, and provide more air circulation. In these, the fan runs at a low speed at all times, and increases to a high speed for a short time when turned on.
Generally, just making sure your fan is energy star rated, the right size for the room, and a reasonable quality will ensure reasonable energy efficiency.
I hope that this answers some of your questions about exhaust fans. If you need a fan or would like to talk to an electrician about exhaust fan installation contact us.