We are under the 2018 IMC. In a multi-family development the plans examiner has asked for 0.35 ACH based on Table 403.3.1.1. The MEP engineer is arguing that since the airflow required is listed under "living areas" he only has to calculate the ACH for the floor areas that are "habitable". E.g. he can just multiply the living room and bedrooms (neglecting closets, bathrooms, and kitchen) floor area times the ceiling height times 0.35. The plans examiner thinks that ACH (air changes per hour) should be calculated based on the full ventilated area/volume supplied by the air handler. Which do you think is correct?
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Calculating Outdoor Air in Dwelling Unit
- Thread starter jc_ks
- Start date
wwhitney
REGISTERED
If the air is well mixed within the entire ventilated volume, the air change rate at any point within that volume will be the same. And it will be determined by the air flow rate divided by the total volume.
Of course, you could have dead-end volumes that have neither direct inflow or outflow and are effectively separated, like a closet with closed doors. With the doors closed, they would not contribute significantly to the volume for calculating air change rate. But can you count on the doors staying closed? The conservative choice is to include such volumes.
It would be rare in modern designs to see a kitchen that is effectively separated from the rest of the dwelling unit.
Cheers, Wayne
Yankee Chronicler
SAWHORSE
When in doubt, check the definitions. 2021 ICC IMC:
Based on that, I don't think closets should be included. That table has separate entries for the kitchen and for toilet rooms and bathroom, so those would also be excluded from the general calculation since they must be addressed separately.
Gregg Harris
SAWHORSE
The MEP is correct; the "Breathing Zone" is only considered for occupied space, 3 inches from the floor to 72 inches above the floor, and 2 feet from walls.
closets, bathrooms, and kitchens only see temporary usage and are not considered occupied zones. Kitchens and bathrooms have specific ventilation requirements due to their creation of contaminants.
Where does it say this in the code?
Gregg Harris
SAWHORSE
Look at the definitions for "Breathing Zone." "Occupied space." It is only in occupied spaces That Bathrooms, kitchens, and closets are excluded from occupied zones. An "occupied zone" is a space where time is spent, not just frequented, as in choosing what to wear, or a storage closet to retrieve a mop, or relieving oneself in the bathroom, or pouring a cup of coffee in the kitchen.
I understand what breathing zone is, but I am just trying to figure out how that relates to ACHs. If you are only ventilating enough for the breathing zone to be 0.35 ACH, then by the time it dilutes with the rest of the air you actually have a lot less than 0.35 changes. Does the code or ASHRAE say somewhere that the ACH is only to be based on occupied space or breathing zones?
Yankee Chronicler
SAWHORSE
wwhitney
REGISTERED
OK, but Table 403.3.1.1 is different for "Private dwellings" than for every other category. For the other categories, it provides Rp and Ra rates, and 403.3.1.1.1 has an equation to calculate the outdoor airflow rate required. Whereas for "Private dwellings" it provides an ACH as well as a total CFM, and both must be complied with. The term ACH is not used anywhere else in the IMC, nor defined, as far as I can see.
So ACH is CFH / Volume. The question arises as to what volume is to be used. It seems reasonable to me to use the total ventilated volume, even if that includes volumes that are not occupiable spaces. The IMC appears to be silent on this question.
Cheers, Wayne
P.S. Based on the first sentence of the OP, I infer that the building in question is over 3 stories tall. As if it is Group R-2, R-3, or R-4 and 3 stories or less, 403.3.2 applies instead of 403.3.1.
Yankee Chronicler
SAWHORSE
I don't understand what point you're trying to make. Are you saying you don't think rental apartments are private dwellings? From the 2021 IMC Definitions:
If a dwelling unit isn't private, it must be public. Individual apartments (and condominium units) are not open to the public, therefore they are private.
wwhitney
REGISTERED
No, I agree they are private dwellings. What I'm noting is that uniquely among the Occupancy Classifications in IMC Table 403.3.1.1, Private Dwellings have both a CFM requirement and an ACH requirement (0.35 ACH). And while IMC 403.3.1 goes into detail about how you are to calculate the CFM, it does not discuss at all how to calculate the ACH. In particular, to convert from CFM to ACH, you need to use a volume, and so the question is which volume do you use?
Cheers, Wayne
How is the ACH achieved? Does that method interact with all of the volume? If the method does interact with all of the volume at any point in time, an effort to kneecap the method makes it a code violation.
Using a calculation to determine a CFM rate for less than the actual volume that is at hand can produce the desired 0.35 yet be meaningless. The actual ACH remains an unknown.
Using a calculation to determine a CFM rate for less than the actual volume that is at hand can produce the desired 0.35 yet be meaningless. The actual ACH remains an unknown.
Yankee Chronicler
SAWHORSE
I regard that as a mixed metaphor. "Ventilation" as defined in the IMC includes both fresh outdoor air and recirculated air.
In table 403.3.1.1, as you point out, there are two requirements: 0.35 ACH and a minimum of 15 CFM/person. The way I read it, the 15 CFM is the amount of outdoor air required per person, but the 0.35 ACH isn't a per person rate -- if a dwelling unit has an occupant load of four people, it doesn't become 1.40 ACH, it remains 0.35 ACH.
But air changes are accomplished with the ventilation system, and I've never seen an apartment, a condo, or a single family house that had ventilation outlets in closets. So why would you calculate the ventilation rate to include the volume of spaces that aren't normally occupied AND that aren't mechanically ventilated?
klarenbeek
REGISTERED
403.3.1.1 says the air need to be delivered to the breathing zone, not that the breathing zone is to be used to calculate the ACH volume. Because this is for private dwellings, you do not use the formula used for other occupancies so using the breathing zone goes out the window for calcs. IMO kitchens and bathrooms are not included in the calcs because they are addressed separately in table 403.3.1.1. In most cases it wont make much difference anyway because the CFM per person will be more than what is needed to achieve the 0.35ACH. They are two separate calc you do. Once the CFM needed to achieve the required ACH is calculated, you compare that to the minimum CFM per person and go with the greater.
Here's an example. I kept the numbers basic for easy math. 1000 square foot 2 bedroom apartment with 8 foot ceilings. Kitchen is 10x10 and two 5x10 bathrooms, leaving 800 square feet for the remainder of the apartment. This is a typical maybe on the large end 2 bedroom apartment. I know I didn't take out closets but this is just a quick example to show it probably wont matter.
800 x 8 = 6400 cubic feet
6400 x 0.35 = 2240 cubic feet of air per hour
2240 / 60 minutes = 37.333 CFM needed to achieve the required 0.35 air changes per hour
Occupant load based on 2 bedrooms is 3 people. 3 x 15 CFM per person = 45 CFM
The higher number of 45 CFM is required. Even leaving in the full volume of the kitchen and baths, its basically a wash at 46.666 CFM needed to achieve the required ACH.
Using the breathing zone only in the ACH calcs wouldn't change what is required since the actual requirement here will go to occupant load, not ACH.
In my experience the only time the ACH is higher than the CFM per person is in large high end luxury apartments/condos, and even then they are usually not too far off from each other.
Here's an example. I kept the numbers basic for easy math. 1000 square foot 2 bedroom apartment with 8 foot ceilings. Kitchen is 10x10 and two 5x10 bathrooms, leaving 800 square feet for the remainder of the apartment. This is a typical maybe on the large end 2 bedroom apartment. I know I didn't take out closets but this is just a quick example to show it probably wont matter.
800 x 8 = 6400 cubic feet
6400 x 0.35 = 2240 cubic feet of air per hour
2240 / 60 minutes = 37.333 CFM needed to achieve the required 0.35 air changes per hour
Occupant load based on 2 bedrooms is 3 people. 3 x 15 CFM per person = 45 CFM
The higher number of 45 CFM is required. Even leaving in the full volume of the kitchen and baths, its basically a wash at 46.666 CFM needed to achieve the required ACH.
Using the breathing zone only in the ACH calcs wouldn't change what is required since the actual requirement here will go to occupant load, not ACH.
In my experience the only time the ACH is higher than the CFM per person is in large high end luxury apartments/condos, and even then they are usually not too far off from each other.
Yankee Chronicler
SAWHORSE
I respectfully disagree. The heading of the third column in IMC Table 403.3.1.1 explicitly calls for CFM/person in the breathing zone. Footnote c does not cancel that out, so the 15 CFM/person for Living Areas is 15 CFM/person delivered to the breathing zone.
wwhitney
REGISTERED
If the closet is closed off from the ventilated volume, then as stated I agree you wouldn't include it in the Volume used to compute ACH.
I also think that as kitchens and bathrooms are "living areas", they have to comply with the "living areas" line item in IMC Table 403.3.1.1. The line item for "kitchen" is an additional requirement for direct exhaust, not instead of the "living areas" line.
But my main point is that ACH is a different sort of requirement from CFM. If you have a single communicating volume, and you draw a line down the middle and say you only care about outdoor air on one side of the line, then it makes sense that the CFM be calculated just using that space. But the CFM required for a given ACH will not change just because you drew that line. The outdoor air provided will mix through the entire volume, so the changeover rate depends on the entire volume.
Cheers, Wayne
klarenbeek
REGISTERED
I see where you are coming from and can see where the volume calcs may be an interpretation thing. Using only the breathing zone will lower even farther the CFM needed to meet the 0.35 ACH. In my example which I think is fairly representative for most apartments, my whole point is it won't change a thing because the CFM needed per person (which is based on number of bedrooms, not square footage BTW) is almost always higher than the CFM needed to achieve 0.35 ACH. This apartment will need 45 cfm of ventilation air regardless of if breathing zone is used to calculate volume or not.
Even adding the kitchen and bathrooms back in will only bump it up to 47 CFM for total volume to meet ACH, figuring full volume. Adjusting that to figure only breathing zone will again bring that lower than 45, so its back up to 45 CFM due to occupant load of 3 people. It's not use either 0.35 ACH or CFM per person, its's calculate both, then use whichever requires the higher airflow rate.