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Heat Pump Hydronic Radiant vs Mini Split in <1000 SF home

PaulNeumann | Posted in General Questions on

I am an Architectural Engineering Student passionate about sustainability, energy efficiency and the impact the built world has—however; little real life building experience so value the input. 

I am currently working on a NZE affordable project and curious to get some opinion. 

In a 81% Heating Degree Dominated small home to me it makes sense to use a high efficiency heat pump hot water heater. When the DHW & Space heating load are about the same through passive strategies/solar gain I feel that using a high temperature heat pump with a high temp tank (DHW) & buffer tank(Heating) would be beneficial & could balance out the cost. 

Most curious about CO2 heat pumps and their increased efficiency and lift temp for DHW but know they are (at least used to be) still very expensive. Also something about code in the US possibly? Has anyone used SANCO2’s product?

With three zones (main, 2 bedrooms) wouldn’t the 3 indoor units for a 800 sf home be comparable to a PEX tubing system. I know a ducted heat pump system would be the most economical but we have alternative motives for a ductless/radiant system. 

That being said does anyone have any opinion or could give some insight to the cost/feasibility in Colorado?

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Replies

  1. Paul Wiedefeld | | #1

    I don't think an air-to-water is economical at all for this situation. Hydronics gets incredibly uneconomic at small scale. You'll need a buffer tank, circulators, valves, an expansion tank, a manifold, possibly a heat exchanger, a dirt/air separator, 1000 feet of pex, etc. The physical footprint alone is challenging in a house of this square footage. This is all before the heat pump itself.

    The Sanden itself is expensive and extremely niche in the US, as are all air-to-water systems here. Should anything go wrong with the system, you'd probably be starting over. Its warranty when used for space heating is only 5 years I believe. It also sends potable water outside, so everything must go right for it not to freeze.

    What's the heat loss for this proposed building? Assuming it's around the Sanden's heating output (only 8k Btu/h) and AC isn't important, 3 minisplits are overkill and would perform poorly. If ducted is ruled out, then 1 unit in the main room and electric baseboard in the bedrooms is probably the easiest path. Use an electric tank for the water heating and solar to wipe out the bill. A single ductless unit will be incredibly efficient and the entire setup should come in <$10k.

  2. DCContrarian | | #2

    Are you going to build this house or just design it? Because we can get a little more outlandish if it's just a design project.

    One of the issues with mini-splits is that the smallest heads available are about 6000 BTU/hr. So if you need three heads that's 18K, which sounds like a lot for a well-insulated 800 SF house. And oversized units tend not to be efficient.

    Have you looked at the Chiltrix air-to-water heat pumps? They sell hydronic heads that basically look like minisplit heads but the medium is water instead of refrigerant. But the smallest head is rated at 3,000 BTU/hr. You can also derate them by running at a lower water temperature, which actually makes them more efficient.

    They offer a domestic hot water option but I think it's kind of hokey.

    Pricing is here: https://www.chiltrix.com/documents/price-list.html

    1. Paul Wiedefeld | | #3

      I really want air to water to work, there could be so many benefits. That said- the chiltrix FOB price in VA for the outdoor unit alone is $4700 and their buffer tank is $1000. I assume both are pre-tax. Probably an installation of ~$15k? I’ve been quoted <$4k for a wall mounted Mitsubishi hyper heat system (9k btu) installed. Until something changes in the US I couldn’t in good conscience recommend air to water.

      1. DCContrarian | | #4

        Pretty much why I asked if this is just a design project or will actually get built.

        1. Paul Wiedefeld | | #5

          Yeah I understood your point and we’re on the same page. Since this question gets asked nearly everyday, I want to make sure it’s clear for others.
          That said, I think there’s a connection between bad designs and projects that never get built. Putting expensive equipment from a minor manufacturer in a house that size definitely makes it less likely it’ll be constructed.

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