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,
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.
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.
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?
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.