Negative pressure from HVAC causing washer drain odors
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  1. #1
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    Jan 2014
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    Negative pressure from HVAC causing washer drain odors

    Hi,

    I live in a condo in the city where the HVAC, water heater, and washer/dryer are all in the same small enclosed room. The return register for the HVAC is built into the door for this room and there is a makeup air handler vent inside of the room.

    I've been noticing on occasion that when my condo is cold and I first begin to heat it, there is a musty smell to the air being expelled from the vents. This is more noticeable when the HVAC room door is closed. I've traced this down to the washer drain by closing myself inside of the HVAC room. When I do that I immediately begin to notice a musty smell and detect air flowing from out of the washer drain. This happens because the negative pressure created in that room when the HVAC is on with the door closed is great enough to draw air from the drain. When the condo is cold, the cold air inside of that room creates even greater negative pressure which would explain why I notice it more when my condo is first heating up.

    My condo is fairly new, so I'm sure there is a ptrap in the standpipe; however, due to the immense negative pressure in that room I believe that it's possible that air could be being drawn from the standpipe itself, or the surrounding area behind the wall. I sealed everything completely with duct tape as a test and the odor inside of the room is barely noticeable, and the musty smell in the condo itself nonexistent, so I'm fairly confident I've identified the cause.

    The problem is - how do I solve it? I obviously can't keep the drain sealed up permanently, and while I could seal the area around the drain, my understanding is that the drain requires air to enter in order to more efficiently drain the washing machine. There's also no guarantee that the odor is coming from the drain itself, as it could potentially be coming from the surrounding area behind the wall.

    Could anyone suggest some next steps I could take to remedy this issue?

    Thanks

  2. #2
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    Aug 2007
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    Northern VA 38 degrees N by 76 degrees W
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    Quote Originally Posted by Chymerix View Post
    Hi,

    I live in a condo in the city where the HVAC, water heater, and washer/dryer are all in the same small enclosed room. The return register for the HVAC is built into the door for this room and there is a makeup air handler vent inside of the room.

    I've been noticing on occasion that when my condo is cold and I first begin to heat it, there is a musty smell to the air being expelled from the vents. This is more noticeable when the HVAC room door is closed. I've traced this down to the washer drain by closing myself inside of the HVAC room. When I do that I immediately begin to notice a musty smell and detect air flowing from out of the washer drain. This happens because the negative pressure created in that room when the HVAC is on with the door closed is great enough to draw air from the drain. When the condo is cold, the cold air inside of that room creates even greater negative pressure which would explain why I notice it more when my condo is first heating up.

    My condo is fairly new, so I'm sure there is a ptrap in the standpipe; however, due to the immense negative pressure in that room I believe that it's possible that air could be being drawn from the standpipe itself, or the surrounding area behind the wall. I sealed everything completely with duct tape as a test and the odor inside of the room is barely noticeable, and the musty smell in the condo itself nonexistent, so I'm fairly confident I've identified the cause.

    The problem is - how do I solve it? I obviously can't keep the drain sealed up permanently, and while I could seal the area around the drain, my understanding is that the drain requires air to enter in order to more efficiently drain the washing machine. There's also no guarantee that the odor is coming from the drain itself, as it could potentially be coming from the surrounding area behind the wall.

    Could anyone suggest some next steps I could take to remedy this issue?

    Thanks
    The chances of the odder being produced from the washing machine drain line if properly installed is probably not possible. The drain should have a two inch trap with a 2 inch water column to prevent what you are experiencing, sewer gasses entering the structure.

    The chances of the air handler producing high static much above 0.5 inches of water column is slim to none.

    This can easily be determined by inserting a camera probe down the washing machine stand pipe to determine the trap assembly and water retention.
    Any plumbing or HVAC contractors technician should have this tool.

    Other thoughts. Is the trap loosing prime because the plumbing vent is improperly installed causing the trap to siphon?
    Is it the residual from inside the washing machine?

    The static across the door can be tested and size increased to eliminate problem. This is probably to small irregardless of the other scenario.

  3. #3
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    Jan 2014
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    Quote Originally Posted by second opinion View Post
    The chances of the odder being produced from the washing machine drain line if properly installed is probably not possible. The drain should have a two inch trap with a 2 inch water column to prevent what you are experiencing, sewer gasses entering the structure.

    The chances of the air handler producing high static much above 0.5 inches of water column is slim to none.

    This can easily be determined by inserting a camera probe down the washing machine stand pipe to determine the trap assembly and water retention.
    Any plumbing or HVAC contractors technician should have this tool.

    Other thoughts. Is the trap loosing prime because the plumbing vent is improperly installed causing the trap to siphon?
    Is it the residual from inside the washing machine?

    The static across the door can be tested and size increased to eliminate problem. This is probably to small irregardless of the other scenario.
    Thanks for the reply. The gasses I'm smelling are more moldy and musty than sewage-like - I wonder if that's indicative of anything. What do you think the likelihood is of the odor coming from behind the wall itself and not from the drain pipe at all?

    Regarding the return register on the door - are you suggesting that it's likely too small regardless of the problem I'm experiencing?

  4. #4
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    The remnants of what is in the washing machine is highly likely. Run empty cycle with hot water and bleach let set one hour and flush then a second cycle with hot water and vinegar, let set one hour and flush.

    As far as the door is concerned 99% are too small.

  5. #5
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    Quote Originally Posted by second opinion View Post
    The remnants of what is in the washing machine is highly likely. Run empty cycle with hot water and bleach let set one hour and flush then a second cycle with hot water and vinegar, let set one hour and flush.

    As far as the door is concerned 99% are too small.
    I just removed the washing machine flex tube from the standpipe to take a closer look, and it was absolutely covered in filth and mold with a smell to match. So it seems you may be right. I removed all grime with nature's miracle odor remover, sprayed some in the standpipe, and put the flex tube back in. The odor is already reduced but still somewhat present. The one thing that I can't figure out is how the odor would be travelling if the flex tube is immersed in the water of the ptrap. But I tried stuffing a trash bag into the standpipe and the odor was gone, so it does seem like it's coming from that pipe. Is there any way air could be travelling through a portion of that standpipe to carry the odor?

    I will follow your suggestion on running the washer with bleach and vinegar - one question, though - what do you mean by "flush"? Run the cycle just water?

    As for the door register - is there an easy way for me to measure the static pressure in order to determine if it's too small? I don't mind spending money on the right tool.

    Thanks for all your help!

  6. #6
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    Quote Originally Posted by Chymerix View Post
    I just removed the washing machine flex tube from the standpipe to take a closer look, and it was absolutely covered in filth and mold with a smell to match. So it seems you may be right. I removed all grime with nature's miracle odor remover, sprayed some in the standpipe, and put the flex tube back in. The odor is already reduced but still somewhat present. The one thing that I can't figure out is how the odor would be travelling if the flex tube is immersed in the water of the ptrap. But I tried stuffing a trash bag into the standpipe and the odor was gone, so it does seem like it's coming from that pipe. Is there any way air could be travelling through a portion of that standpipe to carry the odor?

    I will follow your suggestion on running the washer with bleach and vinegar - one question, though - what do you mean by "flush"? Run the cycle just water?

    As for the door register - is there an easy way for me to measure the static pressure in order to determine if it's too small? I don't mind spending money on the right tool.

    Thanks for all your help!
    The J tube off of the washing machine should be inserted into a 2 inch stand pipe that should be between 18 to 42 inches, (usually around 30 inches to the trap.

    Start the wash cycle on small load and ad 2 cups of bleach leave lid up, this will stop cycle after agitation. let set for one hour. close lid and let cycle finish. Repeat with vinegar.

    For the door turn unit on with door closed. turn knob on door gently and determine if door opens on its own.

  7. #7
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    Quote Originally Posted by second opinion View Post
    The J tube off of the washing machine should be inserted into a 2 inch stand pipe that should be between 18 to 42 inches, (usually around 30 inches to the trap.

    Start the wash cycle on small load and ad 2 cups of bleach leave lid up, this will stop cycle after agitation. let set for one hour. close lid and let cycle finish. Repeat with vinegar.

    For the door turn unit on with door closed. turn knob on door gently and determine if door opens on its own.
    Thanks for the clarification. This is proving to be a very informative thread - I appreciate it. I cleaned the washing machine with bleach so far and I also removed the return register on the door completely and the smell is totally gone. However, even with the register completely removed, the door cannot overcome the pressure and open on it's own. With the register in place, the door usually slams itself shut, so it's definitely looking like it's too restrictive... even without a register at all.

    What's the least restrictive register I can buy? I suppose I could start there and then take some static pressure measurements to determine if I can get by without enlarging the return hole on the door. It's a 20 x 24 inch hole. I was thinking of purchasing the Fieldpiece dual-port manometer SDMN5 for taking static pressure measurements - would you recommend this tool or do you think it's overkill?

    Thanks again for the help.

  8. #8
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    Lots of devices sucking out of a small room. Sounds like when the door is closed you are pulling a pretty good vacuum on that room for it to be pulling air through your plumbing trap. Sounds like Architect didn't understand/consider airflow in design.

    The door register is probably way too small. May need a door with slats, and may even need to remove some slats.

    Since sewer gasses can be flammable this is not just an indoor air quality problem, it's a fire risk.

    Do you have other than sealed combustion appliances? (particularly furnace and water heater) If so this also becomes a CO risk from back drafting.

    Recommend leaving the door open until you get this deemed safe by someone qualified in pressure diagnostics.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  9. #9
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    Quote Originally Posted by tedkidd View Post
    Lots of devices sucking out of a small room. Sounds like when the door is closed you are pulling a pretty good vacuum on that room for it to be pulling air through your plumbing trap. Sounds like Architect didn't understand/consider airflow in design.

    The door register is probably way too small. May need a door with slats, and may even need to remove some slats.

    Since sewer gasses can be flammable this is not just an indoor air quality problem, it's a fire risk.

    Do you have other than sealed combustion appliances? (particularly furnace and water heater) If so this also becomes a CO risk from back drafting.

    Recommend leaving the door open until you get this deemed safe by someone qualified in pressure diagnostics.
    I don't think it's sewer gasses, as cleaning the mold and build-up off of the laundry drain and running the wash with bleach removed all odors (which never smelled like sewage, only smelled like mold); however, it does seem like the door register is too small. I've had the the company responsible for servicing my HVAC in and out a number of times for an unrelated issue and they never commented on the register being too small, so I'm not sure what they'll say if I suggest to them that it is. Having some concrete pressure numbers to approach them with might be a better approach. The technician I've worked with has been really nice and knowledgeable, so I'm bit surprised he didn't bring it up if it's such a big issue.

    For now I'm running things with the register removed from the door. If there's a good instructional thread available for measuring pressure drop across the air return as well as some information about acceptable ranges, that would be helpful for me to take a look at. I'll do some digging on these forums as well but if anyone has a link that might help please let me know.

    Thanks

  10. #10
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    Jan 2014
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    27
    Without the register I'm measuring a pressure drop across the return of around .02 inWC and with the register .07 inWC. Do these measurements seem acceptable?

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