Bathroom exhaust letting in outside air & smells

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RamblinMan

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Hello. Hoping someone may have some advice.

We built a new home about 2 years ago. Reputable builder and generally happy but we have an issue with a single bathroom exhaust. There are 5 bathrooms in the house with 6 exhaust fans (powder room/shower separate) and we have a problem with a single fan.

In that room you can clearly smell the outdoor air and feel cold air blowing down from that exhaust. All the others are fine. In the winter, the temp is consistently 6 to 10 degrees cooler and you can feel the draft. If we have a fire outside (fire pit/bonfire) you can smell the smoke coming inside.

Builder tells me that dampers are not customary on these things.

He replaced the fan with a new unit and a different type of outside part. No change.

As a short test, we covered the exhaust fan with plastic wrap to confirm this was the source of the air. It seemed to be confirmed.

One friend of mine is wondering if the vent hose/pipe is too short. This bathroom is against the side wall to the house and the vent goes directly outside just above that wall. He opines we may need to extend the pipe. Would this really help?

The area above this particular exhaust fan is an unfinished area with opening to the outside. That area is insulated but when you go out there you can smell the outdoors as well but I would assume that's normal.

Any advice on what to try here? Thank you.
 

breplum

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Dampers are built in to virtually every bath exhaust fan. I've never encountered any without in over fifty years. It is not true what your builder claimed. If there were not dampers, that would be a path for cold air directly back to the dwelling.
There are vent terminal fittings available that have dampers built in as well.
It is fairly easy for an inadequate installer to mess up the damper during install.
UMC dictates that environmental air terminus be min. three feet away from windows or other openings.
 

Fitter30

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Most exhaust fans have a damper as part of the housing. Motor and wheel come out as one assembly damper then can be inspected. These dampers are made of thin sheet metal and are not air tight.
 

jadnashua

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RamblinMan

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Depending on where the outside terminus is, you can add a vent hood there with a built-in damper to aid the one that should already be there. This is what I have in my place. It would work on the roof, but not in a soffit, which would take a different type so the seal would work Broan-NuTone 3 in. to 4 in. Roof Vent Kit for Round Duct Steel in Black-RVK1A - The Home Depot


Thanks for the response. For this bathroom they have the vent going to the side of the house. This is a 3 story house when you count the finished attic/third floor and this particular bathroom is on the 1st floor master. We've had the builders guy come back and out and then we paid our own HVAC person to look at it based on the rec of a friend. This HVAC person claims dampers aren't used on bathroom exhausts and that the original guy has the hose too short. He said were it longer it would take more of the cold air from making it back in. I simply don't understand how these folks that are supposed to be the experts are telling me they can't fix this draft coming in the house.
 

WorthFlorida

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Causing the cold air dump into the home, and as fitter30 mention, the interior of the home has negative pressure. Turning on a clothes dryer, bathroom exhaust fans, etc is pulling air out of the home and the air is replaced by any outside air leaks. A very tight new home it can be a problem where exhaust fumes from a gas/oil fired furnace chimney replacement air can be sucked down the chimney. Another is wind and it is usually the culprit. The side where the wind hits the home will have high pressure, the downward side will be negative pressure. Air gets sucked into a home, not blown in.

On very windy days you usually will hear the dampers in the bathroom exhaust fans rattle around. They usually are flimsy and work only by gravity. Sometimes closing an exterior door while all of the windows are close such as during the winter, you'll hear the damper snap open and close.

Why this one exhaust fan doesn't seem to have damper is a mystery. It is possible that it was removed to make room for the exhaust tube since it is very close to the exterior wall. Dampers are usually extended out a bit from the fan housing. When this exhaust fan is on, can it hold a tissue paper onto its grill? Some exhaust fans are called fart fans since some use small little fans and the air rushing in may overtake it. If it is a drum fan, that is the best kind and should mot be an issue while in use.

This may be the best trick out there, the Cape Damper. Google "Cape Damper" and Tamarack Technologies has several videos. What jadnashua found looks like a well made bathroom exhaust hood and maybe easier to retrofit. Notice it reads "Not intended for Dryer Vent usage", yet is looks just like one.


ExpertsVent.jpg
 
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jadnashua

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A dryer will have much higher air flow than a typical bathroom vent, so a damper designed for one of those may not open properly if used for a bathroom vent.

In some places, they've been requiring the house to be quite tight and as a result, require energy recovery ventilation systems to be incorporated that when sized properly, should prevent negative pressure when any exhaust vent is being used. Most clothes driers don't pull external makeup air, but that air that gets exhausted needs to come from somewhere. Older, less efficient boilers and furnaces it's the same. A good range hood that actually exhausts outside versus just running it through a filter can move a lot of air, too. The range hood I have, on high, can move close to 800cf/min. that's a lot to pull through cracks in the house! Older, less efficient homes, that wasn't really an issue, but as newer ones are built, they tend to be required to be more efficient, and they don't leak as much. To maintain a healthy environment, pulling in some external air without losing all of the conditioning means some specialized equipment.
 
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