Three Methods to Size a Bathroom Exhaust Fan
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:
10 ft X 12 ft X 8 ft = 960 cubic feet.
960 divided by 60 = 16.
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.
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).
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
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