GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Sizing an ERV

MSSUSR9501 | Posted in Mechanicals on

So I think I am starting to be able to read and understand the ratings in the HVI spreadsheet.

The HVI ratings include net airflow at the max SRE
Max SRE appears to occur at lowest flows and temperatures.
It looks like SRE declines as outside temp increases – this makes sense since the delta T or temp difference is lower.
It looks like SRE declines as airflow increases – this also makes sense as the residence time in the core is reduced with higher airflows

In my area (zone 5, western MI just north of Grand Rapids), code requires airflow based on square footage and room count.  After reviewing floorspace and room count with my local mechanical inspector it looks like I need to move 75 CFM continuously.  I would like to be able to move more on demand to exhaust humidity from bathrooms.  I plan to exhaust from three bathrooms and a powder room.  Fresh air will be returned close to an existing cold air return.  Two baths are in the basement.
ERV  will be in the attic.  Ducting is all 6 inch rigid with an 8″ oval to get from basement to attic.  From the ERV to the fresh air inlet and the stale exhaust is relatively short, perhaps 15-20 feet of ducting with a couple of bends.
The stale air inlets will be on much longer runs to get to the ERV with a max effective length of 220 feet (after factoring in pipe length, bends and turns etc.).
Based on other suggestions found here, I plan to also put a stale air inlet in the attic and put vents from the living room cathedral ceiling to the attic (conditioned space, 5″-6″ closed cell foam on the roof deck).  Mechanical inspector indicated both of these aspects would be acceptable.

It seems to me that I need to find a unit that moves 75 cfm at its lowest speed and accept that the only time I get the max SRE is when it is really cold out.  Most of the time the achieved actual SRE (or TRE in summer) will be closer to the lower levels advertised for each unit.

When I run the boost after a shower, the SRE or TRE will decline as I am pushing more air through the core.

Am I understanding all this correctly?
Thanks for any comments.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Akos | | #1

    Yup, you pretty much got it. I wouldn't worry too much about efficiency during boost, these tend to be short lived an hour or two at most. Putting the fresh air feed near your return is a good option as it can be pretty cold once winter sets in.

    Since the unit will be running at max during boost, it will make more noise, so I would recommend at least a length of flex on both supply and return ducting to limit noise (you can also use an in-line duct silencer).

    I would only put stale air pickups in the most used bathrooms. You need at least 40CFM to properly clear a bathroom, so you'll be limited on the number of bathrooms you can exhaust. The rest of the bathrooms can have a standard exhaust fan.

    A better option is to use the extra exhaust capacity for a stale air pickup in your kitchen area. Cooking tends to be the biggest source of indoor air pollution, a larger stale air pickup there really helps with this. This should be at least 8' away from your range.

  2. MSSUSR9501 | | #2

    Thanks very much for your comments. I understand your point regarding boost operation, makes sense.
    Code requires some sort of exhaust from each bathroom. Adding a ducted-to-exterior fan in the powder room will be a pain. The basement baths have existing ducted-to-exterior fans and are a future remodeling project so in the short term that will work.
    What would be sweet would be automatic dampers that would open when boost is requested or the light is turned on for the less used baths. Not sure how to find and deploy that capability though or if it could be integrated with the ERV control.

    I had thought about short flex at the ERV connections to reduce vibration and noise. Am also slightly concerned about transmission through the attic trusses the ERV will be mounted to. Will probably try it per manufacturer instructions and if it is still an issue, look at adding anti-vibration pads. ERV will be wall mounted on cross members attached to truss vertical members rather than sitting on bottom chord.

    Thanks again Akos for your comments.
    Happy New Year.

    1. Expert Member
      Akos | | #3

      You can do flow control for each bath but the cost would probably be more than a standard bath fan. The idea is to install a normally closed damper but adjust the closed position to still supply some nominal amount of flow. Wire the actuator to a motion sensor so when you enter the bath, the damper opens up. Because this significantly reduces the pressure drop at that location, most of the flow would be diverted there.

      An ERV should have minimal vibration, the only time I've had issues is when one of the blowers was defective and unbalanced. Vibration pads won't hurt but tend not to do much.

      Spring mounts are the best for this, any local industrial supplier sells these for hanging pipes. A proper vibration mount such as this can also work:

      Air flow noise with hard pipe is a real issue, even an inline air filter doesn't do much to attenuate it. 4' or so of flex or duct silencer works the best.

  3. MSSUSR9501 | | #4

    Thanks very much.
    Yes I understand the cost may be higher but I am a little bit of a tech geek and it would be interesting to look into the damper solution you mentioned.

    I'm starting to wonder if 6" duct is large enough. It will handle the low speed (which is 90% of the time to be honest) but when the boost mode is on, the velocity in the duct is going to be pretty high. I just saw some guidelines suggesting that 6" round is only good for about 85-100 CFM. At 85 CFM air velocity must be a little more than 7 feet per second. Looking at the EV Premium M from RenewAire, the top flow is over 200 CFM at a static pressure of .25 in wg

    1. Expert Member
      Akos | | #5

      At 200CFM, you need larger ducting. You generally don't want to hear when the unit is put on boost, if the velocity is too high at any of the supply or pickup registers, you'll definitely notice. High velocity at the main trunk is not a problem as long as you are not hitting your static pressure limits. You have to be careful with is the wall termination, some of the standard wall caps are extremely restrictive.

      The EV Premium doesn't seem to be too bad for the price and performance. Nice that it comes with ECM blowers. I don't see any information on how defrost is handled with the unit though, I would check with the manufacturer if they are fine in cold climate.

      Their Premium L unit is not much more than the M unit and at lower flow rate it gets you close to an extra 10% efficiency bump. Might be worth it just for that plus you also can get a bit higher flow on boost. The filter on it is also 2x the size of the M unit which means it should last twice as long.

      There are higher efficiency units out there, if you don't mind a bit lower flow, you can't been the cost and performance of the Panasonic Intellibalance 100.

  4. MSSUSR9501 | | #6

    I have to say, it is comforting to hear some of my own thoughts being expressed by another.

    I have been bouncing between the EV Premium M and L and also looking at the Panasonic units. The Panasonic Intelli-Balance 200 is quite a bit more expensive though.
    The incremental cost of the L over the M is low enough to make the unit very appealing and as you point out, the flow and efficiency at normal state are a better fit for my needs.

    I notice that the EV Premium M and L have 8" flanges on the ports but the actual port size is still only 6". They make no mention of snapping or breaking out the 6" insert if using 8" ducting.

    My wall terminations are Famco units with a built in damper. I chose 6" but have not trimmed out the outside yet. It would not be an insurmountable challenge to remove and replace with 8" units. For the inlet side, I simply removed the damper.
    They are these units:

    My struggle is whether it is worth the added effort and cost. The ducting in the basement and up to the attic should only be pulling half the total flow and 6" works for that even on boost. The only place the full flow is experienced is in the attic where the upstairs and downstairs branches join the main trunk and run to the ERV and of course the ERV inlet and exhaust.
    I think I'll plan on 8" for the attic, leave the basement at 6". If the 6" wall terminations are an issue for noise or flow, I can swap them out in the spring with minimal effort.

    Thanks again Akos for your thoughtful comments. I appreciate being able to bounce my thoughts off another.

    1. Expert Member
      Akos | | #9

      Famco doesn't provide free area or equivalent length, but they do look to be not too restrictive. The bad ones the slim low profile triangular shaped ones. A 6" one will be fine for your nominal flow rate, if it is a bit restrictive on boost, so be it.

      Going up to 8" feels a bit overkill, but 7" would not hurt for any long runs. There is no need to change the ports on the unit, you don't want to mess with these as the balancing damper is usually located there. Use a 6" to 7" reducer right on the unit and run 7" duct from there. Once the feeds starts splitting off to the different zones, smaller pipe from there is fine.

      The Panasonic IB 200 does seem to be overpriced. You can also look at the Broan/VanEE/Venmar ERV250/G2400E ECM (they are all the same unit). They are a bit less money, higher efficiency and are rated for cold climate. Maybe with the cross flow core the Renewair can get away with no defrost control, but it seems suspect.

  5. exeric | | #7

    Maybe I missed this. Is your attic conditioned? ERVs should exist in conditioned space to minimize temp loss in the exchanger. A related aspect is that you usually want it in the living space, e.g., a closet, so you have easy access for changing it's filter.

    1. MSSUSR9501 | | #8

      Yep, attic is conditioned space.
      We thought about putting it in the ceiling but decided it was best fit in the attic. It is close to the gable wall with both the OA inlet and RA exhaust terminations. It ends up being over the main bedroom. The access hatch is in the walk-in closet and code required a 24" walkway from the hatch to the appliance. Not ideal but not a total pain either.
      I plan to follow some of Akso's suggestions from other threads also, putting a stale air intake at the top of the attic and an open vent from the living room cathedral ceiling to the attic. This should provide circulation of air through the conditioned attic.
      I also plan to put a filter box inline on the inlet side of the ERV to spare the ERV filter and reduce the frequency of filter changes.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |