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Community and Q&A

Why is my brand new HRV filled with water?

erica99 | Posted in Mechanicals on

We did a DER in MA recently (complete September 2015). Part of the mechanical system design includes a Zehnder 350 that is designed to run 24/7 on medium flow to have sufficient air exchange. We have really high heat right now and high humidity. I noticed just now that it was making a strange noise. I opened up the cabinet (following the instruction provided) and found the right side (leads to the house air handlers) was full of water. There is a condensate outlet drain on the left but there is no way for water to get from the right to the left. I got in there with a shop vac and thoroughly cleaned out the water and dust. Turned it back on and it was fine.
It appears that the Contractor installed per the instructions (drain is supposed to the be on the left) but I suspect something is amiss with the installation. I reached out to the contractor to look at it and possibly fix whatever is wrong but I am hoping someone can offer an opinion or some thoughtful questions to ask or investigate to make sure this doesn’t happen again. Can you help us?

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  1. Regulajo | | #1

    I don't know a lot about HRV's, although I will be installing one in the home we are building.

    Could it be as simple as being installed out of level, or the HRV needs to tilt to the left just a hair so it drains to that side. Seems too obvious, but that is what came to mind first. In my research, Zehnders are supposed to be about the best.


  2. GBA Editor
    Martin Holladay | | #2

    Your contractor is the first person to turn to. If you don't get adequate service from your contractor, you should contact Zehnder. Here is the contact information:

    Zehnder America Inc.
    6 Merrill Industrial Drive #7
    Hampton, NH 03842
    [email protected]

  3. charlie_sullivan | | #3

    Zehnder is very responsive and I'm sure they can help you.

    An HRV can have condensation dropping out of the inside air in the winter, as it gets cooled on the way out. In the summer, with air conditioning, the reverse can happen--the condensation drops out of the outside air on the way in, as it gets cooled. Your description of where the water is sounds reversed from that, but I might be understanding that wrong without having it in front of me to look at. But my point is that the expected location of the condensation is reversed for summer vs. winter.

    In Germany, the summer humidity isn't as bad as the eastern US, so they can probably get away without ignoring the summer condensation. So it's possible that the instructions for the single drain location are good for some climates but that your location really needs a dual drain.

    Another option might be to switch from an HRV core to an ERV core. Many Zehnder models have swappable cores. The HRV core exchanges only heat, whereas the ERV core exchanges heat and humidity. That means that some of the humidity in the incoming air stream gets taken out and goes into the exiting air stream, usually enough to prevent condensation. And then you have drier air coming into the house from the ventilation system in the summer when the A/C is running. In your climate, that could save significant air conditioning energy in the summer as well as leave you with lower humidity indoors. You could even consider swapping cores seasonally, since you have a unit with that capability.

    I'd be curious what Zehnder says--I hope you'll keep us updated even though they can probably be more helpful than we can.

  4. erica99 | | #4

    Follow up to original posted question: we spoke to Zehnder and really appreciate their time and interest in looking for a solution to this technical problem. Unfortunately there does not seem to be a smoking gun and Zehnder could only help us to a point because we did not have them design the independent ducting distribution system and they did not commission the system. There is a reason why we only purchased the HRV from Zehnder and not a whole distribution system and commissioning support: our project house is a small footprint renovation and an early design decision was made to optimize what little space we had and utilize the next forced air ducting to distribute the HRV throughout the house. Zehnder does not commission our type of ducting set up.

    What did come out of longish phone conversations with the folks from Zehnder:
    1. Controlled 8-in. dia. make up air on ducting system for Kitchen range hood would likely through off the humidity and pressurization such that the condensation could occur within or adjacent to the HRV and build up significant water. It has been a really hot and humid July & August in the Northeast.
    2. Flexible ducting is not preferred and could exacerbate the issue in 1 above.
    3. The metal cabinet for the Zehnder comes with punch outs on both sides so an additional drain and trap can be retrofitted to relieve the water without compromising the HRV.

    Another thought occurred to me: maybe the hybrid hot water heater is making this all worse. Because of the tight footprint, the basement mechanical equipment is close to each other. Not all the ducting (most of it) leading up to the HRV is fully insulated. The hybrid hot water heater chills that basement nicely during the summer and I will bet it lowers the temperature of the uninsulated ducting enough to induce more condensation.

    Proposed solution(s):
    A. Add drain on other side of HRV using Zehnder supplied part.
    B. Change over flexible ducting to rigid.
    C. Insulate the ducting everywhere.

  5. GBA Editor
    Martin Holladay | | #5

    I'm sure that Zehnder is giving you good advice.

    A few observations:

    1. Dedicated ventilation ducts are always preferable to using forced-air heating and cooling ducts for ventilation.

    2. If Zehnder didn't commission the system, the question arises: what method did you use to verify the ventilation rate? Needless to say, if you are operating a ventilation system for 24 hours per day during hot, humid weather -- a time of year when it is always best to minimize your ventilation rate -- it's essential to know your ventilation rate in cfm, because you don't want to overventilate.

  6. verygood | | #6


    If I had to venture a guess, I would say that your solution 'C' would be the right one. Since you have a heat pump water heater nearby, it is likely cooling down the air around the supply duct, and since the air is warm and humid it is causing condensation on the inside of the supply duct. If it were insulated all the way from where it exits the house to the unit, my guess is that would solve things. We had a similar issue, but in reverse on a Comfo air 550, where in the winter (climate zone 4a) we were getting condensation on the outside of a supply duct where a few inches of the duct was left uninsulated after a repair. As soon as we insulated it, the problem vanished.

    Hope this helps!


  7. charlie_sullivan | | #7


    It does make sense that Zehnder can't take full responsibility without full control of the system. I'm not sure I buy their explanations of how the system would need to be thrown off for condensation to occur. With an air-conditioned house, an HRV, and high humidity outdoor air, it should have condensation inside the unit. The heat transfer going on in the core of the HRV is a lot more than you get from the uninsulated ducts. So I don't think that changing duct types or insulating ducts would make a significant difference. So I would recommend your option A.

    Did the discussion include considering the option to switch to an ERV core in your unit?

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