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Stack effect discussion

Gary Richardson | Posted in Green Building Techniques on

I find it interesting and a little frustruting that in all the discussion on stack effect I have not been able to find one article discussing the contribution of leaky ducts located in attics to stack effect. I have a home owner who reported excessive dust in her home. She lives in a 22 year old two story home in central California built before the requirement for sealed ducts. I noticed she had about 12 can lights in the ceiling of the upstairs rooms which were not air tight and with the other typical air leaks inthe upper portion of the shell of her home I suspected some duct leaks. She has 3 HAVC systems all with ducts in the attic and in portions of the interior walls. Testing the duct systems I found her duct leaks averaged 21% of nominal air flow which is fairly typical of some 22 year old homes here in the Valley. She has a natural draft fireplace on the first floor and no other gas appliances within the envelope. I recommended duct sealing and replacing the can lights, together with weather stripping attic access and exterior doors. She allowed me to seal the ducts reducing the duct leaks to less than 6% of nominal air flow and no other work. She would not pay for blower door testing. She now complains that I have caused air to flow down her chimney introducing an ash smell and creating a health hazard. I cannot help her to understand that because I have reduced the flow of air out the top of her home I have esentially imporved the condition of the air within her home.
Comments please.

Gary Richardson
Rare Service Heating and Air Conditioning, Inc.
Inactive BPI Building Analyst
Envelope Professional
A/C and HP Professional
Heating Professional

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  1. David Meiland | | #1

    A couple of possibilities. The duct leakage to the outside (which you have significantly reduced) used to help remove the contaminants she is now smelling... and/or... she may have had return leakage dominant, so that the HVAC was pressurizing the home when in operation, pushing some air out the chimney. If there are still a lot of leaks at the top of the house, then air will be looking for leaks at the bottom, and will use the fireplace. You may have reduced energy waste but changed the IAQ dynamics.

    I'm not clear whether she has a wood or gas fireplace, but either one can be an issue if backdrafting. It's hard to know what to say if she won't allow you to do further work.

  2. Robert Hronek | | #2

    I view stack effect as a natural condition and not dependant on mechanical systems. I think what you are describing is throughly discussed in blower door and air sealing training.

    It should be easy to test with a manometer to see if the house goes into a negative pressure condition when the HVAC kicks on.

    Although you have sealed the duct work it appears that the supply side is still pushing air out of the house.

  3. Gary Richardson | | #3

    In answer to her insistance that I have exacerbated her issue my wish was to clear her thinking of what was actually happening without confusing her in the most simplist of manners. I laid a DG-700 on a table near a door and pushed the "reference" hose thru a small hole under the door and observed the resultant reading in pascals asking her to look as well. I put the outdoor end of the hose in a can to diminish the ill effects of any breeze though there was no wind. The indoor pressure with no mechanical fans on fluctuated from -.7 Pa to -1.1 Pa. The outdoor ambient was 54 degrees F with indoor at 68.Very small pressure indeed and very slight fluctuation. I turned on each A/H fan alone and with each unit running the pressure read -1.5 Pa to -1.7 Pa. and with all three fans on the pressure did not go greater negative than -2.2 Pa. She always closes all bedroom doors upstairs so with the fans on and doors closed we discussed the issue for a few minutes and monitored the pressure. There is one small return in the master bedroom all other returns are in the main body of the home. The greatest pressure this situation was -2.5 Pa. Very small pressures but admittedly still results in stack effect. I have tried to get her to understand the concept of performing a blowerdoor test and sealing air leaks until we need to install a balanced ventilation system. She will not spend another dime for fear the smell will become worse. With these limitations I feel I am beating my head against the wall. I must admit my failure with her was in not measuring the house pressure before sealing the ducts so she could see the difference when we sealed the ducts. You see she firmly believes I have made her wood fireplace expel more ugly air (her words) into her home by sealing her ducts. In cases like this I have relied on third party verification of the principles of building science by presenting articles describing the issue at hand. I could find none describing stack effect related to air leaks as the result of attic duct systems.
    I know what the issues are for I have been studing and practicing building science for the past 15 years I am just unable to explain it to some one who will not allow me to show her with additional testing. She will not accept my offer to do this testing at no charge. I would still like to present her with perhaps some reading material written in simple enough language for her to understand so she want tell 10 other people what a terrible job I did for her.

  4. GBA Editor
    Martin Holladay | | #4

    It's far from clear what is going on here, and I'm not sure why you keep referring to this phenomenon as the "stack effect."

    As I'm sure you know, this is how the stack effect usually works in an unused chimney: air is drawn into the flue through the clean-out door at the bottom of the chimney or through the fireplace, and air exits the flue at the top of the chimney. Unless other forces overwhelm the stack effect, it's not typical for air to enter the top of a chimney flue and come out the bottom of the flue inside the house.

    In the case of this house, it's possible that the homeowner is correct, and air is being pulled down the chimney into her house. If that's happening, however, it usually means that the house is depressurized with respect to the outdoors; this is usually caused by exhaust fans (bathroom fans, range hood fans, clothes dryers, power-vented water heaters, etc.) or an unbalanced HVAC system.

  5. Gary Richardson | | #5

    In this home there is no cleanout. The ashes are removed by the homeowner from within the fireplace. Contributing to the ash smell I believe is the fact the homeowner has previously plugged every single crack in her many exterior doors leaving the fireplace chimney as the single largest contributor to infiltration. There are other small leaks such as exterior wall receptacles 22 year old dual pane metal frame windows etc. I spent two hours with her yesterday discussing how air moves into and out of the home and during this discussion she stated they are overwhelmed with the smell while sitting in their living room with no mechanical fan running except periodic running of the furnace.
    I might add here that she insists the smell began immediately following the sealing of her duct systems which was in mid December. December 2011 was our coldest December in many years so I simply put two and two together, very cold outdoors with interior temp set at 68 degrees. Her original complaint was excessive dust in her home and after discussing the many contributing factors she allowed me to do a duct test which showed excessive leakage and with her permission we did seal her ducts. With greatly reduced duct leaks, if what she is saying is true the air entering the home in a static condition (meaning no mechanical fans) should have been reduced not increased as she insists. And any other fans such as bath fans, clothes dryers, range hood fans etc. have not changed.The operation of those are as they have always been. The fireplace damper remains open at all times during the winter so they can have a wood fire periodically when we have permission to burn.
    I know there are many contributing factors to air infiltration such as age and construction of the home
    number and strength of mechanical fans, intergriy of duct systems, wind conditions, etc. The bottom line remains she firmly believes she did not have this condition before I sealed her ducts and now she does and I am the target.

  6. David Meiland | | #6

    Are there fresh-air intakes to the HVAC systems?

  7. John Brooks | | #7

    Is the Fireplace masonry or "metal prefab" ?
    Is it interior or on an oustide wall?

  8. James Morgan | | #8

    Gary - my two cents:

    1. As others have commented you are confusing the issue by describing this problem as stack effect. Most articles you are likely to encounter to support your case are likely to be using the term differently than you are here so will be no help in communicating with the homeowner.

    2. I suspect the key to the problem may be contained in this sentence - "She always closes all bedroom doors upstairs". If the door undercuts or other return paths are inadequate this will result in the negative pressure you measured as the return register sucks more air than is being delivered on the supply side, the balance of the delivered air being pushed out of the bedrooms to the exterior through the various gaps and cracks. The chimney will then supply a return air path from the exterior, completing the loop with the resultant odor. This lack of balance in the system could well have only shown up only after duct tightening. This hypothesis can easily be tested by a reference pressure test with the bedroom doors open. If the negative pressure is reduced with the doors open the fix is easy - either increase the door undercuts or install transfer grilles.

    Good luck!

  9. Aj Builder, Upstate NY Zone 6a | | #9

    Gary, seems one choice would be to see if your customer has a solution that she would agree to.

    They should close the flue and install quality glass doors on the fireplace if they are willing to work past this.

    I would write a letter to them that you are attempting to work with them but that they have to allow you to do so.

    Add my two cents to James's two cents and you have a winner of a solution. Open the path from the rooms and close the path from the chimney and write it all in a letter to "dear customer"...

  10. Gary Richardson | | #10

    I will try to gain some understanding of this issue by this note. I don't believe I am confused regarding my intent here. What I have tried to say all along is that my frustration with the home owner is that she believes I have caused a condition that allows the ash smell from her wood burning fireplace to enter her home when before I sealed her ducts it did not. I believe I did not cause the smell to be more evedent for this reason.
    I said before I thought duct leaks should be included in a discussion regarding stack effect because all other actions on the house being equal the serious leaks in her duct system located in the attic are no different when no mechanical fans are exausting air from the home than any other air leak in the highest surface of the envelope and because of this I believe those duct leaks in that situation contribute to the severity of stack effect. When any one of the FAU units are running the conditions change dramatically depending on many factors such as whether the supply side leaks are greater than the return leaks, where the return terminations are located, whether there are any closed interior doors closed in the affected zone, on and on. The fact remains those duct leaks in the attic are simply more penetrations in the envelope. Hopefully this clears up my meaning concerning stack effect and duct leaks.

  11. Robert Hronek | | #11

    What you are desribing is not the stack effect. The stack effect is warm air rising and leaking out at the top and being replace by air leaking in at the bottom. It is not air coming down a chimney and the
    leaking out the top of the house.

    What you are describing is referred to as backdrafting. After air sealing a worst case scenario should be tested. That is turning on all fans and vented appliances to see if negative pressures causes a backdraft. In your case it sounds as if backdrafting is being caused by the operation of the HVAC.

    In order to solve the problem you need to be able to properly analyaze what is happening. What you have is a mechanically induced pattern of exfiltration and infiltration.

    If air is being pulled down the chimney then there is an inbalance in return air to the furnace. I see two causes. Supply side leakage with ducts communicating to the outside such as the attic or a crawlspace. The other is inadequate return air because of the lack a return path.

    Something else to consider are the chimney dampers. Are they damaged or otherwise not blocking the air. A chimeny ballon could be used to block any leaks in the dampers. Have you monitored the air coming down the chimney.

  12. Gary Richardson | | #12

    Then is it called backdrafting even in a cold fireplace? And the degree of backdrafting is then relative to the strength of the forces opposing normal drafting? Am I then not to believe her when she states she has this smell even with no exaust fans running nor any FAU? I'm beginning to believe she really does not wish to know the exact cause of her problem for she will not allow me to perform any further testing. She insists I correct this issue for which I am responsible so I guess I will have to punch some new holes in her newly sealed ducts! NOT REALLY.

  13. GBA Editor
    Martin Holladay | | #13

    Q. "Then is it called backdrafting even in a cold fireplace?"

    A. Yes.

    Q. "The degree of backdrafting is then relative to the strength of the forces opposing normal drafting?"

    A. Yes. Air won't come down a chimney flue and enter a house unless the interior of the house is depressurized with respect to the outdoors (or due to certain wind conditions). If the homeowner allows you to return, you need to use your manometers to determine why the house is depressurized.

  14. Robert Hronek | | #14

    I would ask her if your could install some chimney ballons.

  15. Gary Richardson | | #15

    I offered to install chimney ballons and she declined saying she would like to comtinue using her fireplace. I did use my manometer for some post sealing testing with various exhausting fans operating but more testing confused her more. She felt I was essentially using "smoke and mirrors" so I was looking for "third party articals in an effort to help her understand her refusal to allow further testing would prevent anyone to help her. She wants me to come back to talk further with her but I really need a good clear written piece on this issue to present to her. I will try by Googling "backdrafting" to see if I can find this.
    Thank all of you for your help

  16. John Brooks | | #16

    Gary, If the Chimney is on an outside wall ...
    this article may be helpful

    If the homeowner did some "tightening up" below the Neutral Pressure Plane...
    The house could be more negative now than it was before....

    I don't see how tightening the supply ducts in the attic could cause
    the house to go more negative and allow more cold air to spill into the cold chimney

    Joe Lstiburek's Builder's Guide shows that loose ducts in the attic cause negative follows that tighter ducts should cause less negative pressure

  17. Bob McWilliams | | #17

    HI Gary,
    Instead of science you might want to try Old House Journal for an article to confirm the purpose of the chimney damper; to stop cold drafts when the fire is out. When she wants a fire, open the damper; that's what people have been doing since Benjamin Thompson (1753-1814) invented the fireplace damper.

    OHJ March 1976 page 3: "A damper is merely a device placed in the throat of a fireplace flue to prevent drafts from coming down the chimney (and heat from escaping from the room) when the fireplace is not in use." Link to full article is below.

  18. Gary Richardson | | #18

    Thank you john and Bob, I have made many suggestions I felt would provide some relief but unfortunately this home owner I believe does not wish to understand what is going on in the home just wants relief. After a fire in the fireplace they sometimes go to bed with warm coals still in the fireplace so they couldn't close the damper until the next day and do you really expect them to open and close the damper periodically during the winter? She doesn't want to do this for fear of getting soot all over her arm! I ready to tell her no one can help her.

  19. GBA Editor
    Martin Holladay | | #19

    In your 3/1/12 comment, you wrote, "I did use my manometer for some post-sealing testing with various exhausting fans operating but more testing confused her more."

    Here's my question: did your post-sealing testing also confuse you? Or do you have an explanation for why the house is depressurized enough to pull air down the chimney flue?

    These types of problems aren't always easy to diagnose. But you can't really dismiss the homeowner's concerns until you are able to come up with an explanation for the current situation. Until you've figured out what's going on, it makes sense to continue your investigations and testing -- if the homeowner allows you to.

  20. Gary Richardson | | #20

    Martin, Though I'm not an engineer I have studied home performance for 12 plus years and I believe I have some degree of understanding. I know that temperature and pressure differences across the shell, height of the building, mechanical fans or other means of exhausting air from the home, size and location of penetrations in the shell, closing of interior doors, and perhaps other forces I haven't mentioned here all have an effect on exfiltration and infiltration. I have been using a blower door, duct blaster, Infrared camera, flow hoods, flow grids etc, to diagnose comfort and energy use, and health issues for a number of years and continue taking classes each and every year. I have done my share of misdiagnosing issues, but if I am not sure I do what I did in this case, I ask for help. Everyone has been very helpful in helping me with this issue. My concern now is that the home owner would like some magical change in eliminating her issue without allowing additional testing. She actually stated she would like me to undo what I have done! What I would really like to do is provide her with some simply written information hoping she will begin to understand her issue is not as simple as she would like to believe and if she is to have this corrected she will have to allow testing and correction of those items responsible for her problem. For example she has about 12 old leaky can lights in the upstairs ceiling that she refuses to believe can contribute to her problem. They are upstairs for heavens sake how can they be part of the problem? (Her words).

  21. David Meiland | | #21

    Gary, go in the attic and punch a new fresh air intake into each return plenum. Then, stop wasting your time on this person.

  22. John Brooks | | #22

    While this is getting sorted out...
    I would advise not using the fireplace
    Make sure they have a working CO detector

  23. Gary Richardson | | #23

    Thanks David and John. CO detector is there and Fresh air is a great idea.

  24. GBA Editor
    Martin Holladay | | #24

    I hope that GBA readers are not misled by David and Gary's attempts at irony or sarcasm.

    It's not true that "Fresh air is a great idea" if the fresh air is supplied by a form of sabotage -- punching deliberate holes in attic ducts in hopes of making a troublesome homeowner stop complaining.

    I assume that David's suggestion was an attempt at humor -- not advice intended to be taken seriously.

  25. Gary Richardson | | #25

    You're right Martin and I appologize. As I said in my 3/02/2012 post at 13:44 I would like to have this corrected in the best way possible. At this point the decision is hers.

  26. David Meiland | | #26

    >>an attempt at humor

    I thought it was a pretty good attempt myself, but I need to be mindful that someone might actually take me seriously, however unlikely and foolish that might be.

    Seriously, I do think that Gary needs to put this client behind him. As presented by him, she sounds uncooperative and inflexible to me. You can't easily do the type of work that Gary is doing for a person like that. There can be unforeseen circumstances and unintended consequences and the owner needs to allow the work to be completed. Since she apparently won't, perhaps she is safest with excess return air to pressurize the house, to wit Gary should undo some of his duct sealing outcome by providing fresh air intakes to the systems.

    I also think Gary probably made a significant mistake by not correctly selling the customer an open-ended package of improvements. Now he's partly done, she's finished paying, and he's stuck. He may not have set the terms correctly for this job by explaining the unforeseen/unintended potential and the need to continue until everything is right. The contractor needs to control the job!

  27. Aj Builder, Upstate NY Zone 6a | | #27

    I repeat my post #8;

    Gary, seems one choice would be to see if your customer has a solution that she would agree to.

    They should close the flue and install quality glass doors on the fireplace if they are willing to work past this.

    I would write a letter to them that you are attempting to work with them but that they have to allow you to do so.

    Add my two cents to James's two cents and you have a winner of a solution. Open the path from the rooms and close the path from the chimney and write it all in a letter to "dear customer"...

    Time to move on.

  28. Gary Richardson | | #28

    I appreciate the remarks and advise from all of you and will take it to heart. I am not easily fooled by a home owner but on this one I missed the mark badly. However it is a very good lesson. You cannot short circuit any attempt to discover all important parts of a clients concerns and unless you are allowed to perform ALL necessary testing you should not proceed.
    Thanks again.

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