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Solar Minisplit Hot water Heat Pump

jberks | Posted in Mechanicals on

I’ve been on vacation last couple weeks, so usually I get future thinking more than usual.

Minisplit DHW isn’t really a thing yet in north america, but I know it’s coming. Probably sooner than we think.

PV augmented heat pumps are also not really a thing in north america yet. But I know it’s coming. Probably sooner than we think.

But, thinking the way I do, I wonder what the viability of incorporating that into a residential hot water system would be.

Looking at fujitsu’s waterstage catalogue ( https://www.fujitsu-general.com/shared/pdf-feur-support-ctlg-3ef023-2008e-atw-01.pdf ), one of their single stage “super high power” units supplies 16kw (54,594btu) of heating, obviously in ideal conditions. That’s about a quarter of capacity of the 199kbtu unit I like to use.

BUT the concept of using a PV panel to provide ‘free’ water heating is interesting.

There’s some caveats here, for instance these have an electric resistance backup heater. Which I’m obviously not interested in when I live in Ontario, the land of cheap NG. Another is how minisplits are roughly a COP of around 3 or 3.5? Comparing that to my matural gas prices it’s cheaper to run gas, but I believe it’s relatively close.

So for a thought experiment, I’d like to ask the group here what they think of a potential setup:

PV split water heater, with it’s integral buffer/battery tank, and it’s backup electric resistance disabled, then to a 199kbtu NG tankless water heater  plumbed in series.

Basically the concept is the heat pump water heater provides ‘free’ hot water when available, then the rest is made up by the NG heater. Used for both radiant heating and DHW.

Obviously in real life this would also have to take into account what a PV minisplit HW system would cost to a brand new market, and then when the ROI happens would have to be taken into account. Or reverse calculating where the system cost would have to be to start getting ‘free hot water’ after say 5 years.

Anybody have thoughts on this?

Here’s hoping some of you with more experience with the numbers or these systems can chime in.

Thanks!

Jamie

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Replies

  1. brad_rh | | #1

    I think you should delete the NG tankless. Size the tank so you can coast through a day without sun. Is grid tied PV an option there? Do you have time of day pricing for electricity?

  2. Expert Member
    NICK KEENAN | | #2

    I don't have numbers to back it up, but my gut is it's more efficient to run a heat pump water heater inside the house. In the winter, instead of doing a big lift from outside temperature to hot water temperature, you're doing two smaller lifts, outside to room and then room to hot water.

    In the summer the heat pump is contributing to the cooling of the house and reducing the load on the AC. That's much simpler than trying to capture heat from the HVAC.

  3. Expert Member
    NICK KEENAN | | #3

    Do you have net metering? If so, just let the grid manage demand for you.

  4. Jon_R | | #4

    One issue is that tankless gas water heaters have a minimum btu needed to avoid uncomfortable cycling. In your case, the boost btu needed can be near zero.

    You are focused on the cost, but I encourage you to focus on what is "greenest".

    Exterior air to water heat pumps plus water tank storage (for DHW and space heating) certainly has advantages (eg, time-shifting, micro-zoning). But cost and familiarity (in the US) are issues.

  5. _JT | | #5

    They do seem to have this on alibaba.

    https://www.alibaba.com/product-detail/Easy-to-install-indirect-system-pressurized_60681601916.html?spm=a2700.details.deiletai6.5.61e31bc0DUjBBW

    I doubt it could keep up with the winter in the US though!

  6. GBA Editor
    Kiley Jacques | | #6

    Hi Jamie,

    Given your interests, you might appreciate this article about integrating solar electric and solar thermal panels for heating and cooling needs—including domestic hot water.

    1. alan72 | | #7

      Hi Kiley - that link isn’t working for me.

      Thanks
      Alan

      1. Expert Member
    2. Expert Member
      NICK KEENAN | | #9

      I read the article twice and I am having real trouble figuring out what they are doing. The narrative sounds like Grandpa Simpson describing a trip to Shelbyville.

      I guess that's a little unfair. I did get that they use a water-to-water heat pump and they run the water through the solar collector during the day when heating is needed and during the night when cooling is needed and they're shedding waste heat. I assume this means they have a tank capable of storing a day's worth of water.

      I also gathered that the system cost something like $150,000 to install.

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