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Broan balanced HRV reduced flow & noise issues.

finePNW | Posted in General Questions on

UPDATE (3/7/24): The grinding was caused by a small piece of loose fiberglass insulation caught in the return blower. Then, adding a few feet of 6″ insulated flex duct (not pulled terribly tightly… may fix this later) to each line at the HRV connection miraculously dropped my noise at the registers to inaudible levels (except when you put your face right next to it… which is not a common location for me, hah!). While doing this, I disconnected all lines, one by one, and it turns out my max flow is ~ 147 CFM per the installation CRM range internal auto-test *with no ducting attached*. I would imagine I should get the full ~161 CFM with nothing attached, so I’ve reached out to Broan. (With all connections attached I get 131CFM max.  Minimum with ducting attached is 65 CFM, which also seems high to me.)

Original Post:
I recently had a Broan B160H75RT auto-balancing HRV installed ( It is rated to 160 CFM, but during its auto-calibration, maxes out at ~ 129 CFM (20% low). It is also quite noisy when > 80 CFM — loud whooshing with some vibration in supply registers at the other end of the house, and loud grinding-sounding noise at the closest-to-unit return/exhaust register primary outdoor exhaust. I will have my HVAC guy out this week, but I’d like to have some notes for him when he gets here, since HRVs do not seem to be his normal work.

Some info about the system:
• Ducts are all hard pipe… I assume this adds to the noise I hear, at least the grinding?
• All connections at HRV itself are 6″ ducts, as are fresh air intake and outdoor exhaust runs. There is some trunking, splitting, and use of 5″ and maybe even 4″ ducting as the system feeds the 6 return registers and 4 return registers.
• Have not measured the duct lengths, but this services a 2300 sqft home, all 1 story, if that helps. HRV is located squarely in one wing of the house (and not in the center of house)
• Supply registers: 6 @ 2.5″ x 10″
• Return (exhaust) registers: 3 @ 2.5″ * 10″, 1 @ 4″ round
• No Manual D performed, to my knowledge.
• Fresh air intake has a sharply angled wall cap on it. (like this:
• Outdoor exhaust has a more gradually angled / more open wall cap on it (like this:
• When in operation, the deflection of the exhaust cap’s damper is almost identical to the more-closed angle of the intake wall cap, which makes me think it’s this cap that’s causing issues with flow.

Things I suspect / want to check:
• Remove HRV from one connection at a time (fresh intake, exhaust, return, supply) and re-check max flow.
• Add 12″ straight lengths of of 6″ insulated flex duct to each HRV connection to isolate vibration from rest of solid ducting… or do I need to make a 90 degree in the flex to better trap noise?
• Switch the less-open fresh air intake wall cap with the more-open type used in the exhaust wall cap

I would love any advice or input from folks who may have performed similar troubleshooting in the past. Thanks!

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  1. Expert Member
    Akos | | #1

    As for the girding, I'm guessing some debris made it into the ducting and got stuck in one of the blowers.

    1. finePNW | | #3

      Oh I hope so! Good call. I’ll check it out.

    2. finePNW | | #9

      Unfortunately (or maybe fortunately?), I just opened up the HRV, pulled out the core, and did some cleaning, and there was a small piece of fiberglass insulation in the exhaust! The house is too much abuzz with contractors finishing up to tell, but I plan to see if it affected the "grinding" noise later this week. Appreciate the lead!

      I suppose I should have my ducts cleaned post construction after all, despite covering vents with filters.

  2. user_8675309 | | #2

    When I installed my HRV I used 6" hard duct everywhere(except for a few tight bends where flex duct was used). During operation it was louder than I had hoped for. I had the ability to add about 18" of flex duct(pulled tight) on both fresh and stale air runs about 4 feet from the HRV. It was noticeably quieter after that.

    1. finePNW | | #4

      That’s great to hear. Did you use a straight run of tightly pulled flex or did you make any S-shapes or other bends to reduce noise transmission?

      1. user_8675309 | | #8

        Just straight.

        1. finePNW | | #10

          Beautiful picture. Thanks so much!

    2. finePNW | | #16

      Well, turns out it was the metal ducting. I added a few feet of insulated flex duct to the *beginning* of each line (at the HRV) and now it's virtually silent at the registers. Amazing. Thanks for encouraging me to move forward on that path!

  3. DennisWood | | #5

    160 down to 129 CFM sounds reasonable given the ducting you have there. You likely will not see much or any benefit to messing with the 6" vent hoods at these flow rates.

    If noise is important, then look at using a duct muffler on the supply side along with short lengths of flex to decouple the unit from your hard ducting. The combination will be much more effective than just a short length of flex. Fantech sells duct silencers, or you can very easily make your own. The LD6 is their 6" version:

    1. finePNW | | #6

      According to the HRV's spec sheet, 129CFM suggests ~ 0.8-1.0" of static pressure in the system, which seems a bit high to me, but what do I really know? (not much, in this case). Does that jive with your initial thoughts on 160 -> 129CFM being reasonable with the ducting I described?

      With the noise coming most intensely from the closest-to-HRV return duct as well as the farthest-from-HRV supply register, I think I'd need silencers on both supply and return, unfortunately, and the budget is currently tight, but your point is well taken and the silencers really do seem to have solid testimonials. Thanks for taking the time to thoughtfully respond and for the specific link and table!

      Are you aware of any tests of the impact of flex duct lengths on HRV system noise? I have been unable to find much, but am thinking a 12"-18" flex duct with a slight, tight S connecting the HRV to the rigid duct lines could provide some noise reduction... maybe at the expense of flow... but unclear if it would even be noticeable.

  4. acrobaticnurse_Eli | | #7

    When I installed my Broan B210E75RS and did the initial commissioning where it calibrates and checks max flow rate I noticed that on the little screen above the controls it indicates whether the supply or exhaust side is more limiting. Running that calibration again could provide guidance as to which side may need work. In my case it said my exhaust side is more limiting and recommended a max flow rate of 195 cfm vs 210 cfm. I used 6 inch insulated flex duct for the ductwork connecting to the outside. Since I have extra 8 inch insulated flex duct I may switch the exhaust to 8 inches to see what difference that would make. Increasing from 195 to 210 cfm isn't really necessary, but if it leads to less wear on the ECM at lower speeds it may be worth  the minimal effort it will take me. I decided to start with flex duct to get myself to install it sooner than later, particularly with the low CFM required, and then switch to rigid if/as needed. 

    I have a single 8 inch fresh air supply going into my upstairs hallway between the three bedrooms and positioned near the upstairs HVAC return for when the heat pump is running. At 140 cfm if I'm standing near that fresh supply it sounds like my Austin Air HEPA filter on medium that I sleep next to, a similar volume level to the white noise machine in a doctor's office. At 70-100 cfm it's barely audible if you're standing near it, and in the two bathrooms air is pulled from it's only slightly audible at ~140 cfm. At ~190 cfm it's significantly louder both at the vents and in the attic by the ERV, and tapering the max speed down to 170 cfm helps. Even though it's just a few feet from my bedroom and it clearly improves air changes as CO2 levels quickly drop in the bedroom, the ERV is inaudible in the bedroom regardless of fan speed.

    1. finePNW | | #11

      This is really helpful. Fairly similar to my experience, so perhaps I should have over-sized my HRV even more than I did to allow for quieter / lower capacity operation at my desired 110 CFM flow rate. If that's the case, that's on me, and my next one in 15 years (or however long) will be over-over-sized... and probably use lasers or something, lol.

    2. finePNW | | #17

      I disconnected all lines, one by one, and it turns out my max flow is ~ 147 CFM per the installation CRM range internal auto-test *with no ducting attached*. I've reached out to Broan. Adding them, back I get 131CFM. Seems like this should not be the case. Minimum with ducting attached is 65 CFM, which also seems high to me.

  5. DennisWood | | #12

    .1 drop is not a lot, and half of that may in fact be filtration on the supply side, if it’s a MERV13. Your Broan should indicate which side is limiting.

    Filters on these devices are small, thin and fairly restrictive which is why going to inline filtration, if you have room, can pay some dividends with respect to reduced ECM motor load.

    110CFm is likely more than you need for a family of four, if you monitor IAQ. 75 to 90 CFM may be a fine, and dramatically drop noise levels.

    Oversizing an ERV with ECM motors does not really have a downside, unless you are over ventilating. The larger core makes for better latent/sensible efficiency at lower CFM. At a given CFM they may not be any quieter than a smaller unit. That said, my experiments with 4” vs 6” ECM fans favor the larger fans for noise considerations as likely the blade velocity is lower for the 6” fan at a given CFM vs 4”.

    1. finePNW | | #13

      Appreciate the follow up! To be clear, it’s a 0.8 - 1.0” drop, not 0.1” drop. We do have in-line filtration via 20x20x5 filter box, fortunately, and removing the filter from the box fortunately did not change the max achievable flow. We will likely be dropping the fan speed to limit noise and keeping an eye on IAQ with some CO2, VOC, and RH sensors. Thanks for your thoughts!

      1. DennisWood | | #14

        Sorry. I did mean 1.0. Aside from filtration, the core itself will be another significant factor in static drop. “Dumb” HRV/ERVs will have a label affixed to the case for balancing with static drop accros the core correlated to CFM.

        20x20x5 filter on supply should be fairly non-restrictive. What filters and MERV rating are in place in the ERV?

        1. finePNW | | #15

          Ah, ok, great stuff. The HRV itself came with (and is currently using) a reusable MERV8 on the intake and window-screen-like filter on the return air coming from the house. I removed them and reran the min/max test, and it did not make a difference in the max available CFM.

  6. DennisWood | | #18

    .1 static drop is very reasonable with filtration and duct runs…I would not sweat a few CFM at boost.

    You can play around a bit here to estimate loss via your ducting. It does not account for elbows though, so add 15 feet to your overall length per 90 degree elbow:

    As an example, 20 feet of 6” duct with two elbows (add 30 feet) is the equivalent of 50 feet of 6” duct. At 150 CFM, loss is .09”w.g.

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