Is a bathroom exhaust fan more effective with the bathroom door open, closed, or cracked?

Factual Questions
1

So, I had Chipotle last night and this morning got to thinking…

Is a bathroom exhaust fan more effective with the bathroom door closed, open, or cracked?

Let’s make three assumptions:

  1. The purpose of the exhaust fan is to remove offensive odors from the bathroom. Removal of humidity is neglected.
  2. Efficacy is defined as the ability to quickly and thoroughly reduce offensive odors in the bathroom.
  3. Offensive odors must be removed through the fan; there is no “credit” for odors that waft into connected rooms.

Here are my thoughts on the various configurations:

Closed: The fan is pulling mostly bathroom air. However, the door closed increases the pressure drop of the system, so less air volume is moved. The most common airflow path of door gap on the bottom and exhaust fan on ceiling, decreases the amount of “short circuiting”, ie. outdoor air moving directly to the fan without displacing odoriferous air. But, could this cause stagnant “dead pockets” of odor?

Open: The fan can pump more volume, but if unpowered, odor can easily escape to the connected room. There’s also increased short circuiting.

Cracked: More volume moved than a completely closed door. However, there’s enough of a flow restriction so that odor will not preferentially seep to the connected room. Also, allowing air entry from the multiple edges of the door could “fluidize” the odor and reduce stagnant zones. Perhaps the best (or worst) of both worlds?

I’ve considered doing a home experiment, but I presume the girlfriend would be unwilling. I’d also have trouble creating a controlled environment, due to variations in bathroom volume, exhaust fan flow, and digestive fauna. I have a Lab mix with a keen sense of smell and chicken allergy, but I haven’t quite figured out how he could contribute.

Surely, anecdotal evidence from other perceptive Dopers exists. If so, please share.

2

If I had to guess I would say with the door open a crack, for the reasons you stated. Sounds like someone may need to do an experiment. BTW, I don’t think those fans are all that effective, especially in a large bathroom. In our case it doesn’t help with the humidity from the shower, but a small cadet heater that we installed does the trick.

3

A lot depends on the size of the bathroom, the size of the gap at the bottom of the door, the capacity of the fan, the differences in the temperature and humidity of the bathroom air compared to that of the adjoining room, and probably other factors I haven’t considered. That is, there are quite a few variables that could change the result.

Assuming the cross-sectional area of the gap at the bottom of the door is larger than the effective cross-sectional area of the fan opening (enough larger to make the reduction in air flow-rate across the fan negligible), and the temperature/humidity effects are also negligible, then leaving the door closed would be fastest. Even more so if the bathroom air is warmer and more humid than in the adjacent room.

One thing is certain, the amount odors that will waft into the adjoining room will always be less with the door closed.

4

IIRC, the standard bathroom fan exhausts 300 cfm. The door opened to allow only this amount of air would be ideal.

5

Be sure the fan exhausts air to the outside and not just into a closed attic.

Then you would also need an outside window or door open a bit for outside air to enter the house and an opening below the bathroom door or the bathroom door to be slightly open.

You can experiment with an outside door opened… And also with all outside doors/windows closed. Stand with wet bare feet in front of your closed bathroom door, if it has a gap between the bottom of the door and the floor.

You should be able to feel air coming in when the fan is on. Perhaps more with an outside door/window open a bit.

Note air can enter the house through other openings like chimneys, natural gas water heater exhaust stacks (can bring dangerous gases into the house), leaks in walls, proper air vents installed for natural gas heating/water heaters, etc.

6

You’re forgetting that it’s also drawing air from the HVAC vent. In most cases it’ll just draw that air from the next room over (however it’s connected). In some houses (such as mine), one of the HVAC returns has a line that goes to the outside. This is there so that when exhaust fans (bathroom/kitchen/whole house) are running, makeup air is brought in through that hose and pass through the furnace filter as opposed to either being pulled in through your dusty attic, cracks in your basement foundation or anywhere else they can find it can find away in (like drafty doors or windows). Also, as houses get more and more sealed, exhaust fans don’t work as well with out something like this.

Long story short, cracking the door certainly helps, but don’t forget that air is also coming in through the vent(s).

7

Don’t you have a window?

8

Maybe you need a new fan? Mine is quite effective, in that if I run it during a shower, I get no mirror fogging, whereas if I don’t, I get mirror fogging within a minute, even with the door open.

For minimizing humidity, I think door open (with fan on) is the way to go. I suspect the same is true with removing odors, but with the caveat that you would then get odor leakage into other rooms. I suspect the leaked odors would be so diluted as to be unnoticeable anywhere other than immediately adjacent to the bathroom, but if you find this is not the case, you’ll need to close the door to some degree.

So how much? I am rather dubious you’d see any measurable change in pressure with the door closed. You’d get too much seepage through cracks under and beside the door. So put a barometer in the bathroom, turn the fan on, and see. If you get any droppage, I agree with harmonicamoon that you want to open the door just enough to normalize pressure again.

9

Ummm… not where I live. Here (and I bet the rest of Canada) 300 cfm is the maximum allowable flow before any exhaust fan has to be interlocked to the Furnace or HRV for make up air. One bathroom fan is always interlocked to the furnace as the Main Exhaust Fan, this can be as little as 75 cfm, though starting to see more at 150 or so cfm 300 cfm would be huge for a bathroom fan. I am not an HVAC guy but I think size of room is the major factor involved.

Cross sectional area of the exhaust vent pipe is more important, not cfm anyway. Typical vent pipe diameter for a bathroom fan is 4", so you have an area of about 12.5 square inches. Typical gap under a door is 1/2 to 3/4", so for a 30" door you get easily 15 square inches, more than enough to supply the fan to operate efficiently.

You should never completely close the door anyway though because then I’m going to think the bathroom is occupied when it isn’t.

11

Your bathroom door is not airtight. There’s likely a gap at the bottom for starters. As long as the door can let enough air through then you should keep it closed.

12

[cough cough cough] … keep the door closed … please … [cough cough] … otherwise you’re going to stink up the whole house …

Use a stick of incense that smokes a lot … that’ll show you the air currents around the volume of the bathroom in your various configurations … just light it outside the bathroom so nothing blows up. You’ll notice right away that you just moving through the bathroom is going to disturb the air flows quite a bit, and that might be the answer to your “slack zones” you’re worried about. Just squirm around a bunch when your sitting the potty.

Use some diaper rash ointment too, you’re welcome …

ETA: Just now noticed this is a zombie thread …

13

My office is right next to the bathroom, and the bathroom door is right across the hall from the return air register on the AC/Heating.

Keep the door closed. I don’t care if you run the fan or not, I don’t want to sample the odor of your bodily functions. I close that door maybe 20 times a day.

14

I’m honestly curious Quartz. Why did you search for and bump this thread from ten months ago to add this comment to it?

15

Perhaps he had an unfortunate experience this morning …

16

After 10 months the zombie was getting kind of fragrant, so I guess turning the fan on was a good idea.

17

I must confess I didn’t notice the date. It showed up in my list of new posts so I guess it was bumped by a spammer or something.

18

Bathroom fans do more than take away odors. That brings me to an easy test for you. Close the door, turn on the fan, and take a shower. When you’re done the mirror will be fogged up, a good indicator that the fan didn’t take out much air. For your next shower, open the door a crack, or wide open. After your shower, you won’t have to wipe off the mirror.

19

Again, not digging at you, but yours was the first new post, so it was not bumped by a spammer. But it showed up on your list of new posts?

I am just curious as to how that happens as it may explain some about the reanimation phenomenon.

I’ll ask in ATMB.

20

ATMB thread here.

21

As I said in the ATMB thread, Quartz’s guess was correct and it was bumped by a spammer.