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Community and Q&A

Split-System Heat Pump Water Heater With Refrigerant

rhl_ | Posted in Mechanicals on

For a while I have considered the Sanden CO2 Heat pump as the bees knees as far as heat pump water heaters are concerned.

I recently realized that the water connections go outside, which makes the safety of the system reliant on heat trace during periods when the outdoor air temp is below freezing.

This further gives a reliance on having electric power for the system to not have a potential for catastrophic failure.

These are non-starters in a cold climate.

The tank only options are nice, with the only drawback being that they cool the conditioned space.

Another alternative I have found is this one:

Which apparently qualifies for the solar thermal federal tax credit, and has an added coil so you can integrate it as a preheat for a ASHP for space heating.

Are there other options for a split system for DHW that use refrigerant as the exchange medium between the units?


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  1. Expert Member
    Akos | | #1

    You are in warm enough climate, I would just go with a standard heat pump water heater.

    The way I would install a Sanden unit is with a glycol loop to a larger indirect. This way you don't have to worry about heat rope and lines freezing. You might need a dual coil indirect to get high enough deltaT to get good efficiency efficiency out of the unit (these units need the RWT to be very low for good efficiency).

  2. rhl_ | | #2

    My understanding is that this isn’t supported.

    1. Expert Member
      Akos | | #4

      The issue is that you need to be careful with your tank, some indirects with the coil on the bottom don't stratify well which will cause issues with the Sanden. I think that is the reason that they want you to use their complete system with tank. Most solar indirect should work as they are specifically designed for stratification.

      You can make it work with glycol ie:

      1. rhl_ | | #6

        Maybe, performance may suffer.

  3. charlie_sullivan | | #3

    In the early days of ground source heat pumps, there was a clever but disastrous idea to run refrigerant tubing directly in ground loops. Those systems then require much larger refrigerant charges, which end up leaking as the buried tubing starts to corrode. The result is a unconscionable release of refrigerant to the atmosphere and a very difficult and expensive problem to fix.

    This system isn't as bad but tends in that direction. I think that we should move in the other direction: towards packaged systems that have refrigerant only within a factory-sealed unit. As we move to low GWP refrigerants, the global warming impact of refrigerant leaks will be less of a concern, but other issues still point in that direction: flammability of hydrocarbon refrigerants, and the high pressure needed for CO2 refrigerants. You are right that running water through the loop is a concern for freeze protection. I think that in your case, a simple indoor packaged tank HPWH is the way to go--you loose a little in the winter but win in the summer, and it's simpler and cheaper.

    If you really want to use an outdoor air source heat pump, you could consider a Chiltrix air-to-water heat pump, which, unlike the Sanden, is spec'ed to run with a glycol loop and an indirect hot water tank, or use a Sanden with the options to manually or automatically drain the outdoor water loop upon a power failure as detailed in their technical guide:

    But those options are expensive and complicated compared to simply getting an excellent indoor only system, and making sure that your heating system is efficient enough that you don't mind stealing a little heat from indoors in the winter.

    Or you could consider using Chiltrix for both heating and hot water.

  4. rhl_ | | #5

    Charlie, while I agree with you, I think that there are many scenarios where a split system makes a ton of sense.

    This seems like a splitting hairs argument. If the refrigerant is CO2, it’s much better, for a once in an install lifetime leak, compared to burning NG for 30 yrs or leaking r410-a the same time, or likely the energy cost of the plumbers truck to arrive every time the water loops break.

    There would be less than 10-20’ of R744 pipe in my home.

    I agree there are many instances where monobloc makes sense.

    Not having the split solution makes the offering worse in places where there is a lot of heat in use, and people are reluctant to switch off fossil fuels.

    1. charlie_sullivan | | #8

      Oh, I'm not worried about environmental damage from a CO2 charged system leaking. With CO2, my concern is just that the skills and materials needed to deal with the pressure involved make running refrigerant lines a challenge: the pressures are often more than 1000 psi vs. less than 200 for typical refrigerants. Standard copper tubing would explode. It's still possible to do it, with new training and materials but it it's hard enough to get conventional HVAC systems installed right.

  5. this_page_left_blank | | #7

    There's very sparse information on performance for this product. No mention of COP at any temperature, no mention of what temperature it works down to. I read through their entire brochure and could not find a hint of what makes this system "solar assisted". It sounds like a conventional heat pump.

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