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Community and Q&A

Help me understand HRV drains

Trevor_Lambert | Posted in Mechanicals on

Can someone help me understand the design principle of the condensation drain on an HRV? My Zehnder unit has one drain, which I’m assuming is typical. A quick examination shows this drain is in the outgoing stream, so condensation in the outgoing stream will go down the drain. What is supposed to happen to condensation in the incoming stream? When it’s hotter and more humid outside, it seems like there is the potential for condensation here too, so what am I missing?

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  1. GBA Editor
    Martin Holladay | | #1

    Condensation occurs when warm humid air contacts a cold surface. The exhaust air stream from an HRV is, indeed, warm and sometimes humid, but the last place it is likely to encounter a cold surface is in the HRV core during the winter. During the summer, there isn't any likelihood that the exhaust air stream will encounter a cold surface.

    More information here: "Preventing Frost Buildup in HRVs and ERVs."

  2. Trevor_Lambert | | #2

    I am genuinely perplexed by your statement regarding condensation in the HRV core, particularly there not being a cold surface there. You link to an article discussing frost in hrv cores. There has to be a cold surface there for frost to form. And frost is basically a form of condensation (may technically be sublimation, but that's a technicality). If the condensation that comes out the drain isn't happening in the core, then I can't imagine where it is occurring.

    Cold is a subjective term. If the surface temperature is below the dew point, isn't that cold enough to cause condensation? Let's say it's 86F and 80%rh outside, the dew point is 79F. If the inside air is 70F, isn't there a possibility of getting a condensing surface in the core?

  3. GBA Editor
    Martin Holladay | | #3

    My answer acknowledged that the HRV core includes cold surfaces in winter. Once the exhaust air leaves the core, however, and is traveling in the exhaust duct beyond the core, there shouldn't be any more cold surfaces encountered as the exhaust air leaves the building.

    HRVs are designed to handle expected condensation. Many HRVS include drip pans under the core. The few HRV manufacturers that omit drip pans under their cores are confident that their design is somehow robust enough to have avoided the condensation problem.

    You told us that your Zehnder has a drip pan and a drain. I'm not sure where your drip pan and drain are located in your Zehnder, but I imagine that the engineers who designed the unit put the drip pan and drain in the right place. Are you experiencing any problems with condensation and strange drips?

  4. lance_p | | #4

    Martin, you said:

    "The exhaust air stream from an HRV is, indeed, warm and sometimes humid, but the last place it is likely to encounter a cold surface is in the HRV core during the winter. "

    The first place exhaust air encounters a cold surface is in the HRV core.

    On another note, I was under the impression all HRVs had condensate drains? If an HRV didn't have a condensate drain, where would the inevitable condensation go? If it's 73F @ 50% RH in the house, the dewpoint of the exhaust air is 54F. Any time fresh air coming in is significantly less than 54F, exhaust air will condense in the core.

  5. GBA Editor
    Martin Holladay | | #5

    Right -- the first as well as the last place. I thought you were concerned about condensation in the duct conveying exhaust air from the HRV core to the outdoors, and I wanted to make it clear that condensation in the exhaust duct is virtually impossible.

    You are correct that all HRVs are manufactured with the ability to connect a condensate drain (although it's possible that you don't need to connect the drain if you live in a warm climate). Some ERVs don't come with a condensate drain because the manufacturer has determined that they aren't required.

  6. Trevor_Lambert | | #6

    The confusion is due to the fact that "last place it is likely" is a common figure of speech which roughly means, "this is the place it won't happen".

    I should have given some context for why I am asking the question. I'm not concerned about anything in the exhaust duct. I really want to understand where the condensation can and cannot occur, and why, so that when I put my home made heat exchanger in series with the HRV, I know how various choices in orientation will affect how I have to deal with a condensation drain.

    I was surprised to see that there is no place for condensation to drain out of the HRV anywhere in the incoming path, because both of the commercial heat exchangers I looked at (Zehnder and UltimateAir) have condensation drains. Both are intended to be in the incoming stream. So if there is no drain on the HRV, why is there one on those pre-conditioning heat exchangers? Is it simply because the temperature differential is potentially greater?

    Based on physical restraints, the best place for my heat exchanger is above the HRV, with the heat exchanger oriented such that the air flow is in line with gravity. Obviously, if condensation forms on the heat exchanger in this configuration, it would be bad; it would fall into the HRV, and the HRV has no drain there.

    In winter, I don't think this is a concern. In summer, it seems to me like condensation will form on the heat exchanger core. Then again, the same seems to me to be true with the HRV itself, and apparently I am wrong there. I suppose the prudent thing to do is to include a condensation drain and ensure the orientation in the summer position directs condensation to the drain rather than back to the HRV.

  7. Jon_R | | #7

    > there is no place for condensation to drain out of the HRV anywhere in the incoming path,

    I've seen other designs where the drain works for both paths. It could be pretty subtle - is it completely clear that this isn't the case for the Zehnder?

    1. Trevor_Lambert | | #9

      Could you share one of these designs? I can't picture how a drain could service two paths separated by air and vapour barrier. On an ERV it would be different.

  8. GBA Editor
    Martin Holladay | | #8

    Q. "Why is there one [a condensation drain] on those pre-conditioning heat exchangers?"

    A. If you are talking about a heat exchanger connected to a buried glycol loop, the answer is simple: The cold glycol that circulates underground can be quite cold -- as cold as 32 degrees F or colder in some conditions. When incoming fresh air hits these cold coils, condensation is possible.

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