Residential hood with makeup air

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brantmacga said:
You’re absolutely right, it should be an ERV, but some you can’t even convince to install a fresh air damper.

Also, everyone down here is using open cell foam in the walls and ceilings now, and sealing every hole.

Even a 50cfm fan will depressurize a bathroom. Most of the newer houses I’ve worked in are uncomfortable due to poor air quality.

Lastly, when you open the window in the house I’m talking about breaking the seal, the smell then goes away. I realize how unlikely everyone thinks this is, but there is no other explanation.


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You have to keep in mind that the toilet and trap are like an hydraulic piston. The surface area of the water in the trap is much smaller than the surface area of the water in the toilet. The force on the trap area will be ratioed per the toilet area. Could be easily 10:1. A 300 CFM fan (which is huge for a bathroom) could easily pull 0.5" of water, and at 10:1, the trap sees 5" of water suction.
 
brantmacga said:
You’re absolutely right, it should be an ERV, but some you can’t even convince to install a fresh air damper.

Also, everyone down here is using open cell foam in the walls and ceilings now, and sealing every hole.

Even a 50cfm fan will depressurize a bathroom. Most of the newer houses I’ve worked in are uncomfortable due to poor air quality.

Lastly, when you open the window in the house I’m talking about breaking the seal, the smell then goes away. I realize how unlikely everyone thinks this is, but there is no other explanation.


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I'd make sure the seal under toilet is good. Bathroom in my shop does this occasionally, pretty sure it is the toilet seal, must only be when air pressure is just right, lower pressure in the room than in the septic system for whatever reason. No fan in there BTW but could probably make it worse if there were one.
 
gadfly56 said:
You have to keep in mind that the toilet and trap are like an hydraulic piston. The surface area of the water in the trap is much smaller than the surface area of the water in the toilet. The force on the trap area will be ratioed per the toilet area. Could be easily 10:1. A 300 CFM fan (which is huge for a bathroom) could easily pull 0.5" of water, and at 10:1, the trap sees 5" of water suction.
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But pulling the water that direction would make it gurgle when it is pulled far enough to create an air gap, should be something else that is noticeable if that is what is going on. If the pull were the other direction you could lose enough water down the drain and end up with a permanently open air gap.
 
gadfly56 said:
Could be easily 10:1. A 300 CFM fan (which is huge for a bathroom) could easily pull 0.5" of water, and at 10:1, the trap sees 5" of water suction.
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That's not how pressure works. When the pressure differential is 0.5" of water column, a manometer will show a 0.5" height difference, independent of the area on each side exposed to the pressures.

If 0.5" w.c. pressure differential is causing sewer smells, it's not from passing through a normal fixture trap. The fixture trap is required to have a seal of at least 2" depth (IPC 1002.4). So the odor is coming from a different route, e.g a faulty toilet seal as kwired suggested.

Cheers, Wayne
 
A half-size bathroom--meaning a lavatory and a water closet (toilet) and a urinal. . . a 50 CFM fan is the regular size.
I’ve seen a 75 CFM for a full--size bathroom with a shower and tub.

For high-end homes with a huge mirror for doing make-up (the beautifying variety)-- it is advisable to have a minimum of 100 CFM to pull the steam out when soaking in the tub or showering.

Our main bathroom used to have a 75 CFM but I replaced it with a 100 CFM .
Took some redoing the ceiling because of the ducting and size of the housing.
Samsung makes a super quiet model - - - cost a little over $300.00 but is worth it.

This is to prevent steam from fogging-up the huge mirror in the middle of winter.

Some huge mirrors come with built-in heaters

Can go a bit higher (if desired) if the bathroom is equipped with a Jacuzzi.

Still, a 300 CFM is overkill.
 
myspark said:
A half-size bathroom--meaning a lavatory and a water closet (toilet) and a urinal. . . a 50 CFM fan is the regular size.
I’ve seen a 75 CFM for a full--size bathroom with a shower and tub.

For high-end homes.... a minimum of 100 CFM.

Our main bathroom used to have a 75 CFM but I replaced it with a 100 CFM....

Still, a 300 CFM is overkill.
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A good rule of thumb is 1 cfm exhaust per square foot of bathroom space.
 
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