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Bathroom Exhaust Fan vents when off

Recently, I had my bathroom remodeled. I selected and had installed a Braun bathroom exhaust fan. I worked great. Until I noticed my central A/C seemed to be running longer to cool the house. I did a kleenex test with the bathroom exhaust fan: With the A/C off, I turned on the exhaust fan and held a kleenex up near the grillwork. The kleenex sheet got sucked up tight against the vent. Turn off the fan, kleenex fell to the floor. I then turned on the central A/C and again held the kleenex up near the bathroom exhaust fan. Again, it got sucked up against the grill work. Apparently, my central air/heat system generates enough positive air pressure in the house to cause the bathroom exhaust fan to vent the conditioned air (read: dollars!!!) to the outside. I checked the vent on the roof while the A/C was on and the exhaust fan was shut off and felt cool air coming out the roof vent.

With the A/C and the bathroom exhaust fan both off, I have never noticed any air coming in from the outside, so I'm guessing the backdraft damper door is working.

I taped a couple of 8.5x11 sheets of paper over the exhaust fan grillwork and noticed over the next few days that the A/C was not taking so long to cool the house--we had consistent 100 degree days over a couple of months.

The grillwork now remains covered and the exhaust fan unused ever since (about 2 months now).

Anyone have a solution for this? Might it be possible to add a little weight to the backdraft damper so it doesn't open when the A/C is on? I was thinking washers or a penny or two epoxied to the lower or mid section of the damper door might work.

Thanks for any info anyone can provide.

Re: Bathroom Exhaust Fan vents when off

It seems that your bathroom fan and vent are doing what they're supposed to do, which is to carry air out of the bathroom. If you have your a/c on, of course the vent will carry cool air out.

The bathroom vent is not supposed to run for extended periods of time, just enough to do its job.

Now to the other issue of the time it takes to cool down the house. This is determined by many factors, including but not limited to: temperature, humidity, time of the day you turn the a/c on, your filter condition, and more. If outside temp is 100, it would take a long time to reduce inside temp to 78.

So while your "experiments" with a tissue are nice, they are not conclusive, as far as your a/c performance.

Also, I suggect you don't alter the flapper on top of the vent, if it's working properly.

Go ahead, remove the paper over the bathroom vent and use it as needed.

BTW, to save money, real money on coolong, make sure your insulation is OK, seal all around windows, doors, electrical outlets, don't let the sun in, etc. There's probably more info available from your power company.

Re: Bathroom Exhaust Fan vents when off

Thanks for the note, dj1, but I think you misunderstood my situation.

First, here's the scenario:
1. A/C off, Bathroom exhaust fan on, kleenex held to exhaust fan grillwork. ok
2. A/C on, Bathrom exhaust fan OFF, kleenex held to exhaust fan grillwork. Not ok
3. A/C off, bathroom exhaust fan off, kleenex falls to the floor.

The central air conditioning system creates enough positive air pressure in the house that it opens the bathroom exhaust fan backdraft damper door open thereby allowing conditioned air to be vented to the outside..despite that the bathroom exhaust fan is powered off.

For all the ballyhoo about energy conservation and economy and sealing up one's house against drafts and heat/cool air loss, this exhaust fan seems to fly in the face of that position.

I, for one, don't particularly like the idea of a 6" hole to the outside letting conditioned air out of the house when the bathroom doesn't need venting.

Re: Bathroom Exhaust Fan vents when off

Bathroom vents don't usually use 6" vents. Could it possibly be that you were looking at a rang hood vent by mistake?

Re: Bathroom Exhaust Fan vents when off

If your A/C is creating high pressure in your home then it's sucking in unconditioned air. If your bathroom fan is the cause of your A/C running longer you have other problems. If you don't like conditioned are escaping or warm air entering your home, then never open a door to leave the house.

Homes are not built like submarines, they leak and some leak more than others. When heating and cooling equipment is calculated for a home, part of the calcs take the envelope into consideration.

The fact of the matter is you need to make up air in your home when air is exhausted from your home. If you push 110cf of air out your home per minute where is it coming from?
You can't fight the laws of physics.

If you had a blower door test I bet your bathroom exhaust fan would be the least of your leaks.

Re: Bathroom Exhaust Fan vents when off

Thanks for everyone's comments and suggestions.

The problem is solved and fixed. I took a hard look at the bathroom end of the fan unit and found that the fan and motor were on a removable platform. I removed it and found the backdraft damper door hanging open. I used expandable foam to seal every hole in the unit frame mounted in the ceiling.

A candle smoke test showed flow only out through the backdraft damper door. I popped the door out of it's mount, and put two small washers on a small wood screw and screwed it near the bottom edge of the door and replaced it. It now hung shut against the door stop.

Another candle test and only a small bit of smoke drifted up to/through the door edges with the A/C on. I turned off the a/c, remounted the fan unit and grillwork, turned on the exhaust fan and did a kleenex test. The tissue was sucked up against the grill. Up on the roof and good air flow was coming out.

I turned off the exhaust fan and turned on the house A/C and did another kleenex test. The tissue just fell to the floor.

On to the next project....


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