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ERV / HRV Details

user-3152143 | Posted in Mechanicals on

The more I read both here in discussion forums and in articles, the more I ponder various concepts, and the more perplexed I end up. I’m a homeowner undertaking a gut-renovation on nights/weekends/holidays. I’m trying my best to balance idealism and the realistic necessities of my budget.

~2000 sf house in Southern CT; CCSPF insulation throughout; Hydro-Air HVAC system. I’m pretty well convinced that an ERV is the correct choice.

Everything in life has tradeoffs and I am having a hard time justifying the budget & effort for a fully dedicated system. My current thought is to have a single stale-air return in the kitchen (space also hosts laundry) and duct the fresh-air supply to a point in HVAC return trunk where it is equidistant to most of the return grills.

My rationale is that when the air-handler is NOT running, those rooms (w/ returns noted above) will receive the make-up air. When the air-handler is running, the make-up air will be distributed through the house. This much I like and am comfortable with. I also like the idea that the make-up air will lose velocity as it enters the larger HVAC return ducts and will be tempered that much more prior to entering a given room. Seems like a reasonable concept as my interpretation is with the ERV you are trying to create a slow current of air from high pressure to low; as opposed to HVAC where you are trying to dynamically mix and circulate air from supply to return.

Concern: my biggest concern is with the potential of condensation from the make-up air…

Question: does the make-up air ducting get insulated?
–>The made-up fresh-air is my question (after the ERV)
–>If not insulated it would allow more tempering of the made-up fresh air but (eg, in summer) will have warm moisture laden air entering into a cool metal pipe. Will this be a sweaty mess?
–>In the event that I insulate this fresh-air supply duct to the point where it taps into the HVAC return trunk, once again you have a condensation potential.

Introducing the potential of moisture into the HVAC ducting doesn’t leave me very confident in my “rationalized” plan.

Any commentary would be greatly appreciated. I really enjoy the GBA articles and discussions within these forums.



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  1. GBA Editor
    Martin Holladay | | #1

    1. ERV manufacturers publish installation instructions. These are available online, so if you know what model ERV you will be purchasing, you should follow the manufacturer's installation instructions.

    2. To distinguish between the two types of ducts serving an ERV, most installers talk about "fresh air" ducts and "exhaust air" ducts. The term "makeup air" is confusing, because you can't use an ERV to provide makeup air.

    3. Yes, it is a good idea to use insulated flex duct to connect an ERV with the exhaust air termination and the fresh air intake.

    4. For more information on your ducting options, see Ducting HRVs and ERVs.

  2. user-3152143 | | #2

    Thanks Martin, I apologize for the confusing terminology. It doesn't appear that I can edit my initial post. So I guess for the sake of communication: "fresh air intake"; "fresh air supply"; "stale air return"; "stale air exhaust". If re-reading above, consider the term "make/made" as being processed by the machine. I had no doubt about the fresh intake & stale exhaust needing insulation.

    I've read quite a bit of documentation from several manufacturers and independent articles, etc. I don't seem to find any discussion about my concern which hopefully is written more eloquently here:

    What about the "fresh air supply" ducting after the machine? Do these machines process the air sufficiently that there isn't a condensation worry after the machine? There certainly is the possibility of temp differentials and same goes for humidity levels.

    Consider the winter: from what I've read it seems realistic you might have 50-degree "fresh air supply" ducts running through a 70-degree space which might have a reasonably high humidity level (I've never lived in a SPF home)...

    So do the "fresh air supply" ducts need insulation? If insulated, where it joins the HVAC system in either a "Simplified" or "Partially Dedicated" system, my mind sees that condensation potential now happening inside the HVAC system. So really, the only logically safe installation with regard to condensation potential is a fully dedicated system with insulated "fresh air supply" ducting throughout. Right?

    Thanks again and I apologize for any confusing nomenclature.


  3. GBA Editor
    Martin Holladay | | #3

    No need to apologize. Once the outdoor air has passed through the heat exchange core, there are no condensation worries for the ductwork connecting the ERV with supply air registers, or for the ductwork connecting exhaust air grilles to the ERV.

    Most HRVs (and some but not all ERVs) require a condensate drain to handle any condensation that occurs inside the appliance.

    In addition to the article that I provided a link to in my last response (“Ducting HRVs and ERVs”), you may want to read a valuable article by David Hansen that provides more information on ducting HRVs and ERVs: “Installing a Heat-Recovery Ventilator.”

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