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  1. #1
    Join Date
    Feb 2007
    Posts
    999

    Negative pressure vs. additional return

    After running Dash's test, I determined that I had negative pressure in the house. I found an open seam in my basement return ductwork, which I sealed. The basement has no supply/return registers.

    I am thinking that adding an additional (first floor) return would be relatively easy, and would improve airflow to a couple of upstairs registers.

    After thinking about it, other than the size (larger) of the proposed return compared to my former leak, wouldn't that cause a negative pressure in the house (all else remains the same)?

    I've read here, courtesy of Beenthere, that reducing return static might cause an increase in supply static. But, in the past, one contractor I interviewed, cracked open the blower door on my (non-ECM) furnace and the airflow to those areas was vastly improved.

    My present furnace has an ECM blower. So, what's right and what's wrong?

    Amp

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,369
    Reducing airflow restrictions on the return side can increase static pressure on the supply side. The centrifugal blower in your furnace/air handler is non-positive displacement. It is possible that if supply side restriction becomes too high, air will actually attempt to flow backward through the blower wheel. The blower makes a very distinctive sound when this occurs, a "huffing" type sound. It is not a smooth, white noise rush of air like you would expect to hear when static pressures are within normal parameters.

    If your "negative pressure in the house" refers to any test you did showing air leaking into the house from outdoors, that is typically caused by a supply duct leak blowing into an attic, crawl space, or basement that is not physically connected to the interior of your house. Return leaks where the return ducting is outside the building envelope will pressurize the house, not cause it to go negative.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by shophound View Post

    If your "negative pressure in the house" refers to any test you did showing air leaking into the house from outdoors, that is typically caused by a supply duct leak blowing into an attic, crawl space, or basement that is not physically connected to the interior of your house. Return leaks where the return ducting is outside the building envelope will pressurize the house, not cause it to go negative.
    Perhaps I mistakenly used the term negative pressure with a return system leak.

    The test consisted of dangling a string at the entrance to the attached garage, and just cracking open the door while the system was running. The string pulled outward, indicating a leak in the return system.

    Using a streamer of tissue, I 'scanned' all return duct seams/connections and found a hidden area that had an opening. After closing off the opening, the string test was neutral.

    While the leak was open, air from the first floor was being pulled into the basement (under the first floor door opening). Using the streamer, I was not able to detect any airflow around the perimeter rim joists or flue area. This isn't to say that there wasn't any.

    Back to my original question, is cracking open the blower compartment door the same as a leak in the return system? And if it increases airflow, is a 'leak' necessarily a bad thing?

    If not a bad thing, then wouldn't it be preferable to draw the air (additional return) from a living area, as opposed to a sometimes humid and less clean basement?

    I appreciate your comments regarding the reverse flow issue, but the return would be relatively small.

    Amp

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,025
    You don't want to draw Return-Air from the basement unless it is a living area sealed off from any combustion sources.

    Since opening the blower door considerably increased the airflow to the rooms, I would consider adding more Return & other options explained on my linked page.

    Yes, it will increase static on the supply-side, but Reduce it on the Return-Side.
    It depends on the engineering design of a backward-curved-blade blower wheel, how much head back-pressure it can handle. Forward curved blades in coml applications can handle all kinds of head back-pressure.

    If the entire Supply-Side is sized sufficiently well, including the diffusers for the increased velocity & static (air noise,etc.) the duct airflow system will function more efficiently, as will the A/C system.

    If SA static appears too high consider using more efficient delivery diffusers or 90-ell turning vanes, etc. Click udarrell if U need a link to more airflow info.

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