I realize this is an ancient topic, but it is a classic problem. Ceiling exhaust fans have to be cleaned or they will spin and spin without moving air. I decided to tackle mine today. We leave it running 24/7 because the bedroom suite it is in has no other source of fresh air. The fan brings in air from the rest of the house. I found the brand name (Hampton Bay) on the grille and Googled it. I know it came from Home Depot and was bigger than I needed. I'm pretty sure it was the 140 CFM model for our 100 square foot bathroom. Found the installation instruction manual online and found that I only needed to remove 3 screws to lower the fan/motor out of the housing. I was about to remove the housing and all, so glad I found the instructions. To clean the fan I used a stiff bristled brush, wooden shishkabob skewer, a serrated PLASTIC knife, and of course a vacuum cleaner. Using metal scrapers or a wire brush would damage the plastic blades. Here's a better picture of the job half done. You can see the partially cleaned blades on the left and the gunk on the bottom. On the right side you can see what I was up against. This fan was installed a year ago. The people contributing to this topic who suggested using the brush attachment on a vacuum, moist cloth, or compressed air clearly have never cleaned a squirrel cage type exhaust fan. The moist air combined with dust forms a solid plaque on the fan edges that has to be cracked and then scraped off. The serrated parts of the plastic knife worked great for breaking the solidified muck off the blades. Unfortunately it seems like hundreds of blades to clean. I didn't count them, but you can see there are a lot. Every blade needs individual cleaning. I broke the surface muck off and then used the stiff brush to help scrape off the rest. That didn't work completely, so I had to hit it with both the pointed and blunt ends of the wooden stick. In the end I went over each blade about 3 times from the front and back sides. Here is the after picture. It is not perfectly clean, but getting the dirt off around the top is especially hard. That is the bottom of the blade when installed but the top when cleaning it. The whole project took 3 hours and was very tedious. Removing the squirrel cage from the motor and soaking it first might have made it easier to clean. Might try that next time. Another idea is to run just the squirrel cage through dishwasher on the top rack.

Building Code Requirement: Minimum ventilation for the bathroom is to be a window of at least 3 sq. ft. of which 50% is operable, or a mechanical ventilation system of at least 50 cubic feet per minute (cfm) ducted to the outside. (IRC R 303.3, IRC M 1506.3) Heating Ventilation Institute (HVI) Recommendations: HVI is an industry association of heating and air conditioning product manufacturers that, among other things, tests and certifies ventilation products and publishes consumer guides on ventilation issues. These are the HVI recommendations for venting bathrooms: Small Bathrooms: HVI recommends that in small bathrooms, up to 100 square feet of floor space, the exhaust fan be sized to provide a ventilation rate of 1 cfm per square foot (about eight air changers per hour). This usually results in a ventilation requirement of less than 50 cfm, so the code minimum fan size must be used. Example 1: The bathroom is 5' x 9' (with 8' ceilings). Multiply 5x9 = bathroom area of 45 sq. ft. At 1 cfm per square foot the minimum recommendation is a fan rated at 45 cfm. This is less than the code minimum 50 cfm, which must be used. Medium and Large Bathrooms: For bathrooms larger than 100 square feet in area, HVI recommends a ventilation rate based on the number and type of fixtures present, according to the following table: Toilet 50 cfm Shower 50 cfm Bathtub 50 cfm Tub/Shower Combination 50 cfm Whirlpool Tub 100 cfm To calculate the minimum fan rating, add the cfm for each fixture in the room to arrive at a total cfm. Example 2: The bathroom is 20'x12'. There is a tub, a separate shower enclosure and a toilet. Toilet 50 cfm Shower 50 cfm Bathtub 50 cfm Minimum Fan Rating 150 cfm Toilet Compartment: If a toilet is in an enclosed stall with a door, then the toilet enclosure is considered by most code authorities to be a separate room. Likewise, if a shower is enclosed right to the ceiling, it will be considered a separate room. These guidelines recommend a separate mechanical exhaust system for "each enclosure", which means a fully enclosed toilet or shower must have its own exhaust fan separate from that of the rest of the bathroom. In most localities, this is also the building code requirement. A steam shower always needs its own ventilation. Strictly follow the manufacturer's recommendation for venting a steam shower. Otherwise you will have mold growing in places you would not think mold could grow. Example 3: The bathroom is 15'x15'. There is a tub, a separate, fully enclosed, shower and a fully enclosed toilet in a stall with a door. The fully enclosed toilet and fully enclosed shower should have their own exhaust fans rated at 50 cfm each. The bathroom fan will exhaust only the bathtub and should also be rated at the code minimum 50 cfm. How Long to Ventilate: When house air is exhausted to the outside, heat goes with it, meaning that your furnace has to work harder to make up for the lost heat. You want to exhaust moist air to reduce the risk of mold and mildew, but once the moist air is gone, you want to stop ventilating. HVI recommends that the fan be left on for a minimum of 20 minutes after use of the bathroom. A timer is a good solution, allowing the fan to turn off automatically at the proper time. There are also fans on the market now that sense the level of humidity in the air and automatically turn on when it is too high, and turn off when it is back to normal. Unfortunately, there is no fan that senses odor, so a manual switch is still necessary.

Is there no roof structure - rafters - which you can run between or mount an interior side fan? If you mount an exterior side fan, look at Fantech to find smaller cfm models suitable for a residential sized bathroom. If you have deep snow, you may want to curb it, or keep the fan interior side with a gooseneck vent cap.

Having the fan come on with the light is a terrible idea. That means it goes off with the light, so if you want it to run after leaving the room you also have to leave the light on. If you don't need the light on while in the room but you want the fan on, then you are wasting electricity then, also. Since your space is a total of 600 sq ft, I will assume that you have a full bath and not just a powder room. You should always run the fan for 20-30 minutes after a shower. Most people turn off the light when they leave the bathroom, so the fan gets shut off as well. Put the fan on a separate switch, with a timer.