Purchasing a new bathroom fan? These fans should deliver eight air changes per hour. Fans are rated in cubic feet per minute or CRMs. To determine what size fan you need, measure the cubic feet of your bathroom (that's length times width times height). Divide by 60, the number of minutes in an hour, and multiply by 8, the number of air changes per hour. That's cubic feet divided by 60 minutes, multiplied by eight changes per hour. The result will be the CFM you need for your bath. The next consideration is noise, fans are rated in sones. One equals 40 decibels. New high-quality fans offer one sone or less. Noisy, cheap ones run at two sones or more. If it's too noisy, it won't be running long enough to clear the air; if it's if too small, it won't have enough power do the job in a reasonable amount of time. And that's the On The House tip for today.