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Is a ducted HPWH a good idea?

Craig Smith | Posted in Energy Efficiency and Durability on

I am planning to build a house in zone 5A. It is to be a high performance home with levels of insulation and air leakage approaching Passive House standard. My builder likes to install a heat pump hot water heater with inlet and exhaust ducting to the outside. The HPWH would be installed in a conditioned basement. Is this a good idea? Is there a “cold weather” HPWH suitable to this application?
This article describes a test of ducted vs unducted HPWH:
https://www.aceee.org/files/proceedings/2014/data/papers/1-1203.pdf
The ducted HPWH reduced home thermal load by 7.8% (5.7 kWh/day), while increasing HPWH electrical load by 4.8% (0.44 kWh/day). So a possibly significant savings. The problem is that the ducted HPWH was referenced to a crawl space that maintained 44±2°F during the heating season, and the temperature of the closet containing the ductless HPWH was depressed to 56° from overall indoor temperature of 71°. The result implies that the COP of the HPWH was nearly the same at 44 and 56°. The conditions are perhaps not so relevant to my situation.

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Replies

  1. Trevor Lambert | | #1

    What is the model they are suggesting? I don't think there is one that can function efficiently in that configuration in zone 5.

  2. Steve Knapp CZ 3A Georgia | | #2

    Craig,

    Where will it be installed? I have a Rheem HPWH in my garage (Zone 3) and, wish I had ducted it so I could route the exhaust to the exterior during the winter.

    1. Craig Smith | | #4

      It will be installed in a conditioned basement. It's a good point and I've edited the question to include this information.

  3. Brian Wiley | | #3

    Hi Craig, I currently have an AO Smith hpwh in cz5 that I’m thinking of changing the location of to a smaller utility closet which would require ducting. I’ve yet to find any source that suggests drawing intake air from the outside is a good idea for our climate zone.

    There is a good article by Martin Holladay that covers things about this topic: https://www.greenbuildingadvisor.com/article/attaching-ducts-heat-pump-water-heater

    1. Craig Smith | | #6

      Thanks for the pointer to the Holladay article, which I had missed. In it, he says not to duct to the exterior in a cold climate because it reduces the efficiency of the HPWH. Unaddressed is the relative value of increased HPWH electrical load vs reduced home heating load achieved through ducting.

      1. Brian Wiley | | #8

        I’ve wondered the same thing. I’ve also wondered if simply switching it to full electric during the coldest 3–4 months of the year—and paying the energy penalty—would be easier on the compressor. I’m any case, just utilizing the heat pump feature for 8–9 months per year certainly is better than running a traditional electric water heater for the full year in terms of environmental concerns.

  4. Expert Member
    Rick Evans | | #7

    Hi Craig,

    I believe you can but the unit's operational range in Heat Pump mode may be limited. Rheem states that the range of operational usage is between 37F and 145F (see spec sheet). But, I've also read in their user manual that temperatures below 15F may inhibit airflow...

    Rheem's HPHWs are designed to meet Energy Star's "Northern Climate Specification" (Tier3).

    From Energystar/NEEA (see attached):

    "Northern Climates" are climates with 4,000 heating degree days
    or higher and average ambient temperatures, below 60 degrees Fahrenheit.

    "Units meeting Tiers 2 [this includes Tier 3, like Rheem] and above are expected to provide
    configuration options for semi‐conditioned, unconditioned, AND conditioned spaces
    such as heated utility rooms. Split system applications are not covered by this
    specification".

    Since they list 'unconditioned' space in this, then I presume this includes ducted to the outdoors.

    Ultimately, I might contact Rheem to be safe. If your contractor has been installing Rheem units in CZ5 with success then GBA should write an article about it.

    I have a Rheem HPHW unit myself in the basement and couldn't be happier with it. You can always keep it within your conditioned envelope and still enjoy the efficiency.

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