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Community and Q&A

Dampers in fresh air exchangers?

Susan91770 | Posted in General Questions on

I have been researching negative pressure effects in houses and wondering if this could be the cause of intermittent sewer fume smells.  I have an ERV / HRV fresh air exchanger (not sure what it’s called but it has an intake and an exhaust between it and the side of the house).

Doing a bit of research on fresh air exchangers, I’ve read that both the intake supply port and the outgoing exhaust port are “supposed” to have dampers on them, and that not having dampers on both could introduce problems with negative pressure.  I found Martin stating the same thing in his post on the following thread regarding balancing an air exchanger…

The fresh air exchanger in my house has a damper on the intake / supply port but, NOTHING on the exhaust port (just an open hole).   When I put my hand in front of the exhaust port, I can feel hot air and humidity blowing in during summer months, and cold dry air blowing in during winter months.    The air exchanger is then connected to the supply side of the hvac unit, but it is manual on/off only, no idea why the installers didn’t hook it up to turn on/off with the hvac unit.    The air from the exhaust port seems to get passively drawn into the hvac system and circulated through the house.

I have experimented for the past several years with blocking the exhaust port from the inside using various materials such as taping a piece of cardboard in front of the exhaust port, or even just stuffing a towel into the port.    Doing this has the effect of lowering indoor humidity during summer months (Iowa gets quite hot and humid), as well as putting a stop to the intermittent sewer fume / methane (confirmed with a methane detector) smells that we’ve had since having the air exchanger installed in 2016 and thus, fewer issues with headaches, dizziness, nausea, blurred vision, edema, tremors, etc. when in the house.  The sewer lines have been smoke tested and checked out fine,  the sewage ejector pit has been inspected and found no cracks / leaks but a dried out lid gasket was replaced and an extra vent to the roof was added.

So, I’m wondering, could an exhaust port that doesn’t have  damper on it, create enough negative pressure, to draw in sewer fumes from somewhere ?  And if air exchangers are supposed to have these dampers on both ports, then why would a manufacturer make a unit that has only one damper?  Is there a purpose for a unit like that ?

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  1. GBA Editor
    Martin Holladay | | #1

    The presence or absence of a damper on the exhaust duct of your HRV doesn't tell you much. What you need to know are the answers to these questions:

    1. Is my HRV system balanced? (That is, is the supply air flow rate in cfm equal to the exhaust air flow rate in cfm?)

    2. Are these flow rates appropriate, excessive, or insufficient for my house?

    3. Does my system have adequate controls, so that the owner can adjust air flows if desired?

    If you look up the installation manual (the manufacturer's installation instructions) for your HRV -- these are available online -- you'll discover that there is a commissioning procedure. The installer is supposed to measure air flows and confirm that these air flows are balanced after the unit is installed. Many installers skip these steps, or perform them poorly.

    If you have any doubts that your system was properly commissioned, you need to have your air flow rates verified. Here is a link to an article that describes how air flow rates are measured: "Is Your Ventilation System Working?"

    You need to hire a home performance contractor or HVAC contractor who knows how to measure air flows. If you interview this person on the phone, ask that person what tools they use to measure air flow rates. Their answer will be illuminating.

    One final point: Systems like yours -- ones without dedicated ventilation ductwork -- are hard to balance, because the HRV fan has to compete with the air handler fan (or furnace fan) during operation. For more on this issue, see "Ducting HRVs and ERVs."

  2. Susan91770 | | #2

    Hi Martin,

    Thanks so much for the feedback.

    The questions that you posed, are the same questions that I've asked myself... and don't have the answers to. Plus, as you mentioned, I would be willing to bet that the contractor who installed the system, likely did not balance it or did not know how to balance it. We've had issues with other systems that the same contractor installed, which is why we no longer work with him.

    I have read a handful of articles that talk about how to measure and balance the air flows on a fresh air exchanger and the equipment (manometer?) needed to do the job. However, I will admit that, while I am mechanically included, this job is above my head.

    I love your choice of the word "illuminating" when referring to the answers that different contractors might give for various questions. I ran into some illuminating responses a couple years ago when I called around to different contractors with questions about our geothermal hvac system that had failed. That situation ended with the decision to gut the system and go back to a traditional natural gas furnace. Seems a lot of contractors know how to install geo systems (correctly?) but, very few know how to maintain or repair them.

    Anyway, your "final point" that HRV systems without dedicated ductwork are hard to balance makes me think that I probably won't waste my energy trying to get this issue solved.

    Thank you for your thoughtful response and suggestions.

  3. Susan91770 | | #3

    Hi Again Martin,

    I just ran across the following article re: HRV's. This is the exact situation that I have happening in my home.

    Link to article --

    I have located the manual for my specific unit and see that there should be a backdraft damper on the exhaust port side . (see image attached). I have inspected the exhaust port and found that no damper was ever installed. This would explain the decreased symptoms (which I mentioned earlier) and decreased indoor humidity during the summers when I have physically blocked the exhaust port with a towel or a piece of cardboard (with the HRV shut off, obviously).

    Note, under "Key points" --- "It is mandatory (to eliminate recirculation) that either the furnace blower run continuously or the HRV/ERV unit operation be interlocked with the furnace blower". (Interlocked - meaning that the HRV comes on with the the furnace / Air conditioning? The HRV was installed as a manual on/off operation.)

    Since the day of installation, these two units have never been interlocked, nor have we run the furnace blower continuously.

    I will be installing a backdraft damper soon.

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