Will a 4", 100CFM bathroom exhaust fan work with 3" exhaust duct?

Question

The bathroom is 112x80x96. The duct to outside will be 3-4' at the longest. This is a 100CFM 25W, 4" fan.

The only reason why I want to buy this one is because it has humidity sensor and timer for under 90CAD.

I could use an adapter installed near the opening in the wall since the ceiling is open but not sure if that will diminish the side effects of the diameter mismatch between fan and duct.

I wouldn't like to increase that opening in the wall.

Will this fan fail prematurely if I use a smaller duct that it's designed for?

Answer 1

Yes it will work. There are many different types of 4" to 3" reducers that will work with your fan. You will have to match one to the material of your vent and the duct. Below is a picture of one sold by Lowes.

Answer 2

The less of the smaller-sized duct you have, the better - so run 4" as far as possible before reducing to 3" for the wall penetration - that reduces the extent of the friction loss from smaller duct, and thus reduces the performance lost.

It won't kill the motor. It will move less air than it would with 4" duct the whole way.

Since this misconception that restricting fans puts more load on the motor comes up rather frequently, I made some tests.

For a "mixed flow axial" fan (radon fan, specifically) unrestricted 4" pipe 18W, restricted down to 2" 16W.

For a "squirrel cage blower" 29-30W unrestricted, 24W restricted.

For 3 different "axial" (propellor-type) fans, power did not change at all with restriction - neither more nor less. While that might negatively affect a marginal design due to inadequate cooling with a severely restricted airflow, a mere 4"->3" reduction would be unlikely to have that effect, and many bathroom fans are, in fact, one of the other types I tested. The idea that there's "more load (or strain) on the motor" is, however, false, as measured by the power consumption of the motor.

Less mass (and volume) of air is moved, and in physics terms that means less work (physics definition of work) is done. A hypothesis I have not tested is that the axial fans are likely moving the same mass of air because it's circulating around the tips of the fan blades when a restriction is applied, so they are moving it in circles (and will also be heating it up.)

Right - I'll have to remember to test a vacuum, as well. Not that it's terribly applicable to this question.

Answer 3

Higher backflow resistance will absolutely put more strain on the motor. That’s why static pressure is an important concept in HVAC with significant bearings on equipment lifespan. An analogy would be driving down the road with a small trailer vs larger and heavier trailer behind your car. With bigger trailer (higher resistance ) your top speed decreases, strain on engine increases, and mpg go up. I used to be a physics teacher in a prior life, and the laws of physics don’t change because we’re dealing with a bathroom fan.

Answer 4

Well if it’s three inch outlet and your duct size is four inch your still only pushing 3 in worth of exhaust and since static pressure would fall you wouldn’t get the right exhausting effect you would if you just went and bought a 4 inch fan instead of your bathroom clearing moisture out in let’s say fifteen mins to say 45 mins cause your still only pushing 3 in just cause the duct is bigger doesn’t mean anything 4 in fan pushes more air than 3 inch fan so if you use a four inch fan in to 3 inch duct your going to have a high static which will ruin you motor due to the bearing going out faster so if your ok with replacing a fan in a few years then fine but if you don’t want to I would suggest getting a four inch fan or waiting till you exit the building to reduce the duct size to 3 inch but different sizes of pipe and air movement differ it works off of back pressure unlike water that works of decrease pipe size to get more pressure ducting is the exact opposite