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Community and Q&A

Weird question about HRV core damage

Trevor Lambert | Posted in General Questions on

I sent this question to Zehnder, and couldn’t get any answer, so I figure I’ll try here.

“Several months ago, when inspecting the HRV core of our CA550, I made the mistake of thinking I could carry the core with the strap that you use to pull it out. It seemed very tight and secure, but apparently that status varies depending on its orientation. It slid out rather quickly and impacted on the floor on one corner. The damage seemed to be fairly minimal, so I reinstalled it without thinking too much more about it. (several rows of fins were wavy, with a small gap between them; this is only apparent in maybe 15% of the area on only one of the four sides of the core).

I’ve noticed recently that the unit does not seem to be as efficient as it once was. For example, here are some temperature readings I noted:
outdoor air 7C
supply air 12C
return air  16C
ambient air 21C

Edit: more recent readings in cooling mode seem to show about the same:

outdoor air 31C
supply air 27C
return air 24C
ambient air 30C

This seems to look like about a 50% efficiency. I don’t have specific numbers from earlier, but I do recall that it used to have no problem delivering supply air of 16C with an outdoor air temp of barely above freezing (so in the ballpark of 80%, by my estimation). 
Does it seem plausible that what I thought was minimal damage is the cause of the large difference in temperatures I’m seeing? I tried cleaning the core recently, and it didn’t make any noticeable difference. I’ve attached a picture of the damaged area (ignore the water drops, as the picture was taken right after cleaning).”

A follow up question, if I decide to order a replacement core, I have the option of getting an ERV core instead of an HRV core. I can’t decide on whether to do this. In the winter, we struggle to keep the house humid enough. So the ERV core will be worse for that scenario. We struggle to keep the humidity down in the summer, so it will help with that but the question is how much? I can already drop the dew point of the incoming air to about 18C with a ground source preconditioning loop, and in fact when that is running the HRV runs in bypass mode pretty much the entire time. What is the simplest way to calculate how much the dew point will change using the ERV core, assuming a couple of conditions (inside DP of 14C, outside DP say at 20C and 25C)? If I assume a latent recovery efficiency of 0.6, is it a reasonable approximation to say the DP of the delivered air will be the higher number minus 0.6 of the span between the two numbers? E.g. 25-(25-14)*0.6= 18.4. I realize the relationship is probably not linear like that, but does it end up in the same ball park at least?

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  1. Ethan Foley | | #1

    I find it really hard to believe that a small amount of damage like that would have a large impact on efficiency. It looks like only a couple plates are spread apart. It's mostly a surface area thing, so a couple plates should be minimal. Could it have damaged it enough to cause more cross-leakage? If you've changed your airflow rates, that would affect the efficiency as well. (more airflow, lower efficiency)
    An ERV should help keep the humidity up in the winter time, it will transfer moisture and heat from the warm moist (relatively) outgoing air to the cold and dry incoming air. Based on the issues you've described, seems to make good sense to me to replace it with an ERV core if you are replacing. Won't help at all in the summer if the HRV is always in bypass mode though.

    1. Trevor Lambert | | #2

      "I find it really hard to believe that a small amount of damage like that would have a large impact on efficiency."

      That was my thought when it happened, and I put it back in without a moment's hesitation. It was only later when I noticed the temperatures that I cast my suspicions back in time. While unlikely, I can't think of another factor. Ventilation rates have not changed. Not knowing exactly what the core looks like in three dimensions internally, I was wondering if the gaps in the plates actually allow cross contamination between air flows. It's also possible there's hidden damage.

      I really confused myself when thinking about our humidity levels in the winter. My brain remembered the general idea that the ERV would be of no benefit in the winter, and I filled in the gaps with some specious information, forgetting the fact that the humidity would be higher with an ERV in the winter, not lower. The fact is that our winter humidity hovers around 30%, maybe varying from 25-35%; on the low end for human comfort, but I think the recommendation for winter time in cold climates is to keep the humidity fairly low, to prevent too much moisture drive into the wall assembly. I think 30% was even listed as the upper limit, although considering how air tight our walls are, I never took that to heart and thought of anything below 30% as "low", hence my feeling of "struggling to keep the humidity up". So I still think the ERV in winter might be a bad change, because a big shift in either direction from 30% is probably undesirable.

  2. Expert Member
    Akos | | #3

    I would be careful with efficiency measurements when the temperatures are close. Unless you use the same meter for all, the accuracy between units could easily be 1C to 2C, which makes a big difference on efficiency calcs.

    If your HRV will run with a single blower you can do a quick crossover test. Partially block off the port on to get around 1" wc pressure and see how much flow you get on the side with the blower disabled. I think it should around the order of a couple of percent (~1%-3%) of rated flow.

    1. Trevor Lambert | | #4

      The temperatures are coming from the machine itself. So for comparison measurements, the only uncertainty is the resolution, which granted is +/-1C for each stream. However, that error should average out over multiple readings, and I have dozens if not hundreds of data points from before and after the incident (I checked it quite frequently when the unit was newly installed, both out of curiosity and to verify the setup and function of my homemade ground source heat exchanger). The numbers were consistently several degrees higher in the supply air in the before time compared to later on.

      I'll try the crossover test, but I don't have a way to measure the pressure or flow accurately. Maybe if it's a big enough leakage I'll be able to tell just by feel.

      1. Expert Member
        Akos | | #5

        If you don't have the proper equipment, I doubt you'll be able to tell if there is too much cross over. Most HRV/ERV units use the blower to pull air through the core which means you won't be able to feel any leaks.

        I would keep running the unit as is into the winter months. If the core efficiency is also low at that time, you can try to block off the channels in question, if that doesn't work then replace the core.

        I would definitely get an ERV core as replacement. I find at home, even with the ERV, my winter RH can get low especially during a cold snap.

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