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Is my ERV setup operating correctly?

Bruce_Davis | Posted in Green Building Techniques on

New construction, cold edge of Zone 4.  I had the guy come by today for the HERS evaluation.  He tested the ERV.  It is only moving 13 cfm.

All medium to high end Carrier HVAC.  Two systems, each servicing about 2000 sq ft. Both furnaces were set “Fan: On”.  Air was continuously moving with no heat or cooling.  He tested at the intake opening because the shutters on the exhaust opening wouldn’t even lift open on their own.

The “Fresh Air to Building” port tees to both vertical return ducts about 6 ft off the ground.  The “Exhaust from Building” port tees to both horizontal return ducts about 10 ft off the ground.  This seems to be correct if I understand the install instructions.  The chart in the instructions indicate I need about 80-90 cfm.  Carrier specs the ERVXXSHB1100 at up to 100 cfm.

The instructions mention a wall control panel, but I don’t have one.  The Cor stat instructions say the ventilation is setup at stat install.  I don’t appear to have access to that to check and see if ERV is setup in stat.  There is an air recirc setting for minutes per hour I can control.  Should I set that up?

What concerns are there?  What action, if any, should be undertaken?  Thanks.


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  1. GBA Editor
    Martin Holladay | | #1

    Something is obviously wrong. You are to be commended for contacting someone capable of measuring air flows so that your ERV can be commissioned. You've learned that your system isn't working.

    It's hard to diagnose what's going on here -- there are several possibilities. I look forward to hearing from other GBA readers with suggestions.

    I'm going to take this opportunity to take a step back and note that your method of ducting your ERV is a method that I've condemned for several years. In my article on this topic, "Ducting HRVs and ERVs," my succinct advice to readers tempted to use this so-called "simplified" system of ducting was, "Don't do it."

    Ideally, every ERV or HRV will have dedicated ventilation ductwork, not shared ductwork, for the reasons explained in my article.

  2. Expert Member
    Akos | | #2


    Turing on the HVAC fan will not turn on the ERV in most standard installs.

    The flow your tester was measuring is the leakage through the ERV caused by the furnace running (which is one of many problems as Martin mentioned this type of setup).

    Somewhere there should be a box that looks like a humidistat, that is for your ERV. If the ERV was installed properly, cranking the humidity set point down (lower RH) should turn on your ERV which should trigger your furnace fan to also turn on.

  3. Bruce_Davis | | #3

    Yes, I had read Martin's article way back when. Adding a third ducting system would have already made a tight utility space tighter and more expensive. Like many other green things, I gave it a try but was met with resistance.

    My installer just responded to this issue with:

    "No need for controller on this style ERV. It made to run 24/7, they use the manual for all the erv’s and they need to remove the part about optional controller for this one. I am out of town next week but as soon as I return, I will come by and see what we can do to fix the problem. "

    Not sure I understand, but guess we'll see. Thanks for the responses.

  4. Expert Member
    Akos | | #4


    With your install, the ERV and your furnace fan need to be interlocked (if the ERV is running the furnace fan needs to also run).

    The one thing you don't want is for the ERV to be running without the furnace to distribute the air, this will not work as the ERV will just cycle the same air. It might even cause the core to freeze up in the winter.

    Once this is fixed (interlocking), you don't want your ERV to run 24/7. Running your furnace blower all the time will use up a fair bit of power a waste of energy.

    What you want is for the tech to set up a controller for your ERV that will run X min/hour (usually 10 or 20 minutes) with option to run it longer if the humidity is high. The number of minutes depends on the size of your ERV and the ventilation needs of your house.

    While he is there, there should be a boost switch/timer installed somewhere. This should crank your ERV to max for an hour or two. I find this very useful when cooking smelly food or wet dog in the house.

  5. Trevor_Lambert | | #5

    Some facts seem to be in opposition. If you want to run the ERV on a 17% or 33% duty cycle, then the amount of fresh air needs to deliver in that time is 3 to 6 times the continuous rate. If he needs 80cfm, that means the ERV has to be able to deliver 240-480cfm. The furnace fan will certainly help to boost the capabilities of the ERV, but it's not going to triple or sextuple it. I'm guessing this gets to the heart of why this setup is frowned upon.

  6. Trevor_Lambert | | #6

    Having looked at Martin's article, if the picture of the simple system is representative of what you have, there's two likely possibilities. Assuming the furnace fans are running, and there is airflow at those registers, and the airflow of 13cfm is being measured at the outdoor intake of the ERV:
    -either the ERV is not running at the correct speed, or
    -you have a pressure differential between the fresh and exhaust ports that the ERV is unable to overcome. Maybe it's not connected properly, maybe it was designed poorly, maybe there's a blockage?

  7. Expert Member
    Akos | | #7


    The fan run time/ERV capacity/ventilation needs to match. If the ERV is small and sized for continuous operation, than the furnace fan also needs to run continuously. Even with an ECM fan the electricity costs for running the furnace fan 24/7 can add up quickly though.

    Like you said, one of the many reasons why this is not a great setup.

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