AirKing Lasko B-Air Blue-dri

Duct it right – how to properly duct your exhaust fan

You did your homework, picked the perfect exhaust fan for your room and are now ready to install it. You have now entered a make or break crossroads in the installation process – how are you going to duct your exhaust fan properly.

Okay maybe that is a bit dramatic, but how you duct your exhaust fan is a key element to the installation process. There are basically two scenarios you will be faced with, existing ducting and new ducting. If you are replacing an old fan, there is probably ducting already in place. It is very important that the ducting be the same size as the new fan’s duct collar. Many older homes utilized 3” or 4” round ducting. Many newer fans are utilizing 6” ducting. Duct reducers are available to make the transition to smaller ducting, but this will increase the sound level of the fan and decrease the performance level – so that perfect fan you just bought, might not be so perfect after all. The best thing to do is to replace the ducting to match the size the fan requires.

With a new installation, you will need to run ducting anyway, so make sure it is the size required by the fan. It doesn’t end there. You also want to use the shortest and straightest ducting possible. This allows the fan to perform at an optimal level. The longer the ducting and the more twists and turns, the less effective the fan will be. While rigid ducting is the best, if you need to use flexible ducting, make sure it is cut to length and as straight as possible. If the ductwork looks like some mythical sea creature, it is probably not an optimal installation. Each twist and turn builds up Static Pressure (SP), which causes the fan to work harder to push the air through. Think of blowing air through a large straw. If it is straight, it’s not all that difficult, but if you bend it a few times, it is a lot harder to blow air through it (learn more about static pressure here). Insulating your ducting will also help with any condensation issues. In colder climates, as the warm air passes though the cold ducting, condensation builds up and can actually cause water to come back through the ducting and into the room.

To finish the ducting installation, you want to make sure the air is making it outside of the home. The worst thing you can do is have air blowing directly into your attic or wall cavity. Air coming from a bathroom will most likely be hot humid air, over time that will build up in the form of mold and mildew. Making sure all seams are sealed is also critical. If air is leaking out of the ducting as it comes through, it is not making it to where it needs to go. Using a properly sized roof, wall or gable mounted cap will ensure that the air is going where it needs to go.

Learn more about ventilation solutions by visiting the Exhaust Fans section of this site.

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