A few thoughts about exhaust fans. I have been on site testing flow rates lately. The following is largely a copy & paste from an email I sent someone. I have had this same discussion enough times to warrant posting it on my blog page for future reference. I hope it’s helpful.
Most exhaust fans operate at about 50-60% of the manufacturer’s stated airflow. The manufacturers are not lying. The fans just face more restrictions under installed conditions than they do under factory test conditions.
- The code minimum air flow for a bathroom exhaust fan is 25l/s (litres per second).
- Most manufacturers state their air flows as m3/hr. (metres cubed per hour).
- (m3/hr divided by 3.6 = l/s)
- A fan will operate more effectively if:
- Window is slightly opened or,
- Bathroom door is open or,
- Bathroom door has a 20mm undercut or,
- Bathroom has an air transfer grille or,
- Make-up air is ducted to the bathroom.
- Fans are best ducted to outside air.
- Restrictions in the duct can affect the performance of the fan.
- Examples of restrictions:
- Flex duct is not stretched before installation.
- Duct too long
- Bends in duct
- Duct squashed.
- Restrictive termination grille or cowl
A good exhaust fan would be one with:
- A decent flow rate (40l/s).
- A 10-15 minute run on timer, (this makes the fan run for 10-15 minutes after it is turned off).
- A 24hr ventilation capacity, (considered in conjunction with a whole-house ventilation strategy).
Remember,
- the purpose of a bathroom exhaust fan is to remove moisture laden air from the room.
- Every litre of air removed from the room is replaced by a litre of air coming in.
- Every litre of air removed from the house is replaced by a litre of air coming in.
Below is a photo of my flow hood and manometer. This is what I use to measure the exhaust fan flow rate. I also use it as a pressure pan to measure leakage in air barriers & ductwork, but that’s a story for another post.
