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How cool is fresh air output from an ERV?

Austin G | Posted in General Questions on

I’m planning my ventilation system now. Mini splits are in, but we need an ERV.  I’m planning to pull stale air from the two bathrooms and dump into the bedrooms as they’re furthest from the mini split heads.

My thinking was that this will also help warm the bedrooms in the winter.  But what about summer?  How cool is fresh air output from an ERV when it’s 90 degrees?

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  1. Cramer Silkworth | | #1

    Depends on the efficiency of the unit. Roughly speaking, a 75% efficient unit saves you 75% of the temperature difference - so for a 75F house with 90F outside, the ERV supply air would be about 78-79F. (take 75% of [90-75], then subtract that from 90).

    ERV won't heat in winter, though. At anything less than 100% efficient the air coming out in winter is cooler than the house average temp, so the fresh air is actually cooling the space. But usually the airflow is low enough that you won't notice, as long as the unit is decently efficient (>75% at least, 85%+ ideal).

    1. Austin G | | #12

      I realize I’m not creating any heat with an ERV, just without a mini split head in the bedroom, my thought process is I would be moving 85% of the warm air from an area of the home that was well heated, and moving it to a room less heated.

      That’s very helpful info though, thank you.

  2. Jon R | | #2

    To repeat what Cramer said, an ERV almost always hurts (not helps) heating/cooling. On the other hand, the negative effect isn't large in BTU terms.

  3. Trevor Lambert | | #3

    You can neither cool a space in summer, or heat a space in winter with an ERV. All you can do is lessen the impact of bringing in fresh air that is differently conditioned as compared to the indoor air.

    85%+ is definitely ideal, in the literal sense of the word because no such ERV exists on the North American market. Even if you drop down to 80%, you're left with only four choices, only two of which flow more than 100cfm.

    1. Austin G | | #13

      For sure. Being a room without a mini split head, my thought process is I would be moving 80-85% of the heat from the warmest area of the house to a room without a direct heat supply.

      1. Trevor Lambert | | #16

        That's not how it works. You can't move heat around from room to room with an HRV or ERV.

        Let's say you've got an 80% efficient unit, and it's 0degC outside. If most of your house is 20degC, and your "cold" room is 16degC, here's what will happen. Let's assume your cold room is a small percentage of the total house, such that the average air temperature is basically still 20degC. The air delivered by the HRV/ERV will be 80% of the way between 0degC and 20degC, i.e. 16degC. So there would be literally no warming happening at all. Taking a more extreme example, let's say your "cold" room is only 10degC, then yes the 16degC air coming in will be warming that room .BUT, given the volume of air you're dealing with, this warming is negligible. It wouldn't be noticeable, and while you're moving some heat from the rest of the house to that room (indirectly), for every 4 units of heat you move you're sending one outside.

        HRVs and ERVs are for bringing fresh air in the house. They limit the amount of energy you lose to do that. That's it. Trying to get them to heat or cool any part of the house is folly.

        1. Jason S. | | #22

          Many ERV's and HRV's have recirculation modes, so saying "You can't move heat around from room to room with an HRV or ERV." isn't completely true. It's not a setting you want to run regularly though and it is true, as others have noted, that not a lot of meaningful heating or cooling distribution happens within the limited cfm numbers in which these systems operate.

  4. Cramer Silkworth | | #4

    @Trevor: Zehnders are in the mid 80%s. Not North American made, but they are available here.

    1. Jon R | | #6
      1. Trevor Lambert | | #7

        So the low 90% numbers you're looking at are from PHI, who clearly have a different testing methodology. You can see this because a couple of Zehnder models have both a PHI certificate and ones submitted to HVI. The CA350 PHI certificate says it's 94%, while the HVI certificate says only 88%. That's a pretty big difference. Note that this is an HRV, not an ERV. Generally ERVs will be several percentage points lower than their HRV counterparts. The only ERV Zehnder has submitted to HVI is the CA200ERV which is 77%. I see no evidence there of any ERVs cracking, or likely to crack, the 85% barrier in the HVI test.

        Why trust HVI instead of PHI? Simply for practicality. Most vendors of HRVs and ERVs sold in North America don't get PHI certificates (I think Zehnder is the only one). So the only way to compare apples to apples is use HVI.

      2. Lance Peters | | #9

        To add to what Trevor posted, Zehnder chooses to advertise their PHI rated numbers simply because they look much better than HVI numbers. They can do this because they're the only manufacturer who've bothered to get PHI certified.

        The reason their claims exist is because they test their units at reduced airflow rates, NOT at maximum airflow like the HVI tests require. When you reduce the airflow rate through an HRV or ERV core the efficiency of energy transfer goes up. This is why they designed their ComfoAir 200 model to throttle all the way back from 156 CFM to just 29 CFM... so they can tell you it's 94% efficient. That's twenty-nine CFM.

        I looked into this in detail and it turns out the Zehnder ComfoAir 200 ERV is no more efficient than a Panasonic Intelli-Balance 100 ERV, which costs less than 1/3 the price for just the unit and FAR less than that even for a typical installation. Zehnder fills their marketing pamphlets and manuals with extended acoustic spectrum (noise) measurements and the aforementioned PHI stats, and people who are not technically minded must look at all that data and think they're buying something really special.

        Zehnder does offer a comprehensive line of accessories and duct work, as well as excellent technical support and commissioning, so the high price isn't without some value. But their prices do limit their products to projects with enough room in the budget.

        I just have a natural dislike for companies who use questionable marketing tactics to push their product and/or promote their product's "superiority".

        1. Cramer Silkworth | | #10

          For what it's worth here's a good comparison of HVI vs PHI testing procedures:

          Neither system is perfect, and yeah you can't make an apple to apples comparison, but I still think the Europeans make better stuff than most of what's available here. But you do pay more for it.

        2. Trevor Lambert | | #11

          "they test their units at reduced airflow rates, NOT at maximum airflow like the HVI tests require"

          I'm not sure where you got the idea that HVI efficiency ratings are at max airflow, but it's not true. The HVI reports have multiple test points, and they use the highest efficiency number as the rating. As an example, the Panasonic unit you cited has a rated airflow of 100cfm, but the rated efficiency (81%) is at 53cfm. At 85cfm it's 75%. The efficiency at max rated airflow is unspecified.

          There's more to an ERV or HRV than the efficiency number. The CA200 is capable of delivering 76% more air than the Panasonic. The Panasonic unit is lacking basic features like a boost mode, and advanced features like night cooling bypass and the ability to automate ventilation based on environmental conditions, features that are all standard on the Zehnder. The Panasonic is too small for 90% of houses being built today. The lack of a boost function tells me they don't really understand the real world ventilation demands of a house (i.e. that it's highly dependent on occupancy, which set and forget does not address).

          1. Lance Peters | | #20

            It's been a while since I looked at the HVI site, and I remembered it wrong. Their wording is "Net airflow @ MAX rated SRE", not "Max airflow" as I thought. Still, my comments are valid and the Zehnder CA200 HRV is not 92% efficient. From their Products page on their website:


            "With a maximum capacity of 125 cfm, the CA 200 has been certified at 92% efficiency by the Passive House Institute."

            PHI isn't even a North American testing facility, but HVI is and has rated the CA200 HRV at 86% SRE @ 66 CFM. That's still pretty darn efficient, but it's a FAR cry from the 92% @ 125 CFM they're implying. That's shady marketing no matter how you cut it. Why don't they just use the HVI ratings like everyone else?

            You have valid points about the Panasonic, which is a rather basic unit and has lower overall capacity. BUT, it's HVI peak efficiency rating is 81% @ 53 CFM. The CA200 ERV version is 77% @ 64 CFM. Those two airflow rating don't exactly line up, but they were both tested at 85 CFM:

            85CFM @ 0C(32F) / Panasonic IB100 ERV / Zehnder CA200 ERV
            Power (W) / 68 / 38
            SRE% / 75 / 75
            ASE% / 80 / 79
            Moisture% / 74 / 73

            So the Zehnder CA200 ERV requires less power to run, but at an equivalent airflow rate is basically the same efficiency as the IntelliBalance 100.

            Most of the advanced features the Zehnder offers are not necessary and are of questionable value (night time cooling for example), and while the Panasonic does lack a boost mode, for the cost of a complete CA200 install with all the silencers, Comfotubes etc., you could have two IB100's and still have enough money left over to upgrade the kitchen from laminate to granite! OK, bad example... how about a serious window upgrade instead?

            I know I sound a bit broken record-ish bringing up the IB100 all the time when people mention Zehnder, but I really think it shows how over-priced Zehnder really is. Don't get me wrong, I appreciate that it's a well Engineered package with a great suite of options and accessories and great support from the company, but it's an EXTREMELY expensive option that seems to depend on creatively deceptive marketing. In my opinion.

          2. Trevor Lambert | | #21

            The reason Zehnder mainly uses PHI instead of HVI is that the bulk of their market is in Europe. All certifications cost money, so if they're going to choose one then it makes sense they go with one that is recognized in their main market. I don't think that's a conspiracy. They are open about where the numbers come from. As for the models that have both HVI and PHI certifications, of course their marketing department is going to choose the higher number. Find me one company that wouldn't do the same.

            And again, the CA200 and IB100 aren't comparable, so comparing the price is kind of pointless. I could point out that there are makes and models out there that cost less than half of the IB100, and throughput more air, but at lower efficiency. It's basically the same comparison you're making, substituting airflow for efficiency as the important spec. But in both cases, it's apples and oranges.

            When I shopped for an HRV, I ended up looking at the Zehnder CA350, UltimateAir 200DX and vanEE G2400. These are all comparable models; similar max flow rates, similar efficiency. There wasn't a whole lot of difference in price. Yes, the Zehnder was more, but not by a lot. As for all the expensive accessories, those are optional and so not germane to the topic. You can hook any of them up with any kind of ducts you like, and the silencers are not necessary.

  5. Doug McEvers | | #5

    Depending on where you live (zone) the supply air from an HRV/ERV can be quite cold in winter. HRV's have a defrost cycle to warm the exchanger. If you put the supply air into the bedrooms only you may find it to be too cool.

    1. Stephen Sheehy | | #8

      I live in Maine and have never noticed supply air from our hrv being cold (or hot.) It's only 24 cfm or less to Any room, so unless the supply vent is close to you, you probably won't notice. Even on boost, I can't feel any airflow.

  6. Austin G | | #14

    Thank you all for the info. I’m seriously considering installing both an HRV for winter, and an ERV with ductwork routed differently in the summer. No central hvac, so fresh air output would come directly from duct.

    1. Expert Member
      Malcolm Taylor | | #15

      A. Gouch,

      Just to circle back to your original question. Neither an HRV or ERV will help warm or cool the bedrooms. Even if the rooms are remote enough from the mini-split that they are appreciably hotter or cooler than the air being returned from the bathrooms, the air-volume is insufficient to make any useful difference.

      1. Austin G | | #17

        Seeing the real world numbers, I understand that now. The purpose of the hrv and ERV would be to minimize the raising or lowering of temperatures in certain rooms depending on the season. Ie. lowering the temp of a room in the winter and raising it in the summer.

        1. Expert Member
          Malcolm Taylor | | #18

          A. Gouch,

          Right, but what the responses are trying to say is that the ERV will not do that to any appreciable extent. So it isn't a strategy worth pursuing.

    2. Jon R | | #19

      > with ductwork routed differently in the summer

      Route/apportion according to ventilation needs, not heat flow.

    3. Charlie Sullivan | | #23

      Having HRV and ERV options is useful. I have an Zehnder with a swappable core, and I do find it useful to be able to swap the core. I run ERV all summer and in a few months of our long New Hampshire winter; the HRV in the shoulder seasons and the rest of the winter. Swapping four times a year is a bit of a pain, but not bad. I could see wanting a way to do that by flipping a switch instead. I might even want to flip a switch to run HRV in the summer for just a half hour after showering--maybe it could switch automatically based on humidity sensors in the exhaust air and the outdoor air.

      However, I have never wanted to change the distribution of supply and exhaust venting.

      1. Stephen Sheehy | | #24

        Charlie. I think I need to clean my hrv core (Comfoair 200). How difficult is it to remove the core? How heavy is it? How do you clean it? Thanks

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