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: 324

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

Steven Lewis replied to Christopher Talarico's discussion Heating with Tankless Water Heater & Hydronic Air Handler vs. Gas Furnace
"It looks better on paper than in real life applications.  Both Amana and Lennox came out with…"
16 hours ago
Dennis Heidner replied to Christopher Talarico's discussion Heating with Tankless Water Heater & Hydronic Air Handler vs. Gas Furnace
"Rinnai has application note on how to use their tankless hot water heater with a Rinnai water to…"
yesterday
Hal Skinner replied to Hal Skinner's discussion How much heat energy is lost through the floor of a house?
"I found the buildingscience article interesting.  Interesting that they noted the wet bottom…"
yesterday
Bud Poll replied to Hal Skinner's discussion How much heat energy is lost through the floor of a house?
"With conduction that ends at the bottom of the joists and convection that is naturally suppressed…"
yesterday
Hal Skinner replied to Hal Skinner's discussion How much heat energy is lost through the floor of a house?
"Good morning Bud, Took a quick look at the site and a couple others listed on the Yahoo…"
yesterday
Bud Poll replied to Hal Skinner's discussion How much heat energy is lost through the floor of a house?
"Hi Hal, Search "Basecalc basement heat loss".  From memory Canada (nrcan) has/had a…"
yesterday
Michael Dunseith posted photos
Saturday
Hal Skinner replied to Hal Skinner's discussion How much heat energy is lost through the floor of a house?
"II just remembered seeing a report by a federal adency, many years ago, that basically said …"
Saturday

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service