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Underfloor Hydronic System performance through 3/4" plywood AND 1.5" concrete subfloor?

bryantroll Member Posts: 29
I am slowly rehabbing a house and a new HVAC decision has been my biggest challenge.
The house originally had a SolaRoll hydronic system that was installed into 1.5" thick gypcrete over 3/4" plywood subfloor on the main level.
I am considering abandoning the original 40+ year old system in favor of a new PEX underfloor hydronic system, but have not been able to find info on concerns of performance with this underfloor system needing to radiate through the plywood AND the 1.5" of concrete, plus tile.
Once up to target temp, will the extra mass of the concrete perform similarly to any other underfloor hydronic system, and just be slower to respond to temp changes, or does that substantial extra mass inhibit the systems performance? I haven't been able to find info on this situation but would like to plan appropriately for it.

(Sidenote: the house is a monolithic concrete dome and is well sealed/insulated with impressive interior temp stability from everything I have seen so far. Exterior walls are 3.5" closed cell spray foam over 2.5" of concrete/rebar with minimal windows/doors.)

Bonus: I am also looking to heat the finished basement below the main floor -- I have heard ceiling radiant suggested however that has been harder to find much info on and what I have found doesn't seem overly positive. The alternative is to install radiators in the basement. The basement slab does not appear to have any insulation and I have about 90" of headroom to work with so I assume that in floor hydronic isn't as attractive of an option. Any thoughts on this as well?

If this entire hydronic strategy has major issues, I will likely end up with mini splits throughout the house which I understand has its own set of pros and cons.

A majority of the install will be DIY (with the help of one paid GC/handyman), but I am always open to hiring a professional for design/planning or specialized work.


  • hot_rod
    hot_rod Member Posts: 20,789
    edited October 30
    You will need a higher supply temperature to drive through the plywood. 3/4 softwood is about an R0.8 so not a huge deal. Hopefully you would transfer plates, 8” on center.

    Has the home heated satisfactory with the system you have now? Any idea what temperature the supply is? Any areas underperforming?

    I doubt you will notice much of a difference in ramp up time. Ideally a constant circulation would all but eliminate temperature swings.
    If you wanted to start at the beginning, a room by room heat load would determine what you need to supply. A dome might be a challenge to calculate😗

    Radiant ceilings work well, you can get a bit higher output than floors as you can run the temperature higher.
    Simple enough to install.

    We have talked about radiant ceilings in a fire different Idronics all the way back to. Issue 6. Free download at this site.

    https://www.caleffi.com/sites/default/files/media/external-file/Idronics_25_NA_Lowering water temperature in existing hydronic heating systems.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • bryantroll
    bryantroll Member Posts: 29
    Thank you hot rod.

    I would definitely use transfer plates, I read some interesting articles about that and was just reading about the thinner stamped plates vs extruded actually. Higher supply temp = lower efficiency / higher operating cost, but likely by a small amount I assume?

    The existing boiler is busted and there were some other small leaks in the system and I didn't pursue that further. I really think I'd feel more comfortable with a new system with a much longer likely lifespan.

    I have only used a couple of 1500w electric space heaters on low/medium setting to keep the indoor temps at about 55 degrees so not a lot of real world testing. Do you see benefit in seeing if/how many it takes to get indoor temp to 68-70, or not worth it?

    I am totally open to having a Manual J performed, or calculating that myself, but I have not done that yet. I am in Evergreen, CO, climate zone 5. Is the heat load calculation something you'd recommend I do myself, or who should I speak to about that? The couple of professionals I reached out to previously were a bit underwhelming.

    Floor joists under the main floor are 16" on center toward the ends and 12" on center throughout the middle of the circle, where the span is larger. I'm sure I can design the radiant tube spacing around both but figured it was worth mentioning.

    Can you tell me how a radiant ceiling is installed? I have found limited info so far -- I see panels which seem to be fairly expensive -- is that the only way? I don't feel that I understand that system clearly enough yet. Would you choose ceiling vs radiators if I'm working in the ceiling already anyway and all the drywall is out?

    Lastly, I also have to replace my hot water heater with something larger that can accommodate 5 br / 5 ba total -- is a combi boiler worth looking into to serve both heating and hot water? I have not researched boilers / hot water heaters / tankless much yet, just wondering if there's a direction I should look for those.
  • hot_rod
    hot_rod Member Posts: 20,789
    Radiant ceiling below the joists. Run 1x2 furious g strips 8” on center. S dew the transfer plates to the strips, Sheetrock or finished ceiling fastened to the strips. Fiberglass batt insulation between the upper and lower plates.

    If you don’t mind the number crunching you can do the load calc. There were some free programs on line.

    The load calc at www.hydronicpros.com Heat Load Pro is affordable. I think there is a free demo, see if it is something you are up for. It is specific to hydronics.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • bryantroll
    bryantroll Member Posts: 29
    I completed an initial heat load calc using the Head Load Pro software -- just used the demo to feel it out and make sure its a good fit but happy to pay to use valuable tools.

    One question I have is for Exposed Floor -- would first floor rooms have exposed floor if the basement space below them is also a conditioned space (separate zone)?

    We entered all of the data as if the first floor had exposed floor space and it came back with a 32k BTU result for the main floor and loft spaces. We did not run the numbers for no exposed floor yet.

    Also -- how crucial is a design temp difference of 5 degrees? I'm right on the border of 2 counties -- Jefferson (5 degrees F) and Clear Creek 0 degrees F). It didn't look like it affected the results very much.

    Any recommendations for ACH value to use? I will have all new windows and doors and the construction is continuous spray foam outside of continuous concrete. Home was built in 1982. We left it at .5 ACH for this initial calculation but I don't know if thats correct.

    Also, it doesn't look like there is anything that delineates a basement below grade vs above grade -- surely that has to make a difference in heat load, doesn't it?

    Link to summary of results: https://drive.google.com/file/d/1KZgb2CK2C_NHEJ1tRs-4lW4wGW1H3unY/view?usp=sharing

    2475 BTU - Guest BR + closet
    3896 BTU - Primary BR + closet
    1083 BTU - Primary Bath
    396 BTU - Hall Bath (interior)
    3604 BTU - Sunken LR
    6772 BTU - Kitchen / Dining (lofted ceilings)
    7401 BTU - Great Room (lofted ceilings)
    2606 BTU - Stairs (2 stories) / Hallway
    2830 BTU - Loft BR
    1431 BTU - Loft Office
    335 BTU - Loft Bath

    32,837 BTU TOTAL

  • hot_rod
    hot_rod Member Posts: 20,789
    Since you are open to buying good tools.

    This is the best resource money can buy for Hydronic Design, piping, and troubleshooting. Chapter 2 walks you through a heat load example with the Pro software.
    It will have details about radiant ceilings and other installation options.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • bryantroll
    bryantroll Member Posts: 29
    Thank you, just ordered! But unfortunately it won’t arrive for a couple weeks. 

    Do you know if/how differently basement heat loss is calculated being below grade?
    Trying to calculate and also make a decision about adding insulation or not to the original uninsulated slab. 
    Currently the basement walls and floor both average about 55 degrees (inside air temp is 55-60 degrees).

    Appreciate the input, thank you. 
  • hot_rod
    hot_rod Member Posts: 20,789
    edited November 9
    Heat Load Pro allows you to select how much of the basement wall is below ground, and also the insulation used. Of course a basement with a walkout on one end would be a different calc also.
    Maybe the demo version doesn't let you into that basement calculator?

    Insulation below the slab, and also around the concrete foundation walls makes some difference.
    Depends also on your climate, how deep does the frost level go?

    The 55° soil temperature sure helps. Ambient air temperature on the above ground sections of the concrete will hit your load. Typically 2" or more insulation down to frost level is worth the $$. Some installers put a 4' band or horizontal insulation around a slab or basement also.

    All this is explained in detail, and with formulas when you get the Modern Hydronic Heating book.

    I'm feeling old now. This little fellow wrenching on the manifold in Modern Hydronics, is now on a NASCAR pit crew. Bigger wrenches :). Probably better pay also.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream