Why are my window fans louder with the bedroom door closed?

[Northern_Piper]

Right, casa Piper is an old house with double-hung sash windows and radiators for heat - i.e., no central air.

So I’ve got widow fans that fit in our sash windows: open window, insert fan unit, lower window onto the fan, plug it in and Bob’s your uncle.

Given the summer heat, it runs a lot, and I’ve noticed something.

If the door to the bedroom is open, the fan is quieter. As soon as I close the door, it gets louder and the sound seems to shift to the bass a bit.

And it’s not just one room. I’ve noticed it in both bedrooms that have fans, and they’re different sizes, floor plans and furniture arrangements.

So how come the change in volume and pitch, just from shutting the door?

[snfaulkner]

I’m assuming it has something to do with cavitation. Like when a prop plane is taxiing, it is quieter than when it is stationary. A closed room impedes airflow so more of the energy gets converted into sound rather than moving air.

I’m drunk so I may not have any clue what I just said.

[Spoons]

Could it be that the closed door is turning any sound back into the room, thus making it seem louder, and with a changed pitch?

[cornflakes]

As the fan blows air out of the room it lowers the room air pressure slightly. The work done by the fan is to move air from a relatively low pressure area to a higher pressure area. Air from the hall replaces some of the air exhausted, but the room pressure stays lower than the hall or outside pressures.

Closing the door causes a restriction in the airflow into the room from the hall. The room pressure drops because there is less replacement air coming from the hallway. From your description, it sounds as if the fan can still move air, but has to work harder to do it, so the blades and motor spin more slowly.

I’m making the assumption that each fan is set to blow air out of the room, but this should work similarly if the fans are blowing air in, except that the fans will raise the room pressure.

Beyond this, a possibility exists of reaching a point where airflow gets restricted to the point where almost no air is being moved from one side of the blades to the other. At this point, the blades can stall, reducing the effort needed to spin the fan blades and allowing the fan to spin faster.

(Aaannd now we wait for a fluids or aeronautical engineer to log in this morning and give a lot more accurate of description.)

[Dickerman]

There is a different (greater) load on the fan when the door is closed.

[Mangetout]

@Dickerman

There is a different (greater) load on the fan when the door is closed.

I think It’s actually a lesser load - when you have a fan blowing air through a tube and you close off the exit, the fan spins faster - it sounds like it’s labouring, but in fact, it’s doing less work moving air (there is nowhere for the air to go), which allows it to spin faster.

[Hurk]

@Mangetout

I think It’s actually a lesser load - when you have a fan blowing air through a tube and you close off the exit, the fan spins faster - it sounds like it’s labouring, but in fact, it’s doing less work moving air (there is nowhere for the air to go), which allows it to spin faster.

From just a lowly shade-tree electrician, I agree with this.

With the door in the room open it is moving the max amount of air, and drawing max amperage. With the door closed it is moving less air, there is less demand on the motor and it is drawing less amperage, thus the difference in sound.

I’m NOT drunk (it’s 7:50 AM) but I still could be all wrong.

[RTFirefly]

@Hurk

From just a lowly shade-tree electrician, I agree with this.

With the door in the room open it is moving the max amount of air, and drawing max amperage. With the door closed it is moving less air, there is less demand on the motor and it is drawing less amperage, thus the difference in sound.

I’m NOT drunk (it’s 7:50 AM) but I still could be all wrong.

Sure, it’s moving less air, but I would assume the setting you have the fan on is telling it how fast it’s supposed to be spinning. And since there’s hardly anywhere for the air to go, it’s got to work harder to keep spinning that fast.

Always sounds to me like it’s laboring harder in this situation.

[CookingWithGas]

I was in a similar sub-discussionin a thread about whole-house fans, where it was said:

@Khendrask

That is backwards. A normal fan, or centrifugal pump only draws current as it is moving the working fluid. Block off the intake, and the motor current will drop as no work is being done. The most common example of this is a shop vac. Block the inlet hose, the motor speeds up, as it isn’t delivering torque to moving any air.

[Hurk]

@CookingWithGas

I was in a similar sub-discussionin a thread about whole-house fans, where it was said:

@Khendrask

That is backwards. A normal fan, or centrifugal pump only draws current as it is moving the working fluid. Block off the intake, and the motor current will drop as no work is being done. The most common example of this is a shop vac. Block the inlet hose, the motor speeds up, as it isn’t delivering torque to moving any air.

Yes, this was my point. Most people think the “speeding” vacuum is ready to blow up because it’s being overloaded, but it spins faster because there is no load on it and is therefore spinning faster and drawing less current.

[Mangetout]

@RTFirefly

Sure, it’s moving less air, but I would assume the setting you have the fan on is telling it how fast it’s supposed to be spinning. And since there’s hardly anywhere for the air to go, it’s got to work harder to keep spinning that fast.

Always sounds to me like it’s laboring harder in this situation.

It’s working less hard - instead of propelling a large volume of air through itself, it’s just whirling a small volume of air around with the blades - it goes from ‘pushing’ to ‘stirring’, in effect.

[beowulff]

@Hurk

Yes, this was my point. Most people think the “speeding” vacuum is ready to blow up because it’s being overloaded, but it spins faster because there is no load on it and is therefore spinning faster and drawing less current.

Note that you can still cook the motor this way, since it’s being cooled by the moving air.

[Snnipe_70E]

@Mangetout

I think It’s actually a lesser load - when you have a fan blowing air through a tube and you close off the exit, the fan spins faster - it sounds like it’s labouring, but in fact, it’s doing less work moving air (there is nowhere for the air to go), which allows it to spin faster.

Unless the fan is a DC fan with a lighter load the fan will not turn much faster. Maybe something like from 1175 RPM to 1178 RPM. The RPM of a AC motor is determined by the cycles and number of poles.
With the door closed the room pressure will increase slightly, and depending on the design of the blades the fan will work harder or easier. With a noticeable increase in noise my swag is air is slipping through the blades counter flow decreasing the pressure, the blades then can work properly and the air is reversed and pressure increases and the whole action keeps repeating.

I have seen this action on large fans 50 to 100 Hp with over throttled duct discharges. It can be a little scarry.

[Mangetout]

@Snnipe_70E

Unless the fan is a DC fan with a lighter load the fan will not turn much faster. Maybe something like from 1175 RPM to 1178 RPM. The RPM of a AC motor is determined by the cycles and number of poles.

A lot of fans and other similar appliances do use DC motors with a rectified power supply built in - anything that has switchable variable speed, and is small enough to carry around\

[kanicbird]

@snfaulkner

I’m assuming it has something to do with cavitation. Like when a prop plane is taxiing, it is quieter than when it is stationary. A closed room impedes airflow so more of the energy gets converted into sound rather than moving air.

I’m drunk so I may not have any clue what I just said.

Not cavitation but turbulence. In a normal situation the fan moves air with very little pressure difference between where is sucks and where it blows. However with a window fan you can have a pressure difference which causes a circulation pattern where the air loops through the blade several times and gets more ejected sideways then ‘forward’ Fan speed will increase slightly also when this happens as it’s just not moving much air.