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ERV installation

Chrisroche | Posted in General Questions on

Hi All,

I am looking for some advice regarding the use of flex duct vs rigid duct in my ERV installation. All of the literature I have read seems to indicate that rigid duct is the way to go due to efficiency and friction loss, but after I ran the math on my design, I seem to be hitting a wall that indicates otherwise.

I built a 1,700 sq ft ranch that is ultra tight and has an unfinished basement below. The dimensions of the house are roughly 26’x63′. I am using the Panasonic Intelli balance 100 self balancing ERV to ventilate and will be moving somewhere between 60-100 CFM depending on amount of inhabitants. I have attached two images that show the floor plan and erv duct plan.

The design I came up with has the ERV suppling fresh air to either end of the house, one in the master bedroom and one in the family room. Each register will supply both upstairs and will be at a T that also supplies the basement directly below with a damper register. So there will be a total of 4 supply registers. There will a return air register in the middle of the house near the hallway ceiling and one below in the basement.

When I run the math on friction loss, I start with 100 CFM of supply that gets split almost immediately after the ERV in basement. So thus I have 50 CFM supply at roughly 50 feet of run with four 90 degree bends. If I am running 6 inch flex duct, this only gives me a friction loss of .02. Everything I have read seems to indicate that I want to keep this number below .1, which this does. At the same runs, rigid duct seems to give a number that is very similar, basically negligible differences in friction loss. Obviously this will rely on me doing a great install of the flex duct with maximum stretching. Am I missing anything here? Is there any advantage in running rigid duct in this application?

I am having trouble attaching the house layout pictures, so here is a link to them at

Thank you for your time!

-Chris Roche

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  1. Expert Member
    Peter Engle | | #1

    The reason for your very low friction loss is the very low flowrate and velocity with 6" duct. If you are targeting a friction loss of .1, you could use 5" flex duct instead of 6" flex, saving some money and space. 5" rigid duct has half the friction loss as flex at this size, .05 vs. .1. This is also a very simple analysis, ignoring the T fitting at the ERV, and treating the T fittings at the ends of the run the same as 2 90 degree fittings. Not quite the same thing. And, like you noted, you will have to make sure the flex is stretched tight and not sagging. Still, with this flowrate and duct size, you will probably do just fine with carefully installed flex ducts and sheet metal turning fittings.

    On another note, you mention that the basement is unfinished, but not whether it is insulated and conditioned. I'm not sure I would share an ERV between an insulated house and an uninsulated and unconditioned basement.

  2. Chrisroche | | #2

    Thank you for the reply Peter!

    The basement is indeed insulated and conditioned with a mini split. 10 mill vapor barrier below slab, with gravity daylight drains and 2 inch rigid under slab and foam on either side of walls as well, so hoping it stays dry. House took a while to build and lots of rain made it to slab so its still hovering around 50% humidity with dehumidifier running. This is 9 months after it became weather tight.

  3. Yupster | | #3

    Connecting the basement supply and upstairs supply in a straight run with a T like that gives a direct connection for sound a light to transfer. Not a good idea. Offset the connections. Also, when you size your ducts, don't forget to include all your fittings. This includes the T, which has an equivalent length of 50' (equal to the length of all your straight duct), the stack head at the top of your riser into the upstairs, the riser itself, the grille, the takeoff off the trunk (also 50' equivalent) etc. You are easily over 200' of equivalent length on some of those runs. You also might want to run supply ducts to your other bedroom and office, those rooms will appreciate the fresh air and it might even be required by code if you don't have any forced air to meet the distribution requirements. The cost will be minimal when you already have ductwork running right under them. It will also mean lower airflow running through your riser, so you can downsize the duct to a 4", which will be much easier to run up a partition wall. If you are running 50 cfm into a riser, you will need at least a 5" rigid riser. Don't underestimate the difficulty of installing flex duct in a tight fashion. There is a reason why professionals screw it up all the time, it's hard! Use rigid duct, it's much more foolproof, especially for DIY. Use insulated flex duct only for the runs that connect to the outside. With 0.4 in.wc. available to you, you should be fine (based off my experience designing, not a real design. And assuming good installation and rigid sheet metal duct with round plastic diffusers) with 6" connections to the outside, and 6" trunks with 4" risers, assuming 20-30 cfm per branch. Using a Y instead of a T at that initial connection to your ERV will help reduce noise caused by turbulence at the the higher flow rate there. It also lowers the equivalent length of that fitting from 50' to 10'. I did not look at your return side but similar considerations should apply there. That's my 2 cents

  4. Chrisroche | | #4

    Great advice Yupster, I ended up going the route that you suggested!

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