Can I Use a Duct Reducer With My Exhaust Fan?

You decide it is finally time to replace your old exhaust fan in your bathroom. You go out and buy the new model and are excited to get it installed. One issue. When you take out the old one you notice that the duct size is different – now what? This is a common issue when replacing exhaust fans. Codes have changed over the years and where 3” duct might have been common, it is now 4” duct. Many codes and builders across the USA are even going up to 6” duct as standard. So what do you do if you have smaller size ducting in your home than the duct coming from your exhaust fan?

A common solution is to use a duct reducer. This attaches to the duct collar coming off the exhaust fan and reduces the duct size down. For instance, if your fan has a 6” duct collar but your ducting is 4”, you can use a 6” to 4” reducer. Seems simple and easy – right. Well, unfortunately it is not that simple. Many fans are not designed to be used with a duct reducer. By reducing the ducting you are creating more resistance and not allowing the air to flow as freely as it was designed to do. This will cause the amount of air you can move to be reduce and in many cases will increase the sound level of the fan. It will also make the fan work harder and possibly reduce the lifespan. This falls into the category of if it is your only option other than replacing all of the ducting and the wall or roof cap, then you might consider it.

A better option is to try and get ahead of the game so to speak. When researching a replacement for your fan, try to determine what size ducting you already have in place. There are a few ways you can do this.

1. Find the model number of the fan you are replacing and do a search online to find out the duct size. This is generally accurate but you do need to keep in mind that the person who installed it could have also used a duct reducer.
2. When you remove the old fan you can see what size duct was used and if a reducer is in place. The issue with this is we know most people like to have the new fan on hand before taking the old one out.
3. If you have access to the fan from above (attic) you can measure the size of the existing ducting.

What about if the ducting in my home is bigger than the ducting from the exhaust fan? In almost all situations going form smaller to larger ducting will not create any issues. You will need to have an adapter installed to properly connect the smaller ducting to the larger but you shouldn’t see any increase in sound or decrease in airflow.

To learn more about exhaust fans and ventilation utilize the link on this site or consult a professional in your area.