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Community and Q&A

Hydronic vs. Minisplit Heating

josh613 | Posted in Mechanicals on

Hi all!  I’m currently planning an hvac retrofit and was hopping you could let me know if I’m on the right track and/or offer suggestions and improvements I may not have thought about.

The home was built in 1977, brick with mansard style roof.  Electric baseboard heat in every room, no air-conditioning, only 1 of 2 bathrooms is vented.  No ducting and no good place to run it now.  Climate: Ottawa Canada, I think that’s 6A?  Hot and humid summers, cold winters [-13F , +86F].

Design loads: 40,500 BTUh heat,  16,500 BTUh cooling. A little leaky: ~5 ACH50.  Insulating the external walls would be cost-prohibitive.  Heating is my primary concern, any cooling and dehumidification would be bonus.

I’m already 100% electric space heating, so I want to switch to heat-pumps.  Mini-splits is the obvious choice,  but the home isn’t open-concept.  Unless I install a head in every room, I’m not convinced they can provide comfortable even heating. Would need 10 heads for that.  If I provide a floorplan, would anyone be willing to try and convince me that minisplits would work?

For distribution, heated floors make a lot of sense.  My basement has open ceiling, so I can easily add pex with plates between the joists.  This led me to look at air-to-water heat-pumps and low-temp radiators for the second story rooms.  I’m looking at SpacePak and Nordic systems.

To not waste the ATWHP during the summer, I’m looking for a small fan-coil unit that I can put into a closet – just to cool and dehumidify the center of the house, not worry about getting it to every room.  Bonus, the additional heat from the fan-coil during the winter means I can probably get away with slightly smaller radiators in the upper level rooms.

So, is it a good idea to go for hydronic system with heat-pump?  or can mini-splits work in a leaky home with many rooms, but without super-insulated walls?

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  1. user-6623302 | | #1

    Have you considered room size fan coils?

    1. josh613 | | #3

      Instead of heated floors? Its definitely an option. I was looking at Jaga Briza 12. I'm a little concerned about each needing its own condensate drain -- that's potentially a lot of holes put through my exterior walls and harder to run insulated lines (can they go through a 2x4? or even a joist?).

      So I was thinking one or two easy to reach fan-coils, with radiators on the external walls for easier install?

  2. paul_wiedefeld | | #2

    Try calculating the heat loss with this: as a second method if you haven't already.

    If you're considering running pex under the floor, why can't you run ductwork there? That'd cover at least the first floor.

    If you go the multi-split route, you'll run into problems with oversizing: there's no system that can handle ten heads with only a 40kBtu heat loss (nor any smaller multisplits that can handle an average heat loss of 4kbtu/head). A workaround might be: cover the highest heat loss spaces with appropriately sized system(s), potentially ducting clusters of rooms if you can (an example would be adjacent bedrooms), and leaving the baseboard for the smallest heat loss spaces.

    Heated floors with an ATW heat pump will be pricey - I'd consider adding whole-house ductwork before that, as you get two systems in that scenario.

    1. josh613 | | #4

      | If you're considering running pex under the floor, why can't you run ductwork there?

      because pex can go through the joists. There's not a lot of head room, and the basement is split up into 3 areas separated by load-bearing cinder block walls.

      1. paul_wiedefeld | | #5

        That's a good reason! Ducts will make life easier, I'd try hard to incorporate them wherever you can, leaving the baseboard for the hardest locations. I think you'll find baseboard will be 3x as expensive to operate but if you're spending $5k/head or so, it remains an easy choice for many rooms.

        1. josh613 | | #6

          The only place for ducts is in the unconditioned attic... I'm no expert, but I can't see any good place to run them.

          Cost wise, I look at the savings potential and compare that to an investment in a dividend stock yielding 4%. Cutting my space heating in half, aiming for 4% tax free cash return per year... I can budget up to ~$30K. More if seasonal COP is better than 2.0

          | Try calculating the heat loss with this: [...]
          This was great! I think I did this a long time ago. I ended up with 30K BTU. That's right around my 'done-by-hand' conduction loss through all external walls.

          The 40K BTU I ended up with is from modelling my home in HOT2000 software:

          That 10K difference is 100% accounted for by the estimated air-leakage by the software! Hard to believe I have no losses from air-leakage with 5 ACH50... I guess that's an advantage of not having a furnace blow air around.

          1. paul_wiedefeld | | #7

            Ha! Air leakage is the Achilles heel of online calculators, which is why they struggle to match reality.

            That'd be about 7900 HDD65F and around 22,000 kwh for heating? I think it can work out if you optimize the equipment - the ATW seems better suited than a multi-split(s) since it fits the loads much better. Let us know what quotes you receive! These costs seem close?
            Current Heating Cost $4,415
            COP 2
            Annual Savings $2,207
            Rate 4%
            Years 20
            Present Value $30,000

  3. josh613 | | #8

    One of us has the math wrong - your cost and savings are double what I have. puts me at about 8130 HDD65F.

    Over the last 12 months, total electric usage: 21,790 KWh. Estimated base load + DHW of 370 KWh/month by averaging summer usage (no A/C).

    So 17,350 KWh for space heating x average cost $0.13/Kwh = $2,256 annual space heating via electric resistance. So savings of about $1,130 per year if I get COP 2. Divide by 4% = $28,250. I rounded up to 30K.

    Given that the electrical utility recently switched to tiered pricing based on usage, savings should be even better.

    > Let us know what quotes you receive!
    Will do! I just need to find someplace that's willing to do it...

    1. paul_wiedefeld | | #9

      Gotcha: it's dependent on the life expectancy of the heat pump - I used 20 years, but in perpetuity, you're right. That's a point in favor of ATW, most of that hydronic equipment lasts a long time even if the HP itself is more mortal.

      That kwh and HDD puts your heat load even lower! 24kBtu or so: 80 HDD x 303btu/hdd/hr.

      If you haven't read this, it's great:

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