hey pup - I'm plenty versed in the building arena and am still in the game as opposed to siting on the bench armchair quarterbacking. "I most assuredly CAN make that statement " yes - but you'd be making an inaccurate one! "Once you know the CFM you then determine the lenght of run from the fan to the point of discharge and use the friction head loss tables to determine the correct size of duct. " Since you're so versed on this - how long is essdana's duct run? Did his/her builder installer use 3" or 4" ducting? What's the static pressure difference between 3" and 4" duct? What was it made from? What's the flow loss when using corrugated aluminum duct vs. smoothwall PVC? Do you need a slide rule for the math calculations ? Are there any elbows/bends in the duct run? How many ? It's clear you know how to do this: "Once you know the CFM you then determine the lenght of run from the fan to the point of discharge and use the friction head loss tables to determine the correct size of duct. " But, since you don't know the sizing, material , length and pressure loss of essdana's duct work install - you can't ACCURATELY say that the 40 CFM fan in question will overcome the static pressure of his/her duct work. Sure it works on paper with your slick tables - but the real world presents some curveballs. You certain learned this on your journey to Master status. And, you're not going to get to use anything but 4" duct for this fan anyway 3" if your builder /customer is cheap. So overcoming high static pressure with a clever oversized duct plan isn't really an option. Another thing - This fan we are talking about - 40CFM, is that a max. rating, average? At what static pressure was that measurement taken or is that maximum free air movement? If it was rated at free air , the second you add duct work (any kind) that rating is going to drop on a pretty steep curve as 40 CFM fans tend not to be high quality and accurately rated from the get go. So - simply going rule of thumb based on your "experience" using a few tables can get you into trouble in a real bathroom. That's my point. Your 40 CFM "idea" sized fan is great in your air conditioned office. What if the maker fudged' their rating and the thing only moves 30cfm when a duct is attached? What if two 90 deg bend are added into the duct run? How much restriction does the roof cap add to the system? How many room air changes is that 40cfm fan really going to produce at essdana's house? You can't answer that- you can only answer what the theoretical maximum would be based on your beloved tables and some educated guesses based on your experience. They are still guesses, and are no better than my guesses - so settle down boy.

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.

I can see why, with a 9 x 6 shower with 8 ft walls you would want a heater fan. That is the size of my main bathroom, minus the 30" x 60" shower/bath. We have a Broan/Nutone heater fan and it makes the room toasty. That is, when it is working. I think we have burned out the heater twice in 10 years and the light once. I always say I am going to replace it with a Panasonic the next time it breaks, then I go online and read reviews and inevitably see that someone has complained of a heater in a Panasonic fan that has failed. Panasonic does not sell parts. With them you have to get a whole new unit. So that is a consideration. I can buy just my blown heater for $100. I have a second consideration for you. Is the Whisper Warm rated for use inside a shower? I am going to doubt it. Find out. You may be limited to good old red warming lamps or newer infrared heaters. There is always a chance of spraying water up there if used in a shower.

Short answer: 110cfm is fine, 70cfm was not technically undersized. Long answer: If your U.S. local building code has not amended IRC M1505 or IMC403, then only a 50cfm (switchable) exhaust fan is required no matter the square footage. The ventilation is associated with fresh air to an enclosed toilet room or entire bathroom, and can be provided via a window rather than exhaust fan. It is not to remove heat. As an alternative, you can have as little as a 20cfm continuous fan when the room is occupied. However, a practical matter is to use the exhaust for steam/humidity. Home Ventilating Institute (HVI) issues guidelines as follows: So those are the requirements vs guidelines for you to follow. When deciding, factor in: - Distance of fan from each plumbing fixture, single or multiple fans - Height of ceiling, e.g. if vaulted - Sones on higher cfm units. Some prefer louder to drown out human noises, some don't want to hear a fan. - remote fan in attic vs room ceiling