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Community and Q&A

Balancing HRV– custom design

fedge | Posted in GBA Pro Help on

So I built in my own HRV and retrofitted it into the house.

I notice most of the literature says I need to sample the mean air speed in a straight section of duct not near a bend or a change in air speed (silencer). I did a combo of the Zehnder tubes and a Fantech HRV. So I have done a few custom things that do not facilitate balancing the system in a standard way.

The inlet or feed into the house goes into a section of straight pipe, which would work… but my exhaust comes from the Zehnder 3.5″ tubes to the silencer to a 8 to 10″ section of flex duct(prevent vibration transfer). I used steel ducts everywhere I could until it came to the last 6″ to 8″ sections that connected to the HRV unit. I decided the vibration would compromise the exhaust and inlet ductwork sealing I had done so I had to isolate it completely from the machine.

The way the basement layout and retro fit are this is the most efficient path (shortest distance with minimal bends) to all the supply areas…unfortunately I don’t have a optimal spot to check the air pressure on the exhaust.

Can I balance with a device on the exhaust and inlet ports on the outside?

I have attached a crude drawing of what i am dealing with. I could move the silencer but it would take up a huge amount of space to do a straight duct there, take up more length in some of the exhuast runs, force me to move a huge shelf (again)–to may reasons not to do that just to be able to balance this thing. there must be an acceptable method to get a decent balance to my system.

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  1. GBA Editor
    Martin Holladay | | #1

    Your post is a bit confusing, but I think I'm close to understanding it now.

    When you explained, "I built in my own HRV," I misread it at first -- I thought that you were saying that you built your own HRV. Then it became clear that you installed a Fantech HRV, not your own homemade unit.

    To balance your HRV, you need to be able to measure airflow rates. There are many, many ways to measure airflow rates. Before we can give you advice, we need to know what type of airflow measuring equipment you own or are using to measure these airflow rates.

    Here is a link to an article that may give you some ideas about the many ways of measuring airflow rates: Is Your Ventilation System Working?

  2. fedge | | #2

    I am going to get an anemometer (rotating vane).

    In my case I think the best thing to do (not easiest) is to take off the outside inlet and outlet covers and take readings there with that device. I "could" do one manometer in the feed to the house on the inside to verify but with that 10% difference I think getting close should be okay.

    My only concern is if it is waaay off.

    When i said built my own HRV system I just meant I frankenstiened together a system that ended up being a lot of extra work to do--still saved me thousands of dollars.

  3. GBA Editor
    Martin Holladay | | #3

    Your approach will allow you to balance (within the accuracy limitations of your chosen equipment) the supply air flow and the exhaust air flow. However, it will not allow you to verify the air flow to the different supply registers, to see that each room is getting the design air flow. For that, you need a small flow hood (see photo below).

    That said, you may not need the level of accuracy that a flow hood provides.


  4. charlie_sullivan | | #4

    Jon's balancing approach works well only if there is no condensation happening in the HRV core. You can check by measuring the humidity of the exhaust exiting the HRV and making sure it is well below 100% humidity.

    Another good check is the use a manometer to measure the indoor/outdoor pressure difference, and make sure it doesn't change by more than a Pa or so when you turn on the unit. That requires a manometer, but if you are having a blower door test done, you can do that check while you have the

  5. Jon_R | | #5

    I would measure the 4 temperatures on a cool day to balance for equal mass flow.

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