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ERV Duct Design

DavidDrake | Posted in Mechanicals on

Based on advice received on a previous question, ( I’ve decided to go with the Panasonic Intelli Balance 100 ERV (cold climate version) for ventilating a 600 SF second floor studio apartment + 250 SF ground floor office and bathroom. I’ll be doing the install and commissioning.

I’d prefer to run flex ducting if possible for ease of install, but am not opposed to rigid if it will be more effective and/or quieter.

Looks like there’s a lot of options in flex ducting, from polyethylene, to aluminum, to something called Fabriflex.

Anyone have a recommendation?

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  1. mr_reference_Hugh | | #1

    David, I did a bunch of research before installing our ERV, which just happens to be the same you are buying. I ended up using rigid ducts but flex ducts can do a good job if installed correctly.

    I would suggest reading this page for the basic to understand how to install flex duct like Fabriflex. Looking at the page it captures just about everything I recall about what i found during my extensive research online.

    If you want the high level summary from that webpage, here it is:
    "Using flex duct for straight runs is usually fine, especially if you're doing it as part of a deliberate and meticulous custom ductwork design for a home. But using it because it's easy to install? Or using it for bends? That's just irresponsible."

  2. DavidDrake | | #2

    Thanks, Mr. Research. That page makes a lot of sense.

    The only previous mechanical systems work I've done (with help from books and other research) has been designing and building a dust collection system for a 2700 SF wood shop, as well as a much smaller material collection and exhaust air filtration system for a hammermill. Both systems work well, and are obviously almost all rigid pipe, with a minimum of flex.

    Did you use rigid rectangular duct or round? And if round, was it the snap-together seam type, or spiral pipe?

    Another couple questions: it seems industry uses Manual D and Manual T for ductwork and grille design. It also seems this is mostly for forced-air heat and AC application, whereas I'm just designing for ventilation. Did you use either manual to design your system?

    My project doesn't have AC, and heat is baseboard electrical resistance. At the moment, for the upstairs apartment, I'm thinking a fresh air outlet directed toward the sleeping area, a stale air intake over the kitchen, and another intake in the bathroom. Then chase intake and outlet branches down the interior wet wall, with intake in the downstairs bathroom (undercut door) and outlet in the downstairs office. Longest duct runs will be about 10'-20'. See attached diagram.

    The IB100 installation manual seems to show using insulated ductwork. If all the ducts are within the conditioned spaces of the building, it's not clear to me why this is necessary.

    1. Expert Member
      Akos | | #3

      You have pretty tight plumbing there. I would do the initial run off the unit with hard pipe and the rest in flex. There is nothing wrong with flex, even ugly bends as long as you oversize slightly. The only thing that needs insulation is the outside connections as these are near outdoor temperature. Any of the fresh air supply and stale air pickup ducting is pretty much at house temperature and can be uninsulated.

      Make sure to get some flex between the hallway fresh air feed and the unit, same for the bathroom stale air pickup. You might have to add some extra bends for the bathroom. Without this flex you'll hear the blower on the unit especially when it goes on boost. That looks like the Panasonic tandem vent, that is only good for about 50CFM.

      1. DavidDrake | | #4

        Thanks, Akos.
        I plan to get the 5" Lifebreath dual vent hood you recommended on an earlier thread; couldn't find a dimensioned drawing of it, so that's just a rough model based on guesswork. The unit appears to be insulated. but I can certainly add a bit more up to the connections with the ERV.

        The layout could be loosened up a bit if that's a benefit, with the exception of the chase through the wall down to the ground floor, which is also going to be tough to do with anything bigger than 4". I was working off the assumption that keeping the pipe runs short (within reason) would reduce friction, therefore loss of pressure, noise, etc. It's possible I could fur out the bathroom wall next to the stair landing enough for larger pipe, but I'd prefer not to.

        Would something like this be suitable for flex? It seems the bends could be made smoother and with larger radii than elbows:

        I found a copy of Manual D (3d ed. 2013) online and am working my way through it. The appendices and worksheets see particularly helpful.

        1. Expert Member
          Akos | | #8

          The Lifebreath dual vent already has insulated fittings, you only need to use insulated flex pipe from there to the IB100. If you have the pressure available, since it is such a short straight run you can use 4" flex between the unit and the wall cap.

          If you look at Man D, most of your losses are from bends and fittings. What you would consider long runs, are pretty short in comparison.

          The semi rigid aluminum is a good compromise for hard ducting. Much easier to run for a complicated run, I have often used it specifically for this reason. If you are splitting the air flow between the office and the apartment, you are looking at 50 CFM through each register on boost, so not a lot of flow.

          50 CFM in 4" is just about doable if you keep the equivalent length down to about 120' but it won't be quiet, not loud but noticeable. If you go to this route, I would add a duct silencer or a flex S bend in the office.

          You can also go down in the wall with 6" oval hard pipe with a pair of round to oval boot 90° s at the end. From there you can run either 6" flex to the unit and to the registers. 6" flex at 50CFM is such a low flow rate that it doesn't matter how ugly of a run you make it. If you want smaller you can try for 5" oval and 5" flex ducting but the 5" oval is harder to find.

          You want to keep your overall losses (including both outdoor and indoor ducting) to about 0.4" to stay within the auto balance range of the IB100.

          Make sure to have a simple to open access panel under the unit for replacing the built in filter.

          1. DavidDrake | | #13

            Thanks again for the advice, Akos. Will definitely consider oval duct.

            The ERV will be located in a mechanical room. Assuming I can mount it with access to the filter facing up, there's shouldn't be an issue.

            Just ordered the ERV and Lifebreath dual hood, so I'll be able to study out both before a final decision.

    2. mr_reference_Hugh | | #6

      To start, I strong endorse the suggestions made by DennisWood in his own reply.

      In regards to the information/drawings you have added and your additional questions.

      Your drawings:
      You attached a sketch or drawing showing your IB100 install.

      My comments
      ***Please read these comments attentively***

      1. Your design looks so much like what I initially planned for my IB100. You are already reading the install manual from what I can tell, but so did I. I missed a few things on the first few reads. (I don't think that IB100 manual is super well written for some reason.)

      2. I was placing the unit in my bathroom until I read this very important sentence in the IB100 manual.
      "Never install unit in high humidity space, such as bathroom, kitchen or laundry room." This is on page 2 of the IB100 manual.

      3. a). You have 2 times a 180 degree turn in your ducts (i.e. U-shaped duct) right near the port on the IB100 unit. If you read Note (1) in Figure 12 on page 11 of the IB100 manual, you will see this sentence.
      "(1) Recommended straight run before elbow is more than 2 ft (610 mm)." They also have a diagram in that Figure 12.

      3. b) Having those U-turns will have a big impact on the performance of your ductwork, from what I have learned. This will also have a significant impact on the noise levels.

      3. c) My IB100 makes almost no noise at 50CMF. It is pretty good at 90 CFM. It gets a bit loud at 100 CFM. Panasonic suggests that this unit is very quiet - but I have not measured sound level. The unit is in a bulkhead in my living space (no basement) I did what DennisWood recommends. I ordered and installed qty 4 of the HVAC mufflers. I would install these on any ERV/HRV I spec in on any future built. BUT YOU NEED THE SPACE!!! Here are some examples (link):

      4. I can't find where I read it but I thought that there had to be a minimum run of duct between the IB100 unit and the exterior hoods. Its getting late in the day for me so I will let you either call Panasonic or check your local code.

      5. I strongly encourage you to -reconsider- where you locate your ERV. Your space is tight like mine (we build only the space we need). Looking at your plans, I would strongly suggest you consider placing it in that spot you have right in front of your hot water tank in the garage. The heat from the hot water tank might be enough to keep that small space above 50F if you create a closet there. I know it would hide the natural light from the window. You could even insulate and heat that small space for very little money. I mention 50F because the IB100 manual specifies on page 3:
      > "Install the unit in the area where air temperature is above 50 °F (10 °C) to avoid condensation in unit."

      You asked:
      Did you use rigid rectangular duct or round? And if round, was it the snap-together seam type, or spiral pipe?

      Comment: I used a combination of both. The I used standard galvanized duct pipe, the stuff available from Home Depot. Here is a link that should show the duct pipe. Hope the link works correctly.!q=duct%20pipe

      If you design the duct runs yourself, I might suggest this site to see equivalencies of rectangular vs. round pipes. The rectangular ducts have a significantly larger volume of air but the elbows need to be selected carefully to avoid ones that create too much turbulence.

      Question: it seems industry uses Manual D and Manual T for ductwork and grille design. It also seems this is mostly for forced-air heat and AC application, whereas I'm just designing for ventilation. Did you use either manual to design your system?

      Comment: Unless you have the mind of an engineer, I would use the manual that DennisWood is suggesting in his reply. I have two examples I will share from my own experience.

      A) One house we built in 2007, I had a "professional" install flex duct and paid him well. As I looked and staired at the ducts, I had a bad feeling. A few weeks after install I started reading the Venmar HRV manual and lots of other research. When I realized that this "professional" had broken every possible rule about duct installation, I ripped it all out. I planned the ductwork using the information in the Venmar manual that DennisWood suggests here. I reinstalled everything with rigid ducts, save a few lengths of flex where it was too difficult to install the rigid stuff. The longest run is about 30 feet. What is my point: after 15 years, everything works just fine. It is a dedicated set of ducts for the HRV because the house only has heated floors as a heat source.

      B) In my most recent build, I wanted an engineer to design the ductwork. It was in the budget from the start and it was not negotiable. So in this case, the engineer would have used Manual D and Manual T.

      You mention:
      The IB100 installation manual seems to show using insulated ductwork. If all the ducts are within the conditioned spaces of the building, it's not clear to me why this is necessary.

      Comment: I agree with DennisWood on this one. He says below "You only need to insulate between the ERV and external hoods". If you closely at the IB100 install manual (and you it is not immediately clear so look closely at the image attached), they certainly do only show insulated ducts between the ERV and the external hoods.

      You mention:
      I'm thinking a fresh air outlet directed toward the sleeping area, a stale air intake over the kitchen, and another intake in the bathroom.

      Comment: You may know this already but be careful with the stale air intake over the kitchen. My engineer was requiring a specific type of return air filter for the kitchen. Here is a link to an example:

      1. DavidDrake | | #12

        Hi mr reference,
        Thanks for the response. It may not have been clear from the drawing, but the space where I'm proposing to put the ERV is a mechanical room above the bathroom, not *in* the bathroom. The mechanical room is within the apartment thermal envelope. I suppose if drywall + paint + good exhaust isn't enough to keep moisture levels low enough in the mechanical room, I could put a vapor barrier under the mechanical room floor.

        I've changed the location of the cooktop versus what's shown in the plans (see response to Dennis Wood below). And the stale-air intake is above a sort of cantilevered soffit over the sink, and apx. 8' from the cooktop (which has a hood not connected to ERV). But it seems like a good idea to at least monitor the grille for traces of grease build up. If there's issues, I'll install a filter unit.

        Not sure if I have the mind of an engineer. I teach architecture in a school of design and construction that's in a college of engineering, so plenty of my colleagues are engineers. They seem to tolerate my attempts at materials engineering :). I read a lot of ASTM standards; Manuals J & D don't seem much more complex. But I definitely appreciate simpler approaches, if available.

        1. mr_reference_Hugh | | #15

          I forgot to include this link in my other reply. I could be helpful if you design the ductwork yourself.

  3. DennisWood | | #5

    I would use flex only to decouple the mechanicals, and use hard duct if you can everywhere else as it looks like you'll need to run a fair bit of 4". You can always transition to 3x14 rectangular in the wall cavities. The snap together stuff is fine. Just tape your joints with a decent quality aluminium foil tape. Duct tape just disintegrates long term.

    Reducing static pressure (hard ducts) will also reduce noise at the ERV as you'll be able to run the EC fans at lower speeds. This is a good read on flex vs rigid :
    For longer runs, it's easier to get it right with rigid. I've seen so many examples of crap flex installs...

    You only need to insulate between the ERV and external hoods. I know you want just one, but I'd use two if you can. Make sure the exhaust has a flap...but if you intend running 24/7, that won't matter.

    Finally, if noise transfer is a concern with the apartment, I'd plan for a few duct mufflers. Easy to make, and they can make a huge difference in noise transfer. DIY , they are pretty easy. Open ducts can be a big flanking noise issue. You can also line rectangular ducts with acoustic duct lining, but in your case with space limitations, this may not be doable.

    Your ERV manual may have a guide for ducting, but you'll find them also in manuals like this one:
    page 12.

    Here are a few duct mufflers I built for a theatre project...

    1. DavidDrake | | #9

      Hi Dennis,
      Thanks for the links. Duct mufflers sound like a good idea.

  4. DennisWood | | #7

    To Mr's comments on the kitchen exhaust you need to consult local code. Here for example you need to follow some rules for "kitchen" exhaust including metal ducting (and metal fan cases) which will be a problem if your ERV is to serve as a kitchen exhaust. Better to sort a kitchen exhaust there that is dedicated and to code.

    I've done quite a few experiments around kitchen exhaust and have sorted a nice setup using an EC inline 6" fan (metal housing) remote to the range hood. I've found that a 30 inch hood at 24" over a 30" induction cooktop required these (measured) air flows for effective evacuation. Fan was a Terrabloom 6" EC unit, so only uses 36 watts at full pop. The EC fan has eight speed settings but 1-4 don't work so well for exhaust in this application:

    Speed Setting 5 - 111 CFM (this is what I have it set at for pretty much 100% evacuation)
    Speed Setting 6 - 124 CFM
    Speed Setting 7 - 131 CFM
    Speed Setting 8 - 162 CFM

    Static pressure is high (by design) to "pop" the Broan Ecovent's outside hood foam ball at temps that can dip below -35C.

    Your ERV won't come anywhere close to this. You don't need a make up air solution under 400 CFM (check local code).

  5. DavidDrake | | #10

    Hi Dennis,
    If by kitchen exhaust, you mean range hood, no plans to use the ERV for that.

    Kitchen layout has changed a bit since the plans posed above—I moved the cooktop to a peninsula parallel to the wet wall. Space works a bit better that way, and there's no blind corners under the countertops to deal with. It does complicate doing a range hood, since I have vaulted ceilings, but I was able to find a Bosch downdraft unit, complete with blower, retractable hood, and duct for $500 at the Habitat Restore. Never been used. I realize downdrafts aren't super effective, but I'm using it with an induction cooktop. And the deal was just too good to pass up. I believe the blower is 600 cfm on high, so will need to deal with make up air, I suppose.

    1. Expert Member
      MALCOLM TAYLOR | | #11

      " I was able to find a Bosch downdraft unit, complete with blower, retractable hood, and duct for $500 at the Habitat Restore"


    2. this_page_left_blank | | #16

      I think "aren't super effective" is a massive understatement. I think "don't work at all" is closer to the truth. Even most conventional range hood setups can probably be described as not especially effective.

      Induction cooktop is certainly better than gas, but the cooking itself throws up pretty massive amounts of particulates.

  6. DennisWood | | #14

    David...I'm a big Restore fan...just picked up 10 x 24"x68" wood frame, metal clad dual pane panels..for $200. Greenhouse project..

    Good score on that setup, down draft or not! 600 CFM is with nothing attached, so functionally you'll be at quite a bit less once installed.

  7. DennisWood | | #17

    This page…agree 100% on the range hood least ones with built in fans.

    Not only is the pickup on many of them poorly designed, but the flow is terrible with any static restriction. An inline fan setup gives you freedom on the hood side for a larger intake. I’ve managed more or less 90% evacuation at just 110 CFM, and 100% ish at 150 or so with a 30’ hood.

    1. DavidDrake | | #18

      Fortunately, the Bosch blower isn't built in, or at least not the same way the under-cabinet landlord specials types are. It's generally sold separately from the hood, and I believe can be installed at some distance from the retractable hood.

      My mother-in-law has the same hood (with inline fan) and I've cooked with it. With short, shallow pots and pans, and less-than-aggressive cooking technique, extraction seems ok. As I'll be renting out this space furnished, I get to choose the pans—no tall stock pots, I promise!

      My wife and I both used to cook professionally. No home hood measures up to what a commercial kitchen is required to have. Yet there wasn't a night I worked I didn't feel like I needed a shower immediately after closing (okay, after the bar I went to after work closed) to get the 'pretty massive amounts of particulates' out of my hair and off my skin.

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