AC duct not sealed

[Michael R. Boyett]

Originally Posted by Lisa Bloemer

Also had a vent connection going from Water Heater/AC closet extending through roof. Can figure out what that is for either.. All electric appliances.. No fuel at all going to building.. Can you comment on that as well? See photos attached.

That appears to be a fresh air/make up air intake duct to mix some percentage of fresh exterior air with the 'stale' indoor air. Newer systems are electronically controlled; this simple one appears to be controlled by the damper shown in the photo.

[David A. Andersen]

I guess we can expand this "guess" even further by bringing in the fire issues!

Jeez, a 4" duct is only rated to carry 40 CFM in good designs.

This is not a duct, it is a take off. 40 cfm in 100 ft of 4" duct only has a static friction pressure of .11. This take off has practicly no resistance. You can get 90 cfm at just .50 inch h20/100 ft.

You are probably correct about the amount of CFM passing through it though, because if they do not have both a supply and return source, and the attic space is fully sealed and pressurized, air flow will be diminished. Not because it's a closed damper though.

Residential duct systems are usually unsealed and not even close to good spec'd work seen in commercial, etc. I have seen studies from Home Energy magazine and others that claim that 20-40% of the air moving through the air handler does not come from or get to the intended grills/diffusers.....the ducts are generally so leaky!!!!! So to me this is not a big flaw but still should be sealed.....I just wouldn't hang a big, red flag on it!

I would say that this is a local issue. There is no reason to accept this amount of leakage in any case.

40%! This is totally absurd! If you have a 40% loss in an air duct system, you need to pull that stuff out of there!

As to pressurizing the attic......I wonder if you could even measure the effect with a micromanometer that this small a leak would have!!(unless it was a quite small space) Since the attic is sealed, any pressure would send air into the house through ceiling penetrations.......conditioned air from above....sounds familiar!!!

If the spray foam installation was preformed as we are assuming (sealing off the attic to be a conditioned space, we could deffinatly measure the pressure! Better yet, pull out an IR camera and just "take a look"!

Other items such as poor duct design (no returns in bedrooms with 2-3 supplies), other duct leaks, wind pressures and stack effect will cause much larger pressure imbalances/variations than this small amount of leakage.

Wind pressure and stack effect are supposed to be reduced with this newfangled "conditioned attic space" scenario, is it not?

Also, a closed balancing damper is not an "insignificant" air leak!

Unlike other duct leakage in unconditioned attics, this air is lost to or returned from the conditioned space. How much efficiency would be gained if this were sealed?

Well, in reviewing the assumptions are making in this particular case, the feeble attempt to condition the space with this design is allowing excessive temperatures up in the attic. Even Lisa mentioned that she was swimming in her own sweat before she got out of there. If were going to accept this air infiltration, you're just making my point in that we are forcing excessively hot air into the condition space of the house.

This condition we are discussing is more about what is acceptable loss rather than how much efficiency loss actually occurs. Were addressing both scenarios at the same time and can't do that. Some say the leak is insignificant and does not affect anything. However on the same hand were discussing how much leakage normally occurs and that this leakage is acceptable, which it is not.

That appears to be a fresh air/make up air intake duct to mix some percentage of fresh exterior air with the 'stale' indoor air.

Very likely, however if the attic is sealed, we are not getting exterior air. The duct should be piped to the exterior of the house.

[Brian A. MacNeish]

Originally Posted by dandersen

I would say that this is a local issue. There is no reason to accept this amount of leakage in any case.

40%! This is totally absurd! If you have a 40% loss in an air duct system, you need to pull that stuff out of there!

EXECUTIVE SUMMARY

Duct leakage in forced-air distribution systems has a significant impact on the energy consumed in residential buildings. It is a common practice to place the ducts outside the conditioned space in a large portion of US homes. This can result in significant loss of energy by leakage to the outside on the supply side and the infiltration of unconditioned air into the system on the return side. Field studies have shown that existing residential air distribution systems can leak as much as 40% of the total supply air. As ducts are often outside the conditioned space, this leakage corresponds to a proportionate amount of energy loss from the duct system.
Several methods for estimating duct leakage have been used in the past with varying degrees of accuracy.
http://www.ncembt.org/portal/Portals/0/downloads/Moujaes%20SF_Duct%20Leakage%20Measurements%20In%20 Residential%20Buildings_NCEMBT-080215.pdf

Why should I be concerned about duct leakage?
In Florida, studies have shown that about 20—30 percent of the air that moves through a duct system is lost due to leaks, holes and poorly connected ducts. There are four
main reasons a homeowner should be concerned with duct leakage 1) Leaking ducts cause higher electric bills. 2) Duct leaks can reduce the comfort in your home. 3) Leaking
ducts can cause poor indoor air quality (indoor air pollution) which can affect your health (radon, pesticides, fertilizers, fiberglass fibers, etc. can be brought into your home by
duct leakage). 4) Duct leaks can cause maintenance problems with your house and your air conditioning system.

http://www.jea.com/about/pub/downloads/DuctLeakage.pdf


Improving Ducts in Southern California
Even in new construction, ducts can leak 20%-40% of their conditioned air,
http://www.homeenergy.org/archive/he...97/971106.html


This is partial results of a cross Canada study I was part of:

The low duct efficiency (only 39.2% of heat is delivered
to the registers) is attributable to duct leakage, radiation
losses and restrictive ducts and registers (register grilles
were typically found to be very restrictive, reducing the
boot area by 50-65%).

Here are some decent articles:
http://www.contractingbusiness.com/2...e/False/43340/

http://www.buildingscienceconsulting...tent_Loads.pdf

[David A. Andersen]

Looks and sounds like they did indeed create (or try to create) a conditioned attic. This duct take off should be sealed but being unsealed is not as big a problem as some may think.

This is a conditioned attic, sealed off from the exterior by spray foam. Is it?The amount of air leaking around this small closed damper may be what...8-10-15 CFM in a system of 1000-2000 CFM......not a big deal and it is into a conditioned space. Jeez, a 4" duct is only rated to carry 40 CFM in good designs.

Residential duct systems are usually unsealed and not even close to good spec'd work seen in commercial, etc. I have seen studies from Home Energy magazine and others that claim that 20-40% of the air moving through the air handler does not come from or get to the intended grills/diffusers.....the ducts are generally so leaky!!!!! So to me this is not a big flaw but still should be sealed.....I just wouldn't hang a big, red flag on it!

Any home inspector that goes along with 40% loss is an acceptable efficiency loss standard and an open air duct connection is okay, is just perpetuating this problem.

Exactly what is your argument?

Your pointing out that this air leakage is a significant energy concern but then say you wouldn't hang a big red flag on it as it is insignificant or acceptable?
Exactly what direction are you going with this and maybe we'll have less disagreement.

As to pressurizing the attic......I wonder if you could even measure the effect with a micromanometer that this small a leak would have!!(unless it was a quite small space) Since the attic is sealed, any pressure would send air into the house through ceiling penetrations.......conditioned air from above....sounds familiar!!! Other items such as poor duct design (no returns in bedrooms with 2-3 supplies), other duct leaks, wind pressures and stack effect will cause much larger pressure imbalances/variations than this small amount of leakage.

Your introduction of minimal information from several different viewpoints at the same time is only clouding the conversation. There are no specific facts, just opinions from different sources.

You say that you can't measure the pressure difference when an open duct system exists. You can measure it, how can you argue this? You can use a micro-manometer to detect air flow through the dirt below a concrete slab sitting underneath a three-story building! If you find that you can't measure it, it's because a pressure differential doesn't exist because of excessive air leakage from an improperly sealed attic space.

Unlike other duct leakage in unconditioned attics, this air is lost to or returned from the conditioned space. How much efficiency would be gained if this were sealed?

Seeing as the air in the attic space is not being properly conditioned due to improper design, "unconditioned air" is being introduced into the living space of the house just as you describe.

Again, we are talking about theory not specific facts (because there are none).

Maybe you're focusing on the use of the new "conditioned attic space" scenario. Again, it's being used, it has a standard, but in many cases is employing substandard workmanship and theory in the process.

Dumping supply air into the attic, or drawing attic air throughout the return of his not appropriate system design.

I don't know where this thread is leading because we do not have any specific facts from the initial question.

The laws of thermal dynamics, as you know are quite specific and do not vary under different circumstances. The circumstances may vary, but the laws do not.

I guess that it doesn't really matter what is actually going on at this house because it is an evaluation way beyond what a home inspector is expected to achieve. However, I think that it is prudent for a home inspector to call out a situation such as this (whether they understand it or not).

I'm not arguing that these conditions do not exist almost everywhere. I am not saying that these conditions adversely affect the building in every case. But to flat out say that it's unimportant is not appropriate. We don't know the circumstances or the test conditions and in no way can make this statement.

I pointed out several situations that could be adversely affected from air leakage in the ducts (regardless of where it exists or how it is designed). We don't know if these situations exist or to what proportion, but to blow it all off because we can't prove it otherwise is unwise.

Several alleged explanations as to what this connection is was brought up, and under each scenario there is adverse potential.

We don't have to make the analysis and provide the facts but we should not underestimate the potential.