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Hydronic underfloor used to effectively enlarge direct solar thermal mass?

FelixCollins Member Posts: 1
I'm in the early stages of planning a new build. I'm considering a heat

pump underfloor heating system in concrete slab. The house will be

carefully optimised for passive solar gain as well. Has anyone heard of

using the underfloor plumbing to move heat around the slab from areas

receiving direct solar illumination to other parts of the slab? The thinking being that this system will effectively enlarge the amount of thermal mass in the house that can be considered direct solar thermal mass for the purposes of the passive solar design.


  • hot_rod
    hot_rod Member Posts: 22,263

    We did a passive home in Utah many years ago that had black, stamped concrete in the south facing great room. We did what you are considering. t didn't end up moving much energy around as we had expected. We watched it for a few heating season with some simple hour meters.

    The cost for the additional controls to allow that, may not have been recovered with any energy savings.

    I think you would be better off using the $$ amount on the building shell upgrades, best insulation, triple glazed windows, top quality heat pump brand, etc.

    With a super insulated, passive home the energy costs should be very low to begin with. You may see heat load numbers in the single digits for BTU/ sq. ft. The back rooms not seeing that passive gain directly, can still be warmed with some air movement from the ERV or other exchange device, with some planning.

    The biggest complaint was overheating. Wakeing up in the morning after a cold night had the slab with a belly full of energy. All that mass needs time to ramp down, it is not a quick responding system, a concrete slab.

    If I were to do it again i would look into low mass heat emitters, maybe some radiant floors in the bathrooms and tiled kitchen floor.

    Most of those early passive homes in the sunbelt mountain areas had their windows and doors open on sunny days, too much of a good thing can be un-comfortable when it comes to a 90° passive home.

    This article may help put some numbers to the question. Thanks to PM Magazine for archiving all this info.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Rich_49
    Rich_49 Member Posts: 2,766
    Moving energy

    Felix ,

      Good to see you took my advice . There are many very informed open minded folks here whom will be more than happy to attempt to help . In another thread titled peer review please is an industry leader who has several ideas to do what you are planning or inquiring about only difference being the energy he seeks to collect and move for other purposes is outside , but moving energy from one place to another is just that whether it is outside or in .

      Passive houses can be funny and there may be other ideas or areas that could be utilized more efficiently than a floor in these homes . When we think about the assembly we must realize that the solar energy will only contact certain parts of the floor , those beginning however many feet into the room that the angle of the sun and the window size and height determine for us  .  I always thought a much better place for this solar gain component would be a wall which had similar mass . That assembly in my mind would solve the problem of the south facing rooms overheating , once heated would transfer the energy to the north also benefitting the whole home as opposed to just one side .

      Some of the experimentation and information on this type of strategy is more than a decade old and the time to re examine them is beginning . With the mainstreaming of ECM circulators , off the shelf controls and a better understanding of things it should certainly be feasible at this time or at least developable . There are too many opportunities missed in the name of the dollar in my humble opinion . Joe Lstiburek said it best in his paper " It's about the energy Stupid" . We are either going to make gains by doing it for real or stop thinking you're gonna save the planet by doing things halfway because that's what someone determined for you . Every individual has to decide what his budget will allow and make a conscious decision about how far he will go and not let someone whom is a self proclaimed or recognized expert tell him what is right or wrong . Believe me , there are plenty of recognized experts out there , most of which became that way because they were around in the beginning and nobody knew anything then . Those are the very same people whom the manufacturers lobbied to push their kind of technology . They have become complacent and comfortable and do all they can to remain that way . I have no loyalty to any manufacturer , group or entity and like others here will speak and think about whatever deserves exploration as long as it is beneficial to the end goal which so many have lost sight of .

      Have a good look around and open your mind further . We wet heads have much to offer but are mainly not utilized by a large segment because they have dealt with monetarily successful folks who may or may not have been worthy of their success . Welcome to the discussion .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • SWEI
    SWEI Member Posts: 7,356
    Tight little houses with good passive solar gain

    We are seeing more and more of these of late.  I've tried to talk the owners into radiant ceilings but the added cost (for slab on grade) has pushed the tubing back into the slab on a couple recently.  I'm trying something new on one right now:  Two zones in a 1500 square foot house.  One for all the rooms with south facing glass and another for the remaining rooms (bedrooms and bathrooms.)  Belimo CCV's on both with proportional control (think electronic TRVs.)  18,020 BTU/hr design day loss, so they get a Thermolec B-6TMB on the wall.  Solar thermal DHW with electric tankless backup.
  • hot_rod
    hot_rod Member Posts: 22,263
    maybe the best plan

    is to talk with as many passive home owners as possible. Nothing like living in a passive home for a few years to get a feel for what works and what doesn't. There is a passive home group in the US and a International association or two also.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream