Hey there,

 

First post on HEP!  Exciting times, indeed.  A question on ACH and N-factors for multifamily:

 

LBNL’s N-factor converts between pressure induced (i.e.- fan measured) and natural air infiltration rates.  The well known N-factor charts max out at 3 story building heights.  In the past, I used these N-factors daily when working on single family homes. 

 

Any idea if there are equivalent N-factors that can be used (or calculated….) for mid/high rise buildings?

 

I would prefer to deal with a natural ACH rate rather than an ACH50, or whatever else.  Any help would be much appreciated.  

 

Thanks a ton,

 

-Grant 

Tags: ACH, factor, n

Views: 329

Replies to This Discussion

Hi Grant,

I checked with our tech support department and here' what they had to say.

The short answer is no. The wind and stack coefficients that are used in the n-factor calculations are empirically determined from many experiments. There is simply not enough data to extrapolate results for taller buildings. It would be recommended to use ACH50, or permeability, or any other accepted result for larger buildings. ACH natural was meant for houses.

Silvie

 

 

Grant,

 

ACHn is irrelevant in a building big enough to have any continuous venilation -- the fans completely change the pressure regime in the building. Many MFBs over three stories (at least in my part of the world) have some sort of continuous ventilation.

Further, the n-factor is fairly local and fairly specific, since it has to take into account stack effect AND wind, across all the weather conditions in the entire year. So the calculation is quite complex, and the n-factor can be significantly different for cities in the same state, with similar HDDs. 

So, when LBL did the original tables, they did them only for low-rise housing. They were (probably) calculated largely to support weatherization work -- and that was entirely focused on single-family, low-rise buildings.

Many mid-rise and high-rise models will accept input based on ACH50 or CFM50. So, if you've actually measured the building, you can use that as a proxy. If not, you just gotta guess, and see what happens when you true-up to actual energy use.

In my part of the world, 1 CFM50 per ft2 of ABOVE GRADE wood-frame envelope is a good guess (even if the building has single-wythe brick cladding). Make it about half that for a masonry-wall building. I don't know WHAT the right number is for buildings in your part of the world.

Thanks folks!

In a past life, I had always used the following formula for determining potential energy savings for reducing infiltration:

 

Q= (1.08 * ACH REDUCTION * [volume/60] * 24 * HDD)/ Boiler Efficiency

Q = annual heat loss due to infiltration

 

Any different formula's recommended for mid/high rise buildings?

 

Thanks.

Hi Grant. This equation should be fine, as a CFM of outside air costs the same to heat/cool regardless of building type. However, I'd recommend considering the cooling costs (sensible + latent) of the associated air leakage as well. Depending on where you are, it could add quite a bit of savings to the pot.

RSS

Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.

Latest Activity

Hal Skinner added a discussion to the group HVAC
Thumbnail

Old ductwork efficiency file

Allison,found the attached file on ductwork efficiency on an old CD I made.  Its definitely dated. …See More
18 minutes ago
Hal Skinner posted a discussion

What an RCC can do with no R- insulation in the walls.

I have had a handful of folks on this forum tell me, in effect,  that an RCC can do a very good job…See More
1 hour ago
Trip Smith posted a discussion

Where to spend my marketing dollars?

Hey everyone, I'm a new home performance company owner. (Originally a general contractor who is…See More
2 hours ago
Profile IconManuel Gutierrez, Chris Baker, Sharon Cannizzo and 1 more joined Home Energy Pros
4 hours ago
Kevin Jordan replied to Kevin Jordan's discussion air infiltration total
"thanks.  but it was just an illustration of a hole 2x2 with money going out of that…"
6 hours ago
Tom White posted a video

Adaptive Thermostats Demonstration Results Installed at UC Davis 6/26/2014

In buildings where variations in occupancy would fool a standard programmable thermostat, new adaptive thermostats have potential for significant energy savi...
9 hours ago
Hal Skinner's discussion was featured

Has Lead Based Paint ever stopped you on an energy upgrade?

In the process of determining all the different things that a home energy rater suggests to a…See More
10 hours ago
George Kopf's 2 discussions were featured
10 hours ago

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service