Different ACH50 Numbers and Duct Leakage Numbers

[tedkidd]

Whatever approach, have a rough target, measure at end.

[pendetim]

Thanks energy_rater_LA.

Yes I am also unclear about the 124ft^3 being both or just the main leakage but lets give the builder the benefit of the doubt and assume it is combined for both.

If I assume the house is 25,000ft^3 and 2900ft^2, Main Unit is 3.5T and Upstairs is 1.5T and leakage for combined systems is 125CFM@25pa, what is the leakage percent? (and how did you calculate it??)


Fixing the leaks could be a major PITA as the system is an "octopus" style made up of almost entirely flex duct with several fiberboard triangular shaped distribution boxes. The only good thing it has going for it is that the flex has R8 insulation, which I understand is hard to achieve with tin.

On second thought, since the flex tube is manufactured continuous with no seams, the place most leaks should occur would be at the joints and D-boxes so may not be too hard for someone to find and fix, or am I off base?

[energy_rater_La]

systems would be tested separately to determine what amount of leakage
is for which system.
at the time of my comment, you had only mentioned one system. wasn't until
later that you wrote about second system.

5% energy star requirement for combined systems 3.5+1.5=5 tons would be 100 cfm
your numbers aren't bad if for combined leakage.
but if the leakage is for one or the other system..then you have an issue.
3.5 ton system is 1400 cfm so 5% would be 70 cfm
1.5 ton system is 600 cfm so 5% would be 30 cfm
calculation is based on cfm supplied by system, and percentage of leakage
allowed by energy star.

finding out if the leakage is for one or both systems is what you'd need to know.

asking the builder without providing him the 'out' that leakage is close to minimum
for both units is the tricky part. if he realizes that he is better off answering that
leakage is for both units..you'll get that answer. at this point I'd think he may
be trying to do some cya. but we won't talk about that.

duct testing of both systems independently would give you the correct answer.
check with local hvac companies to see if they have flow hood..that is an easy
way to verify what % of air supplied by tonnage of hvac system is actually
entering the house, and not leaking into attic/basement?


here is a study on Accuracy of Flow Hoods in Residential Applications
http://epb.lbl.gov/publications/pdf/lbnl-49697.pdf
from the article:"active flow hoods are the most reliable and consistent for measuring distribution
system airflows if the measurements are to be used in estimating duct leakage, air handler
flow, and individual grille flows for room load and comfort."

there are other methods of determining duct leakage.
subtraction method with blower door
duct blaster testing
pressure pan testing
flow hood (balometer)

as to your other questions..
Fixing the leaks could be a major PITA as the system is an "octopus" style made up of almost entirely flex duct with several fiberboard triangular shaped distribution boxes. The only good thing it has going for it is that the flex has R8 insulation, which I understand is hard to achieve with tin.

R-8 duct wrap is available and code in most places (except La.) for hard pipe ducting.

On second thought, since the flex tube is manufactured continuous with no seams, the place most leaks should occur would be at the joints and D-boxes so may not be too hard for someone to find and fix, or am I off base?

you are correct. getting to all the leakage areas to seal them is usually what costs.
materials are cheap. leakage is usually same areas.
the job is labor intensive..so that is where the cost comes in.
common leakage sites are:
plenum connections to equipment
duct collars on plenum & tri boxes
duct attachment to supply boxes (supply boots)
and supply box to attic floor (if ducts are in attic)
return leakage is different, depending upon what type of
returns you have.

only mastic materials should be used. paint on mastic, or mastic tape.
I only use hardcast brand mastic 1402 tape. I've found it is strong
enough to seal & hold supply boxes in place when sealing them prior
to re-installing supply grills.
duct tape, foil tapes fail quickly. if you invest in duct sealing..
insist on mastics only.

to give you more specific info..where is system & ducts located?
where are your returns? ceiling? low on walls? floors?

personally..I'd look for hvac company with flow hood..and experience
in using it. have them do supply by supply test keeping track of air flow
from each duct. then return air (s).
supply numbers should add up to air flow per tons of system..in a perfect no
duct leakage system. cfm not being supplied is duct/return leakage.
flow hood can tell you how much from each duct & return.
its my favorite tool for measuring duct leakage, but not approved by RESNET.
so..hvac companies are the next resource IMO.

answer the questions & we'll try to help you determine your best next step.
best of luck.