City is requiring a “whole-house mechanical ventilation system”

[Dustin Le]

i tried google, but know so little that I can't tell what's what. I'm setting up a basement ADU and the city is requiring a minimum 30 cfm mechanical ventilation system. Does anyone have any idea what this is, where I would locate it in my house if it existed, and what it would involve to install?

I appreciate your help!

[Wayne Brooks]

@Dustin Le I’m guessing an hvac contractor would know. 30 cfm is nothing, I think a typical window AC puts out 100 cfm.

[Bill Brandt]

Bathroom vent fans have 80+. Simply put one in the bathroom. 

[Ethan McCoy]

Quote from @Dustin Le:

i tried google, but know so little that I can't tell what's what. I'm setting up a basement ADU and the city is requiring a minimum 30 cfm mechanical ventilation system. Does anyone have any idea what this is, where I would locate it in my house if it existed, and what it would involve to install?

I appreciate your help!

Hi @Dustin Le,

I’m a mechanical engineer with a few years in industrial/commercial HVAC. 1 cfm=1 cubic foot per minute, which is a volumetric flow rate. Typical residential bathroom exhaust fans are in the ball park of 70 cfm and can vary greatly. 

Depending on what a space is used for, code requires a certain amount of fresh air(outside air) based on occupancy or # of people per sq ft. You’ll need to provide air directly from outside at a rate of 30 cfm via mechanical means. The simplest solution would be to provide an opening directly to the outside and install a bathroom exhaust fan greater than or equal to 30cfm somewhere(ideally at the far end of the space to provide mixing) in the space, that would operate during occupied hours or however the city defines it. You could also put a blower fan at the opening pointed into the space, without the exhaust fan. The point is that you need to ensure the makeup/replacement air comes from the outside at minimum rate of 30 cfm. If you put an exhaust fan without a opening, the makeup air will come from building infiltration(window, door cracks) which doesn’t count towards your fresh air requirement.(it can but it’s tedious to calculate). Because it’s a basement it can be argued that the space infiltration can come from the space above which is occupied and thus has a diminished outside air percentage. 

Bringing outside air directly into the space without conditioning it may cause thermal comfort issues or worse, introduce moisture and an increased potential for unwanted bacterial growth. 

I’m not terribly familiar with the residential line of products but I do know that heat exchangers can be had to use the outgoing air to ‘condition’ the incoming air. I’d reach out to a residential mechanical contractor.

Probably more info than you wanted…best of luck. 

[Henry T.]

Quote from @Bill Brandt:

Bathroom vent fans have 80+. Simply put one in the bathroom. 

 Roger that.

Put in a bathroom fan.  

[Nathan Gesner]

Quote from @Dustin Le:

i tried google, but know so little that I can't tell what's what. I'm setting up a basement ADU and the city is requiring a minimum 30 cfm mechanical ventilation system. Does anyone have any idea what this is, where I would locate it in my house if it existed, and what it would involve to install?

I appreciate your help!

What they are asking for is a system that exchanges fresh, outside air with the stale air inside. This is not unusual if you have a gas water heater or furnace in the basement. 30cfm seems low, so maybe it's for ventilation in a bathroom to remove the humidity?

When the city requires something, you're allowed to ask them questions and they will usually be happy to explain to you exactly what is needed and why.