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Community and Q&A

Ducting Plan for ERV

Andy719 | Posted in Mechanicals on

Sorry for the length of this, but I’m going crazy with the amount of available information on ventilation systems, and I’m hoping you would all be willing to help me out again on decision making for new house options (I took Akos advice to skip flash and batt and just go with R21 fiberglass cavity insulation on another post).

I realize that an ERV with dedicated ducts is best.  The problem is the track/spec home builder I’m buying from only offers a few custom items.  They will install a Trane ERV on one of the two Trane air handlers for $3,800.  I don’t believe I can get their sub to do a standalone ERV system. Base price includes the code minimum exhaust only system with a bathroom exhaust fan on a timer.  I guess these are my options:

1. Get the Trane (rebranded RenewAire I think) ERV.  I realize this isn’t the most efficient design, but keep in mind I will have a variable speed ECM air handler.   It also saves me the headache and time requirement of the other options, which is equally as important to me as the upfront cost.

2. Do nothing now, have just the exhaust only system, wait until I move in and test for CO2.  Then at that time if needed do one of the following:

2a. Install standalone ERV (likely Panasonic intelli-balance) which will cost $1000 plus materials for ducts and bulkheads. Then take the timer off the exhaust fan.

2b. Install supply fan for balanced system in forced air return duct.  Control based on either exhaust fan or air handler activation?  I’m thinking have it turn on when heat/cool comes on so it is always treated, realizing that I will have an exhaust heavy system on shoulder seasons.

2c. Install supply fan for balanced system into open area, controlled by exhaust fan timer.  My house is an open concept with an open stair at the end of the house that connects conditioned basement, first, and second floors without doors.  I was thinking of dumping the outside air here if I go this route.

Assuming those are options, which would you recommend? Option 1 is quick and easy but pricey and not perfect.  Options 2a-c may not even be needed if the exhaust only system works well enough, but that is a risk.  They are then cheaper but either less efficient than option 1 (2b-c) or require almost as much money but extra time and retrofit effort (2a).

Thanks for taking the time as I assume it will be beneficial for lots of buyers in the mid-Atlantic area.  Some of my specs for consideration:
Maryland – Zone 4A
2018 IRC/IECC design
Around 4,800 square feet not including conditioned basement.
Around 6,500 square feet including basement.
All electric, no combustion appliances
1 air handler for top floor (unit in closet, ducts in attic (sorry))
Bedrooms for the two kids, the wife and I upstairs.
1 air handler for main floor and basement
Apartment for the elderly in-laws on main floor.
Exhaust fan on timer is in buddy bathroom shared by kids with doors opening into each of their bedrooms (not hallway).

Thank you all again for the great content and advice
Andy

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Replies

  1. Doug McEvers | | #1

    When using the forced air duct system for the distribution of ventilation air in a cold climate there was a concern. Introducing the (HRV, ERV) supply air into the furnace cold air return could be a shock to the furnace heat exchanger. I am not an expert on this but might be better to tie in on the furnace supply side.

    1. Andy719 | | #3

      Is that the case for heat pumps? I don't have a gas furnace.

  2. John Clark | | #2

    Supply only ventilation I guess (Option 1). You'll be able to amortize the cost of the ERV into the mortgage.

    btw..The only way to build a balanced system is via a dedicated duct system. You can't do it by tapping into the HVAC duct work.

    What does Maryland code allow?

    1. Andy719 | | #4

      Isn't an ERV by nature a balanced system, and not supply only? Volume of exhaust is still equal to fresh air intake, right?

      My area has adopted the 2018 I-codes. I don't know of any amendments related to ventilation systems.

      1. John Clark | | #10

        In a dedicated duct system the air/in doesn't physically mix with the air/out. I can't imagine how an HVAC installer would prevent mixing of the air within the HVAC ducts since the same duct system is used to move both fresh and stale air. I suppose they could install motorized dampers within the primary return and supply line so that they bypass the furnace itself. I would also wonder about the efficiency of the ERV when the blower motor is running.

        1. Andy719 | | #18

          The Panasonic and RenewAire instruction manuals show two methods of integrating with HVAC ducts. Either connect both fresh/stale ducts to return side with fan interlocks, or you can split them, or you can put the stale intake on the return air side, and then put the fresh air outlet on the supply side with a backdraft damper in the fresh air duct. This second method does not require fan interlocks according to the manufacturer.

    2. Josh Durston | | #6

      I'm not sure if I agree that you can't balance via shared ductwork. There are different goals of balancing: balanced airflow and balanced building pressure (versus outside). Generally you may need to have slightly unbalanced airflow to achieve a neutral of slight positive building pressure.

      There is certainly a effectiveness penalty to shared ductwork (since your diffusers are situated for heat/cool instead of ideal ventilation points), and a energy penalty since you are running more fans.

      Some ERVs will react in real time to changes in ESP, and should be able to maintain balance even if the furnace fan is varying it's speed somewhat. I would avoid manually balanced ventilators in a shared ductwork system. These are ERVs/HRVs that rely on manually positioning internal dampers to restrict/balance flow. Some (but not all) ECM fans will target a flow setpoint and will vary their RPM to try and achieve a constant flow even with some dynamic variables.

  3. Jon R | | #5

    In #2, there is an assumption that measuring CO2 will indicate if ventilation is adequate. But when it comes to other pollutants, this isn't true.

    Some ERVs are fairly resistant to becoming unbalanced when the supply or return pressure changes.

    1. Andy719 | | #19

      Thanks, I agree. I will use one of those off-the-shelf multimeters that covers CO2, VOC, particulates, humidity, etc.

  4. GBA Editor
    Kiley Jacques | | #7

    Hi Andy,

    It has been said many times—in some way or another—on this site that connecting an HRV/ERV to forced-air ductwork is an inferior installation compared to a unit with dedicated ventilation ductwork. You should definitely read what Martin Holladay wrote about Ducting HRVs and ERVs. In short, “delivering ventilation air through heating or cooling ducts is a bad idea.”

  5. Expert Member
    Akos | | #8

    My sister went through a similar dilemma with a renovation and eded up installing whisper comfort ERVs in all the bathrooms. The extra charge for 3 including install was about 1/2 the cost of the HRV connected to the AC system. Again, not ideal but better than exhaust only ventilation.

    Interlocking ERVs with non modulating heat pumps is not the best as you'll get pretty bad humidity removal. Since the fan will continue to run when the compressor cycles off you will get re-evaporation of moisture from the AC coil.

    If it is a modulating system, it can be made to work but I would still look at least upgrading to one of the auto-balancing ones (ie https://www.vanee.ca/en/products/residential-air-exchangers/88-erv-150-cfm-75-sre.html). The cost delta is small and it is actually less work for the trades as they don't have to do much commissioning.

    1. Andy719 | | #11

      Thanks, you are the best. I will have variable speed blowers but not compressors. I was hoping to have them, but I thought the prices were downright criminal. I meant to add the whisper comforts or lunos systems as item 2d, but I figured my post was too long anyway, so I let it go.

      Interesting that someone close to you went with 3 whisper comforts. Did they have any issues getting them approved as bath fans since they are only 40cfm, or did they set them up as continuous operation? Is the filter cleaning a big nuisance compared to just changing filters? Do they handle the shower humidity well?

      1. Expert Member
        Akos | | #17

        The units run all the time for the required ventilation capacity. They have a switch input to select the higher flow rate on it when needed to clear the bathroom. There is still a single exhaust fan for the needed total ventilation capacity.

        The filters are not the cheapest but pretty easy to swap out.

        1. Andy719 | | #20

          Gotcha, thanks. I interpreted the user manual to be saying that they wanted you to vacuum the filters. It gives the impression they are not disposable.

  6. Andy719 | | #9

    Kiley,

    Thank you for responding. I love the FHB podcast by the way. I'm no building pro, but I learn a lot and have a lot of laughs as well.

    I know the mantra about independent ducting, and I've read that article. However, Martin also says, "If the furnace has an energy-efficient blower with an electronically commutated motor (ECM), the energy penalty of continuous fan operation is reduced but not eliminated." However, he doesn't state the penalty reduction amount. The 2018 IECC now requires that any new construction HVAC air handler that is also used to distribute ventilation air must be an ECM.

    My post was also about the economics of it to the regular buyer. Should I wait until construction is done and spend $1500-2000 cash on equipment for a self-installed ERV, or $500-1000 on a less efficient supply air fan setup without recovery, or go with the builder option of the ducted ERV which adds about $20/month to the mortgage. If I had the option of a builder installed fully ducted ERV, I would do that, but it will be pulling teeth.

  7. Andy719 | | #12

    To the moderator, I see my post title was changed. This post wasn't intended to be a discussion of best practices to duct ERV equipment. It was intended to be about the comparative economics of installing an ERV by builder's rules, retrofitting a better one later, or turning a standard exhaust only system into a balanced system with a non-recovery supply fan.

  8. Eric Habegger | | #13

    I agree with the general advice to keep the AC and ERV ducting separate for the same reason others have enumerated. It may be inconvenient for you now to accomplish that in terms of time and labor but you have to weigh that against the inconvenience later of having a bastardized system of interconnected ducts. If the combined system doesn't work well or is less efficient than you expected the headaches and inconvenience maybe even greater. It's just that they will be delayed so its easy to ignore those possibilities NOW. When you have interconnected systems of great complexity it will be very difficult to get it right later if it doesn't work perfectly from the beginning.

    I like the Panasonic ERV you favor as I have one and they are very efficient. However I think you would have to install at least 2 of them for the size of your house and the number of occupants. It's still not a bad price for 2 - about $2000. If you elected to go that route it is well worth installing a large filter box that takes a standard merv13 filter in the outside air supply line for each ERV. It will recoup the cost of the installing the box in no time because of the much cheaper filters. They will also increase the efficiency of the Panasonic units considerably by being less restrictive.

    1. Andy719 | | #14

      Eric, thanks, point taken. I watched the BS&Beer episode where the rep from Panasonic advised doing the same thing with the filter boxes. In my mixed climate, I like how they market this ERV with the ability to seasonally adjust the supply vs. exhaust balancing ratio.

  9. Andy719 | | #15

    Ok, so now this will turn into an ERV duct planning post. Here is what I'm leaning toward right now: Take the advice of most and don't have the builder install an ERV tied to the air handler, but just take the code minimum exhaust-only system to start. Later on I will install one standalone ERV in my basement. I have an oversized chase (standard on model, but not needed for gas flue piping since opting for all electric) that runs right up the middle of the house. I will run a single rigid duct up this chase (6" to first floor 6x4x4 tee/wye split with 4" supply register on each 1st and 2nd floors). Supply registers will just dump fresh air in open first floor and 2nd floor hallway. ERV stale air exhaust will just come from basement. I will then just let the forced air systems distribute that air, but there will be no direct duct connections or system interlocks. Since one intellibalance won't cover the entire ashrae requirement, I will reduce the exhaust fan timers to cover the leftover cfm requirement by intermittently exhausting stale air from the upstairs bathroom.

    1. Expert Member
      Akos | | #16

      That is a reasonable plan. If the builder would let you, I would snake some 3" semi rigid aluminum duct (hvac places sell this in long length) to each of the bedrooms. This easily fits into standard 2x4 walls. You can then add in the registers and connect it to your trunk in the fireplace chase down the road.

      I would also see if they can install the intake and exhaust hoods for you. Much easier to have these in before the siding goes on. Panasonic sells and tandem hood that needs only a single hole and pretty quick install.

      1. Andy719 | | #23

        Just saying thanks again for your help, but the builder rejected me on this one. At least their sub did. They are unwilling to install any ductwork for future systems, or allow me to install any myself. Funny how the plumbers and electricians have no problems with doing rough-in work, but the mechanical refuses to. I either over pay them to put in their simplified ducting ERV, or I get the standard exhaust only bath fan.

    2. Doug McEvers | | #21

      Andy,

      I replied to the original title of your post, utilizing HVAC ductwork for HRV/ERV. Somehow it was changed. As a builder involved with energy efficient housing (installed first HRV in 1983) your plan sounds fragmented for ventilation air. Get it all done right before you take occupancy. Can't provide much detail but exhaust only systems are not allowed in MN for a number of years now.

      1. Andy719 | | #22

        Thanks you for the help. GBA changed the post title, that wasn't me. We are definitely in different climate areas, as I am in MD. Exhaust-only is allowed, and is what I will probably get at occupancy. I don't intend to leave it that way. My original long question could be paraphrased to ask which makes the most sense in my climate zone, spend a few hundred on a supply fan and interlock it with the exhaust fan or air handler, or spend a few thousand on an ERV? I was hoping someone might have an idea of the actual energy penalty/payback.

        I do feel pretty good about my plan, even though fragmented. The post-occupancy ERV will provide the majority of the necessary ventilation. The hvac ducts will provide the distribution during most of the year. I can then use a couple of smart switches to control the bath fans to pick up any extra necessary ventilation at night during the short shoulder seasons we have, and I don't have to spend the money on a fully ducted retrofit ventilation system.

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