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How to Size a Bathroom Ventilation Fan

As the trend to larger spa-like bathrooms continues to gain in popularity, the need for proper ventilation becomes more important.  While many people are tired of their noisy and innefective bath fan - not many people know how to properly size a fan to their needs.

There are a few different ways to calcuate the number of Cubic Feet Per Minute (CFM) of air movement needed to properly ventilate a bathroom. Make sure you check out the end of the article and read about static pressure.  A long duct run from the fan to the outside can skew your calculations.

METHOD 1

This method works on the assumption that the goal for bathroom ventilation is 8 complete air changes per hour.  Take your square footage x ceiling height to get the total cubic feet to be ventilated.

Example:  Bathroom measures 10 feet wide and 12 feet long.  It has 8 foot ceilings.  So 10 x 12 x 8 = 960 cubic feet.

We take the cubic feet and divide by 60  - which is the number of minutes in an hour.  We take the result and multiply by 8 (remember, our target is 8 complete air changes each hour).  The complete equation is as follows:

STEP 1
10 ft X 12 ft X 8 ft = 960 cubic feet.            

STEP 2
960 divided by 60 = 16.                 

STEP 3
16 x 8 = 128


So - we need 128 CFM of air movement to properly ventilate this bathroom.  Assuming there is not and excessively long or twisting duct run to the outside - we would select a fan that moves somewhere around 130 CFM or higher.  Examples would be the Panasonic FV-15VQ4 or Broan QTXE150FLT, both of which are rated at 150 CFM.  If an inline fan is desired - the Fantech FR 110 would suffice.

METHOD 2

This method is a simple one - for bathrooms under 100 square feet.  According to guidelines of HVI (Home Ventilating Institute) baths 100 square feet or smaller require one CFM per square foot of bathroom - with a minimum of 50 CFM.

So - if you have a 7 foot by 9 foot bath - you need 63 CFM.   It doesn't get any simpler than this.  Most conventional ceiling insert fans, however, are rated either 50 or 80 CFM - so you will have to round up or down (rounding up preferred).

METHOD 3

This method of calculation is designed for bathrooms over 100 square feet and factors in the number of fixtures in the bath (toilets, showers, tubs).  The rules are:

* Allow 50 CFM for each standard toilet, shower or tub
* Whirlpool and jetted tubs need 100 CFM

So - if you have 1 toilet (50 CFM), 1 shower (50 CFM) and 1 whirlpool tub (100 CFM) - you will need a 200 CFM Fan.  The Pansonic FV-20VQ3 would work well in this bathroom.  An inline fan kit that would work well in this bathroom is the Fantech PB 270-2.  This kit includes a 200 CFM fan and allows for 2 exhaust grilles in the ceiling.

Static Pressure and Duct Run:
A ventilating fan must overcome resistance when pushing air from the inlet, through the duct, to the outside of the building. This resistance is known as static pressure. The amount of static pressure depends on the duct length, type of duct, elbows and the roof jack or wall cap.  Essentially, the more elbows, turns, duct length, etc. that you have - the more static pressure will be present and less effective CFM you will get out of your fan. 

Panasonic provides some tools to calcuate the model needed to ventilate properly after factoring in duct lenth, duct type, elbow, type of exterior vents, etc.  As you might suspect - the end result of the cacluations only leads you to select their fans.  While it is possible to use air duct calculators and other tools to precisely determine effective CFMs you will get out of a fan - it is probably OK to simply round up to a larger fan if you know you don't have a straight duct run to the outside.