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Community and Q&A

No Condensate from Heat-Pump Water Heater

88Clayton | Posted in Mechanicals on

Just moved into my new home a few weeks ago.  I had a heat pump water heater installed (in basement).    During the first week or so, it was producing condensate, which could be seen dripping out into the drain.   However, it’s been bone dry every day the last two weeks.  No observable condensate at all.  Surely it’s not clogged after only 3-4 weeks.   Is this normal?   I’m having no trouble with hot water.  All good there.  The fan and compressor run pretty much all the time.  I figured there would always be some level of condensate.   I’m using this HPWH in efficiency mode, which relies on the heat pump exclusively.

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  1. bfw577 | | #1

    Thats normal. I use a bucket and use the water for my houseplants. I have had no condensate the last few weeks. Put a temp humidity sensor down there and you will probably find the humidity to be very low. Cold air in the winter usually has very low dewpoints.

  2. _jt | | #2

    I also have no condensate in the winter. Basement temperatures are about 60 degrees 33% humidity.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Clayton.

    I'll third the last two posts. I had the same experience in the winter with my heat pump water heater. Very little condensate.

  4. Expert Member
    BILL WICHERS | | #4

    You may find you have more condensate being generated during spring/summer months when the humidity levels tend to be higher. In the winter, humidity tends to be lower so there is less moisture in the air and that's where the condensate comes from. Basically if you have the cold coil of the heat exchange at a temperature below the dew point of the air, you'll condense moisture out of the air which is what the condensate is.

    Eventually, as the humidity level drops, the amount of moisture condensed out of the air won't be enough to actually run off as condensate since the airflow of the fan can evaporate small amounts back into the air. At some point, the dew point will drop below the temperature of the cold coil and then you'll get no condensation at all.

    As others have said, you're probably seeing normal operation for the conditions you're currently experiencing and have nothing to worry about.


  5. Joel_Charles | | #5

    Has there been any research done on well-sealed well-insulated homes into how a) how much heat pump water heaters in cold climates drop indoor humidity in winter and b) whether a certain level of humidity is required to ensure their efficient operation? There was a comment on a separate article about HPWH's in which a user in Minnesota noted that their HPWH seemed to operate less efficiently at lower humidity. The question above seems to be indicating that the HPWH performance did not suffer (though it doesn't mention whether it consumed more juice to get that performance). I will have HPWH in a well-sealed, well insulated home without a basement and I'm concerned about it dropping humidity too far.

  6. Expert Member
    Dana Dorsett | | #6

    >" I will have HPWH in a well-sealed, well insulated home without a basement and I'm concerned about it dropping humidity too far."

    Unless you live in the arctic or something that's not something to worry about.

    But for the record, what is your location?

    1. Joel_Charles | | #7

      Thanks Dana. Wisconsin (Zone 6).

      1. Expert Member
        Dana Dorsett | | #8

        In zone 6 Wisconsin the outdoor dew points average in the teens to low 20s in winter, which is dry, but not arctic dry. Very tight houses usually NEED dehumidifcation even in winter in zone 6 climates, which is easy to manage by bumping the ventilation rates to suit.

        As a practical matter a heat pump water heater needs to stop pulling humidity from the air when the indoor dew point is in the low to mid 40s F, otherwise it would need to incorporate defrost management- the coils would frost up. Assuming a worst case of dehumidifying to a 40F dew point, at 70F that corresponds to a relative humidity of about 33%, which is still in the human-healthy & comfortable range.

    2. Expert Member
      BILL WICHERS | | #9

      Actually Dana, extremely low humidity probably does reduce the effectiveness of an HPWH at heating water. It’s like thinking about an air conditioner “backwards”, where the extra energy needed for latent heat removal actually helps you. That extra energy energy from condensing some water out of the air put a little more heat into the water in the tank.

      I’m used to thinking about it the other way around where high humidity levels result in lower sensible BTUs from an air conditioner (in datacenters we only care about sensible BTUs), but for an HPWH it actually means there is more available heat to pull out of he environment and pump into the tank.

      How low the humidity would have to be to actually see a real impact on HPWH performance I don’t know, but I’d guess something less than maybe 20ish percent or so.

      Any thoughts?


      1. Expert Member
        Dana Dorsett | | #10

        >"Actually Dana, extremely low humidity probably does reduce the effectiveness of an HPWH at heating water. "

        Of course the heat of vaporization recovery makes a difference, but I don't know of a study with the data showing how MUCH difference it makes. It has to be reasonably humid for a standalone room dehumidifier or heat pump water heater to be extracting much latent heat. They are a slam dunk solution for locations/climates with high latent cooling loads.

        What is pretty clear is that an HPWH won't over-dry a home in winter (which was was one of the concerns) without providing some sort of defrost function in the controls, a function that SFAIK doesn't exist, and that below dew point of 35-40F the efficiency won't change very much, since it's sensible-only heat being extracted. Only in places or situations where active humidification would be required to keep the humidity in a human-healthy range even in a TIGHT house would there be any reason to be concerned about how much the water heater was removing from the indoor air.

        1. willymo | | #11

          Dana, this brings up a question I have about safe basement humidity levels. In Spring/Fall, with basement temps in the mid 50's, to get to a DP of 40° you would need 60% humidity. Is that problematic from a health standpoint? I know for conditioned space (~70°) this would be a problem, but how about for a basement?

          Thanks, Bill

          1. Expert Member
            BILL WICHERS | | #12

            High humidity itself isn't bad for your health, although it might be uncomfortable, what happens is high humidity makes it possible for stuff like mold to grow more readily and that's where the health issues come in.

            It can be difficult to control humidity levels in a basement unless you have a vapor barrier under the slab, and insulated perimeter walls (basically an insulated foundation). The only remaining options are either ventilation or a dehumidifier, both of which will consume energy -- especially the dehumidifier.

            I run a small dehumidifer in my basement and have noticed it runs less and less as I gradually complete my basement insulation and air sealing project. My home was built in 1978, so before most of the energy codes related to such things.


  7. willymo | | #13

    Bill, I was using high humidity as a proxy for mold/mildew health issues- should have been clearer. I know that, for instance, 70° and 70% humidity in a basement will lead to problems. What I am less clear on is in a 50° or 60° basement, what is a dangerous humidity level?

    My experience with old house basements, is that first you expel the water with drains and sump pumps, and then you dehumidify. (actually, first you drain ground water away from the house!) I installed the HRV this fall, and so far it is drying out the basement much better than the dehumidifier ever did, and at MUCH lower energy use. Of course, in the Northeast this only works in the fall/winter/spring, when outside DP's are lower than the basement DP.


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