I have a bathroom fan that doesn't seem to be doing the job. I'd like to replace it with something that has much higher CFM (jet speed); is there any reason not to do this?
Is it possible that it can cause more problems?
While this would just be short term use. If you have natural gas, propane or oil appliances, this could & even would suck noxious fumes into the house which could eventually or quickly kill you. That's the bad news.
Other than that, an exhaust fan does, of course, remove heated & cooled air. If you're single I don't see this being a big deal. But, in a family situation this could evacuate the house & therefore waste a good deal of money on a daily basis.
If you're talking about something over 300cfm by a good deal, then partially open doors could slam shut. Like, you don't have your keys & your front door's knob is locked when you take out the trash, retrieve the mail or chat to a neighbor.
For bathrooms less than 100 square feet, determine the room’s CFM (cubic feet per minute, that's how they are rated in the U.S.) requirements by measuring and multiplying the length, width and ceiling height of the room, then use the multiplication factor of .13 and round up to the next “ten.”
For example: 10’ long x 8’ wide x 9’ height x .13 = 93.6, select a bath fan with at least 100 CFM.
If the bathroom is larger than 100 square feet, www.HVI.org recommends that you add the CFM requirement for each fixture present - a toilet is 50 CFM, a shower is 50 CFM, a bathtub is 50 CFM and a jetted tub is 100 CFM.
Once you know the minimum recommended CFM requirement, compare that with the CFM rating of your existing fan. If you want/need more CFM, than buy one rated for more. Pick the quietest one you can afford that has the features and aesthetics you desire. Try to get one that utilizes the same vent size, to reduce complications and expense related to duct replacement.