GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Does running a forced air system create negative pressure on the living space and attic?

suect | Posted in General Questions on

I’m trying to understand the dynamics os the a/c and furnace on the positive and negative pressures in the home.

If a system was to remain off would it remain neutral pressure?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    A properly installed HVAC system won’t affect the air pressure inside your home compared to outside. An HVAC system typically just recirculates air within the home.

    Things that could potentially cause pressure differentials would be leaky ductwork in unconditioned parts of the home (attics, crawlspaces) that would allow air to be either drawn in from outside (in the case of leaky return ducts, which would increase the pressure in the home), or expelled to the outdoors (in the case of leaky supply ducts, which would reduce the pressure inside the home).

    HRVs that aren’t balanced could potentially affect pressures too. Note that any of these pressure differentials will be very small and probably imperceptible. It’s sometimes beneficial to run a slight positive pressure inside the home since that can help you to keep pollutants (pollen, etc) out.


  2. Jon_R | | #2

    > If a system was to remain off would it remain neutral pressure?

    And even if off or ducting is leak free and balanced, stack effect and wind will mean that most of the home isn't neutral pressure.

    1. suect | | #10

      Would it be negative on the lower level, positive on the upper level? Would fresh air add to the positive pressure on the upper level?

      1. Jon_R | | #11

        When the house is being heated or cooled? Is your attic sealed and conditioned or vented? Best to fully describe your setup.

        Measuring/logging attic humidity is usually helpful.

        1. suect | | #13

          The attic is semi conditioned as it is foam insulated. In September a whole house dehumidifier was installed in the attic.
          The humidity and temperatures decrease at night possibly with the system working less? I notice when the system is idle in the morning as well humidity is in the low 50’s and temperature in the low 70’s but as the unit is working more the humidity increases reaching to the mid 60’s. I’m tempted to raise the a/c temperature so it does not cycle so much.

          Would it be a negative pressure pull in the attic or a positive pressure push from the main level…or both?

          Humidity is definitely higher on hotter sunny days.

  3. suect | | #3

    Thanks Bill,

    Is there a method that is best for measuring if a unit is balanced correctly?

    1. Expert Member
      BILL WICHERS | | #4

      “Balancing” in regards to HVAC systems generally means measuring and adjusting airflow rates from each register. This is done with a device that fits over the register and measures airflow in cubic feet per minute (CFM). While this is common practice in commercial buildings, I’m not aware of it being commonly done in residential structures.

      Balancing doesn’t generally refer to air pressure differentials between indoors and outdoors. It would be possible to make such measurements with a differential manometer, but I’ve never heard of that being done.

      Is there any particular reason you’re concerned about air pressure differences between indoors and outdoors?


      1. suect | | #6

        Thanks Bill,

        Actually the pressure difference between main and attic levels are of greatest concern.

        You helped greatly with 2 opinions I received:
        An energy auditor who measured all returns and supplies in the fashion you had discussed. He mentioned there is a 330 cfm deficit on the main level. Theories he proposed are:
        - the remainder is taken from the attic
        -conditioned air is being pushed up into to attic and wall cavities creating a humidity issue
        His recommendation is to add an additional 10” return to the main level.

        I provided this information to our HVAC contractor who stated return measurements cannot be measured by this method. He used a probe, inserting it into the return (I believe the measurements was 0.113), after the filter 0.336) and supply but the calculation was not provided. His recommendation was a less restrictive filter. This was done with little change.

        The goal is to decrease the humidity in the attic, I’m looking to find the root cause. Could it an additional return help?

        1. Expert Member
          BILL WICHERS | | #8

          I don’t see why you couldn’t measure return CFM with the same test rig. I’ve only ever been concerned with supply air though, I’ve never tried measuring a return.

          An additional return may help, but if the problem is due to leaky ductwork the additional return is a band aide. The additional return will reduce pressures elsewhere in the system, but if your primary concern is humidity getting into the attic (or being pulled inside from the attic), the additional return may reduce, but will not completely solve, that problem.

          If you have leaky ducts in your attic, you need to seal them. The usual solution would be duct mastic. The product itself is cheap, but the application of the mastic to the ducts is very labor intensive, especially in areas with limited access like attics. Any HVAC contractor should be familiar with this kind of work, but specialty performance contractors that specialize in energy efficiency may do a better job with something like this. What you want is someone physically small who cares about quality and will be diligent when doing the work. As a tall guy who has done this kind of work, I can tell you that height is not an advantage here! :-)

          Pressure differences between levels that cause nuisances like rattling doors when the system starts and stops will be helped with additional registers where needed to balance air flows. Ideally you want each room to have both supply and return registers to keep air flows balanced. Sometimes this isn’t possible and doors are cut with a gap on the bottom, or a grille in the door, to allow air to circulate.


  4. Deleted | | #5


  5. Jon_R | | #7

    > conditioned air is being pushed up into to attic ...creating a humidity issue

    This is a Winter problem? In warm AC weather, conditioned interior air entering the attic dries the attic.

    > the remainder is taken from the attic

    If this is an interior balance issue between closed off areas, you are on the right path - add/adjust supplies and returns to balance. If you have return ducts in a vented attic that are pulling in attic air, the right solution is to seal those ducts. And insulate them.

    Checking room-to-room and whole house pressurization (vs flow) should be far more common than it is.

    1. Expert Member
      NICK KEENAN | | #9

      Negative pressure in the attic in summer would draw in humid air and raise the humidity. That would happen if there was a leaky return in the attic.

  6. suect | | #12

    Thank you. I will have this checked out

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |