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Leveraging multi-zone VRF for domestic hydronic heating

mark_gil | Posted in Mechanicals on

There have previously been a number of discussions here about using existing heat-pump infrastructure to also provide auxiliary hot water (for radiant or other purposes).   

A couple of years ago, there was a promising discussion around Mitsubishi’s PFWY module (available in the light commercial City Multi system) suggesting that they might get into this space.

I’m currently working with our contractor to specify our HVAC, and we’re planning on using Mitsubishi VRF equipment (likely 1 x 3 Ton and 1×4 Ton unit) with multiple indoor Zones; we have a 450 sq/ft space which we’re planning to add a Hydronic underfloor radiant system to, but ideally, we’d avoid either separate outdoor units, or tankless water heaters so i returned to the PFWY discussion.  Looking online, it appears that the unit is still made, but perhaps not compatible with the smaller P-series outdoor units; and it looks like it’s a 32k BTU capacity unit, and I suspect we need much less, perhaps closer to 12k.   I wonder if anyone has had any luck exploring options in this space?

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Replies

  1. DC_Contrarian | | #1

    Just spitballing, but heated floors might be good for 10 BTU/sf, with 450 square feet that maxes out at 4500 BTU, probably less because you never get to use 100% of the space. So everything you're talking about seems way, way oversized.

  2. mark_gil | | #2

    Thanks! Yes, the systems themselves are sized to heat and cool a 6,000sq/ft building, but will have a little excess capacity (at design temperatures). They're VRF and so that's fine. The PFWY is way oversized - you're right to do the amount of water heating that we would need - i was basing my calcs on 25 btu/sf vs 10; but your point stands. It's actually the relatively low heating load that made me want to explore it - i'd rather avoid adding a gas tankless; and we're getting close to our max electrical draw on the house with the electric car charging!

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