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Community and Q&A

Negative pressure in 15.7 ACH 50 home – does it matter? Zone 5a CT

jenniferz5 | Posted in General Questions on

I am tightening up my home after a licensed RESNET HERS rater came out and gave me the bad news (see title of this post).  Our furnace has also given up the ghost, so (fortunately?) I am able to replace that at this time, too.

After reading several articles on GBA regarding negative pressure in a home, I realize mine is guilty of this when the bath fan is running (we have only one bath in the home).  It is a Panasonic WhisperGreen, which we run during daytime hours on high (110 CFM) as a way to improve our IAQ.

We will be getting an ERV (probably the Minotair) in the near future, but is there a way that I should be running the fan to both improve IAQ, but minimize negative pressure?  Does it matter that there is negative pressure during the day?  Does it help or hurt the IAQ?  

Forgive me, but this part of the HVAC puzzle is all new to me.

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  1. Expert Member
    NICK KEENAN | | #1

    What do you mean when you say "improve IAQ"? Poor IAQ is usually used to mean odors or other contamination. Those should be addressed at the source.

  2. Expert Member
    Dana Dorsett | | #2

    >"Our furnace has also given up the ghost, so (fortunately?) I am able to replace that at this time, too."

    Have you run this math yet?

    Furnace replacement time is an opportunity moment to right-size the equipment, which has considerable comfort consequences. Most existing furnaces have ridiculous oversize factors, and never break a 25-30% duty cycle all season, heating the house, but delivering the opposite of comfort: The hot flash followed by the extended chill, with some rooms never fully getting up to temperature.

    It's worth reviewing Nate Adams' freebie chapters and short videos on the topic:

    If you're improving the air tightness and insulation levels of the house, don't be afraid to go with a low oversize factor from the fuel-use based load calculation rather than the ASHRAE recommended 1.4x. A 15.7ACH/50 house usually has a lot of low-hanging fruit on the air-sealing front, which could easily knock 15-20% off the load numbers if made super-tight, and 1.4x would be sub-optimally oversized.

    eg: if the fuel use calc in the leaky condition comes in at, say 35,000 BTU/hr, the ASHRAE 1.4x upsizing factor would put you into a ~50KBTU/hr furnace, but if air-sealing & insulation upgrades knocks 15% off that to ~30K you'll really be better off with a ~40K furnace, only about a 1.15x oversize factor from the fuel-use estimated load of the house in it's air-leaky condition. But a 50K furnace for a 30K eventual load is a 1.67K oversize factor. It will meet it's AFUE numbers at that level of oversizing, but even at design condition would only see a 1/1.67= 60% duty cycle instead of north 0f 70% (at 1.4x oversizing), and during average mid winter conditions is likely to be running well under 50% of the time, which is where those cold comfort issues start creeping in. An oversize factor of 1.67x is more comfortable than the typical 3x-4x for hot air furnaces, but not as comfortable as a 1.2x oversize factor running north of 80% of the time at design condition.

  3. Expert Member
    BILL WICHERS | | #3

    My first concern with negative indoor air pressure would be the possibility for any naturally aspirated natural gas appliance to backdraft. This is most likely with water heaters, but may also affect your furnace. Sealed combustion appliances would eliminate the possibility of backdrafting, and they're usually more efficient too!


  4. jenniferz5 | | #4

    Thanks for the replies! DCContrarian, IAQ to me means anything in the air that should not be breathed in by the occupants. This can mean many things, but my concern is mainly PM 2.5 and VOCs.

    Dana, I have done the math and will probably be going with a Minotair with a duct heater and heat coil from the Sanden CO2 HPWH. Our Hearthstone Clydesdale wood-burning fireplace insert and a few well-placed electric heat strips will back that up.

    Zephyr7, if I am understanding you, I should worry about my LP gas water heater backdrafting when the bath fan is running (it hasn't as yet).

    So, do I run the fan for 20 minutes every few hours? Only after showers? What is the answer here that will help with my goal of improving IAQ but not backdrafting my water heater?

    1. MattJF | | #12

      What did you get for a heat load calc?

      How big is the house and where are you located?

      Minotair is a very expensive ERV, with relatively low heat output. You only get 5700btu at 17deg. Sanden is also very expensive and I seem to recall it not being efficient with the high return temps in space heating.

      I would consider a ducted air source heat pump installed in place of your furnace.

      Spend the rest of your money and efforts on sealing the place up. After that is done, get an ERV like a Panasonic whisper quiet ($950) or an S&P ev90.

  5. Expert Member
    NICK KEENAN | | #5

    I just don't see a 110 CFM fan making any difference in a 15.7 ACH home. Either way. It's not going to create measurable negative pressure, and it's not going to increase the overall ventilation of the house. How the wind is blowing is going to be a much bigger factor.

    The purpose of the bath fan is to clear humidity when the shower is running and to clear objectionable odors in the bathroom by replacing the air in the bathroom with air from the rest of the house. Most are marginal at best at their purpose.

    You don't need mechanical ventilation until you get below 5 ACH. Have you tested for indoor air pollutants or do you just assume they are there?

  6. walta100 | | #6

    I see no reason to add more ventilation until you air seal your house and retest at less than 2 ACH you have plenty of ventilation now. This will be a herculean task.

    The real mystery is how much negative pressure do you have and why do you have it. Is your ductwork in your located in the unconditioned attic or crawlspace? Leaky ductwork in your attic would depressurize your house.

    An interesting question would be what fuel does your current furnace use and there a better option today?


    1. Expert Member
      NICK KEENAN | | #7

      It's not at all clear that OP actually has negative pressure, she just thinks she might because she's running the bathroom exhaust fan.

      1. jenniferz5 | | #8

        According to the information on GBA, it seemed fairly easy to determine whether I had negative pressure in my home. With the bath fan off, I felt no air movement at the main door (my only test sight, as all windows are "shrink wrapped"). With the bath fan on, air was pulled into the house through this same door.

        Please do let me know if this is normal, or if it signifies negative pressure. Also, again, does it matter? No issues have popped up, but as I tighten the house, will I need to adjust my use of this fan as an exhaust-only ventilation strategy?

        1. Expert Member
          MALCOLM TAYLOR | | #10


          I think what DC was getting at was that every house with exhaust fans has to draw make-up air from somewhere. It takes the easiest path into the house, which in your situation was the open front door, but with the door closed might be all sorts of other air-leakage paths. That in itself isn't unusual or necessarily a problem.

          1. jenniferz5 | | #13

            This makes perfect sense. Thank you.

          2. Jon_R | | #18

            > make-up air ... takes the easiest path into the house

            This belief causes enough problems that it should be corrected. Air flow takes ALL available paths - but easier paths see more flow.

          3. Expert Member
            MALCOLM TAYLOR | | #21


            Yes of course. But in the context this discussion it isn't a useful distinction.

  7. walta100 | | #9

    Without knowing the size of the home we can only guess but and average size home with 15 ACH would indicate you have opening in your house about the size of a large window opening.

    The amount of negative pressure a 110 cmf fan could produce in a home with 15 ACH is unlikely to be detectable with the very best interments. How the wind is interacting with a house is likely to have much bigger effect on an open door than your bath fan.

    If you are going to chase down the leaks get 2 box fans and fit them to 2 windows blowing out then seal them in with blue tape. Light an incense stick, hold the stick near any possible leak watch the smoke with a bright LED flashlight. When the smoke pattern changes you have found a leak, mark it with some tape and fix the leak. There will be hundreds of small leak to patch.

    I am not trying to discourage you. I want you to understand professionals with decades of experience and the best equipment would need well over a hundred man hours to seal your home to less than 2 ACH. This has become an obsession for some of us I wish you luck with your hunt.


    1. jenniferz5 | | #14

      Yes, I am getting a bit obsessed with this myself, Walta! Once the weather is a bit more mild I will get the box fans in place and start sealing. Thank you.

      1. MattJF | | #15

        If you can get your hands on an IR camera, doing this work in the cold is a bit faster than probing with a smoke pen as you can scan a bit faster.

        Our library has a “library of things” section that includes an IR camera.

    2. Expert Member
      NICK KEENAN | | #16

      Just to toss some numbers around, let's say the leakage in the house is 20 square feet and the front door is another 20. At 110 CFM the air is going to be moving through those openings at 2.75 feet per minute. That's 0.03125 MPH. I don't believe that is perceptible.

    3. kenmarcou | | #17

      Walta, given the time you’re referring to for sealing up a house like that would you say that the best approach would be to use a combined air/water control layer on the outside? Fluid applied/stick on/ if structural sheathing is necessary, taped zip? Seems the exterior approach would cover it better.

      1. walta100 | | #19

        All the stuff you mentioned is great for new construction but almost never applies to old homes. Getting an old home below 2 ACH 50 is a herculean task in an old house I think your only option I would to pull the trim and caulk the gaps.


        1. kenmarcou | | #20

          Yeah getting that low is probably impossible. But if you’re going to strip the siding off and need to do a new roof and roof deck because things are leaking and it’s been 40 years anyway….like I am going to have to do….it seems as though, put taped zip on the roof, and coat the whole thing with fluid applied wrb and Bam, for the first time in 2 centuries, there’s a water and air control layer. Rather than the hundreds of man hours you mention trying to wild goose chase it on the inside.

  8. tommay | | #11

    As DC says, the poster's situation is a minor and possibly insignificant. I like to leave the bathroom door open and let the heat and humidity stay in my house in the winter and open the window the rest of year. But the indoor and out door pressure is a good topic and is rarely discussed. What can cause these differences and what happens when they are unequal can greatly contribute to heat losses and heat gains. Higher indoor pressures in summer keeps outdoor air from coming in and forces indoor air out in winter. Equal pressure minimizes flow. Temperatures are directly related to pressure and work in the same way.
    So using a lot of these new systems with fans and the like, or using temperature swings, kinda messes with the natural balance. Just some food for thought.

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