Are Kitchen and Bathroom Exhaust Fans Sufficient to Ventilate a Home?

[kyphysics]

If one didn't want to open their windows a lot to ventilate a house/living space, could a viable substitute be to run the kitchen and bathroom exhaust fans (which presumably don't pull in outdoor humidity, mold spores, and other noxious substances the way an open window could)? For example, if you ran a kitchen exhaust fan all day, could it replace all the air in that kitchen at least? And if you ran your master bedroom exhaust fan all day, could it replace on the air in the bedroom. ...And assuming you have good circulation of air throughout rest of enclosed house, could this process of using exhaust fans eventually ventilate a home decently well to replace a lot of the air?

...Or, is my logic wrong and exhaust fans pull in all the bad stuff from outside just the same as opening a window? Or, maybe there are other drawbacks?

 

[berkeman]

exhaust fans pull in all the bad stuff from outside just the same as opening a window?

Where does the air come from if not from outside? The only thing that trying to keep the windows closed while you run the exhaust fans will do is reduce the airflow out of the exhaust fans compared to when the windows are open.

 

[OmCheeto]

According to my calculations, a typical 50 cfm bathroom fan would replace the air in my house in about 2½ hours. Seems like overkill to me.

Although this may sound like a thread hijack, I'm curious as to why my house has a fairly consistent level of CO2 at ≈700 ppm, with no forced ventilation?

If I'm emitting a constant volume of CO2, and my house is well sealed, why doesn't the CO2 level go through the roof? Top of my head answer is leakage.

 

[jrmichler]

You can ventilate a house by running those fans. Just be advised that those fans have cheap motors that are not designed to run continuously. And they are noisy.

My own house is tight enough that mechanical ventilation is necessary. I have a Fantech SH704 heat recovery ventilator that has been running continuously since 2011. I cheaped out and bought a single speed unit, then found that it moved more air than I really wanted and was noisier than I liked. So I added a ceiling fan speed control, and set it to the lowest speed. It's dead quiet, and the house is never stuffy.

 

[Lnewqban]

...the kitchen and bathroom exhaust fans (which presumably don't pull in outdoor humidity, mold spores, and other noxious substances the way an open window could)?

Your presumption is not correct.
Those fans are only able to exhaust as much volume of humid dirty air as it can leak-in through windows/doors gaps, electrical conduits-outlets, dryer duct, crevices in exterior walls, and any open path it can find between the atmosphere and the interior volume of your house.
Strong wings against the exterior envelope exacerbate that leakage effect.

 

[OmCheeto]

You can ventilate a house by running those fans. Just be advised that those fans have cheap motors that are not designed to run continuously. And they are noisy.

My own house is tight enough that mechanical ventilation is necessary. I have a Fantech SH704 heat recovery ventilator that has been running continuously since 2011. I cheaped out and bought a single speed unit, then found that it moved more air than I really wanted and was noisier than I liked. So I added a ceiling fan speed control, and set it to the lowest speed. It's dead quiet, and the house is never stuffy.

I made a DIY 'heat recovery ventilator' unit soon after I bought my house back in 1990.
10/10 stars!

 

[sophiecentaur]

could a viable substitute be to run the kitchen and bathroom exhaust fans

I remember a Which Report on extractor fans many years ago. They reckoned that any but the most powerful fans can be beaten by a moderate wing from the appropriate direction. IMO, to be certain of good extraction then it may be best to use a vertical flu and a cowl with a weathervane to keep the output pointing down-wind. Probably over the top but it could be done passively in anything but flat calm.

 

[DaveC426913]

...Or, is my logic wrong and exhaust fans pull in all the bad stuff from outside just the same as opening a window? Or, maybe there are other drawbacks?

Why do you assume that only outside air has "bad stuff"?

Have you considered the bad stuff in your home that you would want to vent to the outside, to be replaced by fresh air?

There's

• fumes from your kitchen,
• off-gassing from materials and equipment,
• radon seepage into your basement,
• spores and other airborne microbes,
• carbon dioxide from living things,
• oxygen depletion,
• carbon monoxide from furnaces and burning things such as candles,
• odours.

Taken to the extreme, a hermetically-sealed house would quickly become toxic.

This had been studied. Look up Sick Building Syndrome. For years, developers tried to seal off spaces from outside air, and they have since learned that this was folly. They actively avoid it now. You don't want to take this too far.

 

[sophiecentaur]

Why do you assume that only outside air has "bad stuff"?

He seems to have forgotten the effect of dilution of all your home's bad air with clean air .... in most cases.

If you live right next to th gasworks then, whatever you do, you will suffer. But that's the job of the public health people to regulate what factories (and motor cars) let out into our public air.

Taken to the extreme, a hermetically-sealed house would quickly become toxic.

Yes and if you sit in your car, you will see a symbol on your heater controls which lets you re-circulate the air back into the cab. Spend a few minutes with a flatulent passenger and you'll change the setting back to normal. The control might just help in a traffic tunnel but not for long.