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Community and Q&A

ERV and flex duct

user-6356169 | Posted in Mechanicals on

We’re planning to install a Renewaire EV90P ERV with dedicated ductwork in our new 1900 sq ft home. It’s going to be a budgetary strain to install hard pipe for all the entire system. So I’m hoping we can get away with a rigid duct for a trunk (running parallel to floor trusses) and then 6 inch flex duct going out to the exhaust and supply registers (through the webs of the floor trusses) – this is what the HVAC contractor is recommending, but I’m concerned about this small unit being able to handle the additional resistance to air flow from the flex duct. We will probably have three return air grilles (in 2 bathrooms and the laundry/utility area) and five supply registers going to a great room downstairs, a kitchen area, and to each of 3 bedrooms upstairs.

Ventilation performance for the EV90P ranges from 108 CFM at 25 Pa ESP to 42 CFM at 150 Pa ESP. I have no idea personally how to determine if the increased resistance from using flex pipe is something we need to be concerned about on this scale. Our HVAC company has a great rep for tight ductwork but I’m not sure about how they design the ductowork for the ventilation systems. Should they be running a Manual D for this, just as for the main forced air system?

The long and short of this is … can I assume we’ll have a decent, workable system with a rigid trunk and flex duct for the branches. If it’s somewhat less efficient than the ideal all-hardpipe system, I can accept that. But I don’t want to spend all this money and end up with a system that’s not ventilating as promised.

Thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    I learned about ducting HRVs and ERVs from David Hansen, one of the Vermont pioneers of mechanical ventilation. David only used galvanized ductwork, because of the low air flows involved in ventilation systems.

    For more duct installation from David Hansen, see Installing a Heat-Recovery Ventilator.

    I'm interested in the response of other GBA readers to your questions.

    -- Martin Holladay

  2. user-626934 | | #2

    All hard-pipe ERV/HRV systems can have noise issues at the registers, in our experience. Despite low velocity airflow, we would always get a "rocket ship" sound at exhaust registers when all hard pipe was used. We now typically spec insulated flex duct for the last ~5ft of each branch for sound control and slightly easier installation.

    Yes, the installer would ideally design the duct layout with Manual D....but ultimately, they should just guarantee they'll be able to deliver your design they do it is up to them. So first, you or they will need to know the design airflow rate. It's unlikely you'll need the full 90+cfm that the EV90P can give you, so they'll need to install balancing dampers on the main trunks from the ERV to the house. You'll more likely want something closer to ~75cfm, and with balancing dampers, you can always go up or down from there. Here's a great balancing damper that also allows accurate airflow measurements (when used with a digital manometer): If you use this, follow the instructions on the necessary section of straight duct that is required upstream and downstream of the damper to ensure smooth airflow at the damper.

    In my opinion, the installer should guarantee the total flow (supply and exhaust) will meet the design value within +/-5-10%, that the supply and exhaust are within 10% of one another, AND that each branch will meet the room design value within +/- 5%. If they don't have experience balancing ERV's, or are uncomfortable offering that kind of guarantee, you should look to a third party design + testing firm.

  3. user-6356169 | | #3

    Thanks Martin, that was one of many articles I read leading me to believe that hard pipe was the best option. And thanks John, so much info in your post ... leading me to some more questions:

    1) Couldn't we also control the ventilation rate by controlling how often the unit runs? Instead of installing dampers. We will have variable occupancy ... sometimes 3 people, sometimes 4 or 5. So our ventilation needs will vary. But maybe you think dampers are essential for properly balancing the system.

    2) Would getting the HVAC contractor to design system with Manual D be an acceptable, though maybe not as perfect, alternative to hiring a third party design + testing firm?

    3) What sort of company would provide such design and testing?

  4. Anon3 | | #4

    There are duct calculator online, with 6 inch flex duct and 100 feet run, the friction loss is 0.08" of water, another words, nothing.

    The intake filter when it get's loaded can easily reach up to 1" of resistance btw, making the house negative pressure. It's sad that Renewaire didn't go with a constant airflow motor control...

  5. user-626934 | | #5

    "1) Couldn't we also control the ventilation rate by controlling how often the unit runs? Instead of installing dampers. We will have variable occupancy ... sometimes 3 people, sometimes 4 or 5. So our ventilation needs will vary. But maybe you think dampers are essential for properly balancing the system."

    Yes, you could use a % runtime controller, but 24/7 is a simpler way to go. If you're using the ERV for bathroom exhaust, most building codes require 20cfm continuous or 50cfm intermittent. You might not be able to get 50cfm exhaust out of each of the bathrooms if you run the ERV intermittently. In addition, during the "off" times, you would want the ducts closed off to prevent unintended air you'd want to install motorized dampers on the ducts connected to the outside. And yes, the manual dampers are needed in ALL cases to properly balance the system.

    "2) Would getting the HVAC contractor to design system with Manual D be an acceptable, though maybe not as perfect, alternative to hiring a third party design + testing firm?"

    Not just design, but also test/balance and guarantee the system will meet the design airflows within +/- XX%. The very best residential HVAC contractors will do this.

    "3) What sort of company would provide such design and testing?"

    Martin just wrote a blog piece on exactly that topic -

    My company provides HVAC design and testing services for clients in central Virginia, but since we insist on testing/balancing every system we design, it limits our range.

  6. user-6356169 | | #6

    Thanks John, super helpful. Hadn't thought about the potential for air leakage through ducts to outside, so yes, seems like 24/7 operation with a balancing damper would be the way to go. I'm thinking that if we go with the Fantech damper you recommend, I could purchase a digital manometer and eliminate the need for the separate Renewaire "PTL" control, and also wouldn't need motorized dampers on ducts.

    Trying to find out from our HVAC contractor if they can do a manual D on this and if they'll check/guarantee flow. If not, how they feel about doing the install that someone else designs/tests. Hope this all doesn't become a budget buster ... but all my research has convinced me that "faith-based ventilation" is not a good idea ...

    BTW, we're not far from you, in Blacksburg, but probably out of your range.

  7. user-626934 | | #7

    You're in Blacksburg? You should have said so in the first place! ;-)

    We occasionally do work in Blacksburg, Christiansburg, and Roanoke. Send me an email if you'd like to arrange a time to chat. Click the "contact us" link at the bottom right of the home page for my info.

  8. Reid Baldwin | | #8

    We have a Renewaire EV130. Renewaire sells a percentage time controller for their ERVs. That is what I use and I do adjust it based on how many people are in the house. I have a CO2 monitor not far from the controller that I use as a guide. We have controls in the bathrooms that force it to run for 20 minutes. The Renewaire installation manual said to allow 50 cfm per bathroom if you are using it for bathroom exhaust. We are a little less than that with 3 bathrooms and 130 cfm.

    You will use less motor energy running it part of the time than running it 24/7 and restricting the flow with dampers. The efficiency will be a little better running at a lower flow rate, but I would be surprised if that was significant enough to offset the difference in motor power. (I haven't done any calculations to check my intuition about that.)

    For balancing, you theoretically only need one damper - on whichever side would otherwise have more flow. Of course, if installing the dampers is how you measure the flow, then you wouldn't know which side that is until they are both installed.

  9. user-6356169 | | #9

    So Reid, do you have motorized dampers on the pipes going out of the envelope, to restrict air leakage, as John has suggested?

    And, being such a newbie to HVAC principles and practices, I guess I don't really understand how you balance with one damper. Seems like with one damper, you could control total flow, but what if you need to balance one branch of the system (downstairs) against the other (upstairs)?

    Against the advice of many here at GBA, we are going to install separate bath exhaust fans. Part of that is just to preserve resale value. We hope to live in this house for 20 or 30 years but one never knows ...

  10. Reid Baldwin | | #10

    Our ERV was installed by our HVAC contractor. They weren't accustomed to installing balanced ventilation and weren't particular inclined to read and follow directions. We don't have any dampers in the system, manual or motorized. I see no indication that they commissioned the system.

    I have plans to redo some of their work. I ordered two of the Fantech manual dampers mentioned above and will install them in the exhaust and fresh air ducts (on the indoor side). I will use them to measure airflows in various conditions and adjust them as necessary to balance the system. By balancing, I am referring to making the incoming and outgoing airflows equal. The exhaust airflow from each bathroom may still not be equal to each other. I am not sure if the Fantech dampers will allow accurate enough measurement of low airflow rates to assess whether lack of motorized dampers is a problem.

  11. Anon3 | | #11

    Unless your ERV is self balancing, filter loading will cause it to go unbalanced as soon as you start to use it.

  12. Jon_R | | #12

    I've been told that the goal of balancing the airflow is to prevent pressurizing or depressurizing the building. Even ignoring the fact that this may be beneficial, I find it odd that one doesn't just directly measure what they are trying to prevent. Too small to measure accurately? Are air density differences typically accounted for in flow rate balancing?

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