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  1. #1
    Join Date
    Dec 2019
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    Exhaust/Intake on high efficiency furnace causing humidity issues?

    We discovered mold in our house and it seems to keep coming back. We have a mold remediator who said he suspects an issue with the furnace. Itís December in northeast Ohio and the humidity in the house is 40* and higher with the furnace running. We donít have a humidifier. When I open all the windows, I can reduce it to around 33* but shut the windows and it spikes right back up. We found mold on the A coil (AC quit working in September) and on the register closest to the furnace. A LOT of mold. Looking at the intake and exhausts for the furnace, I noticed they ran side by side until just before the basement wall where they are joined together and they exit the house maybe max 24Ē above grade. Right behind the outside AC unit. The intake and exhaust both vent into a single pipe through the basement wall which is a straight short pipe with a small amount of standing water on the bottom of the pipe. I plan to call our HVAC guy Monday but should I even be running the furnace like this? Obviously itís been like this for a while so I guess whatís a few more days, but I worry about the exhaust getting drawn right back into the house. Weíre already experiencing a lot of health issues that have just continued to multiply and weíve only lived here 5 years. Iíve been opening all the windows several hours a day but of course that just makes the furnace run more even though at least the air seems fresher. Ugh! Is this even an acceptable thing to have the intake and exhaust run together like that? Should I continue to run the furnace or pay extra to have someone come this weekend?

  2. #2
    Join Date
    Jan 2004
    Location
    PA
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    78,336
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    Its called a concentric vent. Its not a problem, and its very common.

    Sounds like you have a tightly constructed home. But they may have failed to install a fresh air system for the home.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    Aug 2002
    Location
    Southold, NY
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    25,663
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    Quote Originally Posted by Christina Baad View Post
    We discovered mold in our house and it seems to keep coming back. We have a mold remediator who said he suspects an issue with the furnace. It’s December in northeast Ohio and the humidity in the house is 40* and higher with the furnace running. We don’t have a humidifier. When I open all the windows, I can reduce it to around 33* but shut the windows and it spikes right back up. We found mold on the A coil (AC quit working in September) and on the register closest to the furnace. A LOT of mold. Looking at the intake and exhausts for the furnace, I noticed they ran side by side until just before the basement wall where they are joined together and they exit the house maybe max 24” above grade. Right behind the outside AC unit. The intake and exhaust both vent into a single pipe through the basement wall which is a straight short pipe with a small amount of standing water on the bottom of the pipe. I plan to call our HVAC guy Monday but should I even be running the furnace like this? Obviously it’s been like this for a while so I guess what’s a few more days, but I worry about the exhaust getting drawn right back into the house. We’re already experiencing a lot of health issues that have just continued to multiply and we’ve only lived here 5 years. I’ve been opening all the windows several hours a day but of course that just makes the furnace run more even though at least the air seems fresher. Ugh! Is this even an acceptable thing to have the intake and exhaust run together like that? Should I continue to run the furnace or pay extra to have someone come this weekend?
    Agreed with Beenthere, do you have any way of bringing in outside air? That's one tight house!

  4. #4
    Join Date
    Dec 2019
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    Thread Starter
    What can we do (or have someone do in this case) to improve air circulation? Could the concentric vent be restricting air flow to the furnace somehow? Oh, it also vents right into the back of the outside ac unit. Not sure if thatís normal or if thatís a potential problem either. There are 3 bedrooms, a large kitchen, and a living room plus 2 bathrooms and there are a total of 4 cold air return runs for the upstairs and none downstairs. Two of the bedrooms actually share one cold air return by having a hole cut straight through the shared wall. We wondered if adding returns might help with circulation?

  5. #5
    Join Date
    Dec 2019
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    9
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    Thread Starter
    The crazy thing is the windows are horribly leaky but the air in here is so stale and stagnant in the winter as soon as we close up the house. I canít figure out how so much moisture got into the A coil either when the AC hasnít run in over 3 months.

  6. #6
    Join Date
    Aug 2002
    Location
    Southold, NY
    Posts
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    You need a way of bringing in outside air. It is recommended a complete air change every 6 - 8 hours too purge the indoor air of pollutants.

    @teddybear is the man to talk to. Hell be along shortly!

  7. #7
    Join Date
    Nov 2008
    Location
    Piney Flats, Tn.
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    the exhaust and intake are a closed loop and can not possibly put humidity in to your house. So what the intake and exhaust do is, the intake sucks air in from outside so the furnace can burn the gas then that same air which is now exhaust goes back outside. So the air in the pipes never touch any air in your house. Now with that said the exhaust blowing into the coils of the ac is not good.

    So your furnace IS NOT putting any humidity in to your house UNLESS the drain is stopped up and draining in to the house. This will be apparent by water being around the furnace.

  8. #8
    Join Date
    Sep 2008
    Location
    VA
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    The humidity in the house is 40% higher? So, you're saying it's around 73% RH with the furnace running? That information doesn't sound accurate.
    Are you discharging your flue gases directly into the home? How exactly did your contractor explain the furnace is raising the RH this high? Or did he just say it's the hvac system? The two pipe concentric kit actually reduces the potential risk of drawing air into your home.
    "The only true wisdom is in knowing you know nothing" Socrates

  9. #9
    Join Date
    Dec 2019
    Posts
    9
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    Thread Starter
    Not 40% higher, it jumps from 33% with the windows open to 40% RH (or higher) with the windows closed and the furnace running. So we assumed the problem was the furnace somehow adding moisture to the air but now I think itís actually more likely that there isnít enough fresh air coming into the house to reduce the humidity in the house. With all the windows open, 33% even seems high for winter air to me with no humidifier running. With our mold issue, we really want the humidity down around 30% and definitely donít want it up around 40-50% but we canít find the source of the moisture either.

  10. #10
    Join Date
    Aug 2002
    Location
    Southold, NY
    Posts
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    No mold issues below 50% RH

  11. #11
    Join Date
    Sep 2008
    Location
    VA
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    Quote Originally Posted by Christina Baad View Post
    Not 40% higher, it jumps from 33% with the windows open to 40% RH (or higher) with the windows closed and the furnace running. So we assumed the problem was the furnace somehow adding moisture to the air but now I think it’s actually more likely that there isn’t enough fresh air coming into the house to reduce the humidity in the house. With all the windows open, 33% even seems high for winter air to me with no humidifier running. With our mold issue, we really want the humidity down around 30% and definitely don’t want it up around 40-50% but we can’t find the source of the moisture either.
    The furnace will actually help to lower the RH% because of air expansion with an increase in temperature. You have moisture coming in from elsewhere. Keeping the windows open with a lower outdoor RH% will reduce indoor RH by dilution. Finding the source can be difficult. Thermal imaging cameras and blower door testing can sometimes help. Adding a fresh air system will improve the issue if it's a very tight home, but if the humidity is due to a primary source of water, the fresh air will be a Band-Aid. You need to have a legitimate building analyst and not a bug company come out to perform a real inspection. Building performance analysts have proper tools to help diagnose these issues, not just throw out random claims.
    "The only true wisdom is in knowing you know nothing" Socrates

  12. #12
    Join Date
    Dec 2019
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    9
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    Thread Starter
    Just had another contractor come out this morning. He tested the RH through the house. At the furnace, RH was 80%, in the part of the basement we finished, 65%, upstairs where the humidistat read 40%, his meter read 60%, and around the corner in the bathroom where we had the mold explosion, it was 70%. So we have a SERIOUS moisture problem and a lack of fresh air/ lack of circulation problem.

  13. #13
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    40,515
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    Sounds like you need one of Teddy Bear's ventilating dehumidifiers. Get the basement dried out and ventilated and the whole house will be comfortable.

    https://www.ultra-aire.com/

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