[nettinga1]

If my bath fan, 150 cfm, runs for 8 hours a day, that would exhaust 72,000 cubic feet of air, right? Does that mean that I would have to replace all that air, and so in turn replace all that heat that was lost during those 8 hours? Where does all that makeup air come from?

 

[Chris Johnson]

Front door opens and closes a few times, the windows leak, the sill plate leaks, the penetrations through the walls are not sealed, the attic lid isn't sealed, the plumbing coming through the slab isn't sealed, the sump pump isn't sealed.

And if it all is sealed, the house may implode

 

[MTN REMODEL LLC]

If my bath fan, 150 cfm, runs for 8 hours a day, that would exhaust 72,000 cubic feet of air, right? Does that mean that I would have to replace all that air, and so in turn replace all that heat that was lost during those 8 hours? Where does all that makeup air come from?

Outside...... or a mexican dinner.

 

[griz]

If my bath fan, 150 cfm, runs for 8 hours a day, that would exhaust 72,000 cubic feet of air, right? Does that mean that I would have to replace all that air, and so in turn replace all that heat that was lost during those 8 hours? Where does all that makeup air come from?

You'll be lucky if the bathroom mirror don't fog up during a hot shower...

 

[Golden view]

If it's a 2,000sf house with 8' ceilings, that's all the air in 1.8 hours or .56 ACH, not that much really.

Plus, is it really 150cfm?

 

[nettinga1]

Thanks guys! I understand all the air leaks, what I can't wrap my head around is loosing all that tempered air, or basically having to warm that cooler replacement air. Thanks for everyone's input.

 

[Rio]

If you're running the fan for 8 hours a day for ventilation requirements and are concerned about throwing away the energy that it took to heat or cool that air use a HRV (Heat Recovery Ventilation) system. That way you get the ventilation and don't waste the energy nearly as much.

 

[SLSTech]

Everyone above pretty much nailed it - with that a rated 150 CFM fan is not going to be exhausting 150 CFM - with a typical install, you will be lucky to exhaust 80% of the rated flow - shoot some that I have tested barely get 11 CFM due to all sorts of exhaust ducting issues not including it's inability to draw enough air

 

[BadgerBoilerMN]

That is why they make ERV's and we use them exclusivley for bath exhaust and yes the mirrors fog up depending on the weather etc. etc.


Who says they shouldn't?