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Community and Q&A

Hydronic radiant in slab heating upper floor?

Michael Schonlau | Posted in Mechanicals on

In a Ranch-style home with a basement (approx 1600 sq ft on each level), can in-floor hydronic radiant loops provide adequate heating to the upper floor also during prolonged winter temps? We live in Omaha, NE. Floor coverings on the slab would be either be carpet or just finished concrete. Plesae assume the house will be well-insulated and air sealed, with “good” windows. Floors on the upper level would be mostly concrete, tile, or stone for thermal mass (passive solar design).

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Michael,
    Yes, but only if you aim for near-Passivhaus levels of airtightness, insulation, and window quality.

  2. Dan Kolbert | | #2

    I dunno, Martin - we've built a couple of houses on slabs with no heat distribution on the 2nd floor. They were good, but certainly not Passive House. One of them had double-glazed windows, not huge R-value, very good solar orientation, but spread out layout. We ran pex to the 2nd floor but after 2 winters (one nasty, one mild), the client has no interest in 2nd floor distribution.

    The other was much tighter, w/ triple-glazed, higher R-value, but 3 floors. Again, heat only in 1st floor slab, but whole house is fine.

    I essentially agree with Martin, though, only disagreeing in what the threshold might be. An energy model (and someone who knows what to do with it) would help make the decision.

  3. David Meiland | | #3

    Are you guys reading this as radiant in the basement only, main floor only, or radiant in the basement AND in a main floor slab?

  4. Dan Kolbert | | #4

    I guess I read it as basement only, wood deck on the 1st floor.

  5. David Meiland | | #5

    It's not clear to me. IIRC my installer generally uses 20 BTUH/SF as a figure for radiant slabs. Do you know what number you guys are using, Dan, and what type of heating load/SF you're figuring in your homes?

  6. Michael Schonlau | | #6

    The question is about radiant in the basement slab only. We would have a mix of wood and concrete/stone/tile for the main floor coverings.

  7. David Meiland | | #7

    What is the heating load for your house?

  8. Michael Schonlau | | #8

    I don't know that yet. My architect seems confident the radiant slab will do the trick, but my wife is skeptical (we don't want her to be uncomfortable). I asked an HVAC contractor the other day for a Manual J and a Manual D. He told me he could do it, but I could probably "wipe my [email protected]# with it" because it wouldn't be helpful. Should I keep looking for HVAC contractors who will do it or is there a better approach? I'd like to know what the heating and cooling loads are up front, so I can better educated about choosing equipment.

  9. David Meiland | | #9

    I'd be looking for a radiant designer/installer that can very carefully do the heat loss analysis, and understands it to be a critical part of the job.

  10. Riversong | | #10

    Michael,

    The advantage of radiant floor heating is that it creates a higher level of human comfort by warming feet and bodies and other physical objects in the space rather than the air, which is secondarily heated by the warmed bodies and by convection off the floor.

    The comfort range for a floor is 66° to 84°F, with the highest satisfaction at 75°. At 75°, a floor can supply 10-14 btu/hour per square foot. Unless your house is very energy efficient (32,000-45,000 btu/hour maximum design load), then a single floor won't be able to comfortably heat an entire house, and you won't get the radiant comfort advantage on the non-radiant floors. In fact, a cool concrete/stone/tile floor will be quite uncomfortable.

    All floors should be heated, and the radiant design is critical to effectiveness, efficiency and comfort. Get yourself a heating contractor who understands this and who is willing to do the calculations necessary to design an appropriate system.

  11. NickJ | | #11

    I have a fairly efficient house (over 3 floors, very open plan), and my wife wanted warm feet. So we installed radiant in our main floor slab and upper wood floors. They heat the house no problem at all (in fact, just the concrete slab would have been enough). The big problem with efficient houses is that to actually feel the heat of your feet, you'd be baked out of the house (at least we are in the pacific NW). So my recommendation is to skip it altogether (or be satisfied that your feet will never be truly warm, although your slab won't be cold).

  12. Riversong | | #12

    There is a widespread misunderstanding that a radiant floor needs to be hot in order for feet to feel warm.

    With shod feet, 90% of people report comfort with a floor that is between 66°F and 84°F, and with bare feet a floor between 79°F and 84°F. Your feet (and most of your skin) are typically 91.4°F, so nothing less than that temperature is going to add heat to your feet.

    But temperature comfort is dependent on the rate of heat loss from the skin (that's what you actually feel), so any floor that is at or above room temperature and not too conductive will feel comfortable. A few degrees above room temperature makes a big difference.

    On bare feet, the perceived comfort is more related to the conductivity and specific heat of the flooring material than the temperature of the floor.

    And whole-body temperature comfort in a space is as much dependent on the mean radiant temperature of all solid materials as it is upon air temperature. This is why radiant heat can feel more comfortable, even at lower air temperatures. But you won't get any mean radiant advantage on a second floor if only the first floor or basement is radiantly heated.

  13. User avatar GBA Editor
    Martin Holladay | | #13

    Micahel,
    If your HVAC contractor told you that you could ""wipe your [email protected]# " with a Manual J calculation, then absolutely nothing he promises can be trusted.

    As I'm sure you know, I've written that many Manual J analyses are done poorly, and a Manual J calculation is not the be-all and end-all to HVAC system design. But to dismiss Manual J entirely shows a remarkable lack of sophistication, so it calls all of the contractors' promises into doubt.

  14. Armando Cobo | | #14

    Michael,
    Manual J is used for forced air systems. Manual GAMA-H-22 is used to design hydronic systems and a qualified and independent third party MECHANICAL ENGINEER should be able to analyze your home and recommend you the proper system required.
    A hydronic system in a basement will never be effective upstairs as it is downstairs. You would need to keep all doors open 24/7 and wait much longer for the temperature to go equally up or down to mantain the whole house comfortable.
    Using a barometric controller can save heating ambient temperature recoup time, but it will never be as effective if you install radiant or forced air upstairs. The first night your wife complains about being cold you’ll get the picture.

  15. User avatar GBA Editor
    Martin Holladay | | #15

    Armando,
    As far as I know, Manual J is used for residential load calculations -- space heating load and cooling load. It can be used for buildings using any type of heating system -- hydronic, forced air, or whatever.

  16. Armando Cobo | | #16

    Michael,
    Also, a common mistake when all radiant heating systems are installed is the lack of fresh air supply in the house. Make sure you install make-up air through your air handler (if using an AC system) or mini-ERVs.

  17. Armando Cobo | | #17

    I know that, Martin, but manual J does not design any system, it only calculates loads, that's it. GAMA H-22 is used to design hydronic systems.

  18. David Meiland | | #18

    a qualified and independent third party MECHANICAL ENGINEER should be able to analyze your home and recommend you the proper system required

    I have rarely seen a third-party mechanical designer on residential jobs. A number of different manufacturers of radiant heating equipment offer design software for contractors, and from what I can tell they are widely used.

  19. NickJ | | #19

    My point was this: don't install radiant if you want your floor to feel noticeably warm (at least this has been my experience). Your floor will feel comfortable, but if you want it to feel warm/hot, expect to open some windows.

  20. NickJ | | #20

    (and this is over the entire slab, not just in a bathroom, where perhaps you can get away with it feeling warm without overheating your house).

  21. David Meiland | | #21

    Nick, I am also in the PacNW and work in many, many different homes in a given year. In more than half I take my boots off outside and wear my socks inside, at least for an initial visit. I can tell immediately which homes have heated floors and which do not, even in the shoulder season. Heated floors are much more comfortable. I installed radiant in my last house in Northern Cal, very near the bay where it's not often cold.... also a major improvement in comfort.

  22. User avatar
    Michael Chandler | | #22

    Third Party Man J
    David, our Energy Star and Green Rating provider (Southern Energy Management) handles all our Manual J and HVAC system design with their in-house mechanical engineer. They also do flow hood and duct blaster testing after the HVAC company has "commissioned and balanced" the system to give feed back on where the problems are and they test all the exhaust fans and make-up air supplies. They don't do it for free, but it's a great service and gives us really useful data on our assumptions and actual performance and helps us make changes to move our quality in a positive direction. Check with your Energy Star provider, good chance they can connect you with an engineer who can take it our of the HVAC installers hands (or [email protected]# as the case may be) I haven't had my HVAC installer do his own Man J for years.

  23. NickJ | | #23

    David,

    I agree - I'm all for radiant. In my case, which may be oddball, for the floors to feel distinctly warm (which is, after all, what my wife wanted) the house quickly overheated. We leave the thermostat at ~68F and the floors are "comfortable"...not cold, but not...ooooohhhh hot floors.

  24. mike maples | | #24

    the correct approach is to do a good heat loss, which will show minimal heat is required in the lower slab and address the" comfort zones" with some other less expensive approach to radiant . staple up with plates under bathrooms,kitchen, euro rads in living,mud entry and keep it simple...

  25. David Meiland | | #25

    "Simple" to me would be tubing in both floors (hopefully gypcrete upstairs). One water temp, no radiators on the walls interfering with furniture placement, and the comfort of warm floors. If backup is needed a direct-vent gas fireplace that works during power outages might be a good choice, if not a woodstove.

  26. Robert Hronek | | #26

    Being in Omaha what are you doing for AC? I see a need for a forced air system.

    Also a basement is protected from weather extremes the heat lost will be lower than the above grade main floor. I know heat rises but you are not going to get enough heat to rise to the main floor. It is not a feasible plan.

    If you are looking for warm floors then you could install electric heating under hard surface flooring.

  27. Mike Jones | | #27

    Warmboard might work better, they'll do the calculations / layout if you send plans, pricey but good value if you want best odds of getting an efficient radiant heat system that delivers consistent comfortable temps throughout your house. "Best odds" here means initial design is not just adequate but optimized and installation is highly likely to follow mfr specs so system performs as intended. The much faster response (up and down) from products that use aluminum plate for heat transfer instead of relying on thermal mass (slow to get warm, slow to cool) gives higher assurance comfortable temps can be achieved and maintained. If you want to bring solar into the system, besides siting the house / window / overhangs properly and getting good windows, consider building a solar collector to preheat water to be circulated in your radiant floor system. Solar preheated water needs to feed a boiler or tankless water heater to assure you'll get the temps needed to heat your house. Lots of good choices for construction will provide a good airtight envelope (SIPS, ICF, etc.). Energy recovery ventilators a must. Good luck.

  28. Riversong | | #28

    I know heat rises

    Heat does not rise. Heat flux is isotropic, that is, it moves equally in all directions.

    Hot fluids rise within a volume of colder fluid. Fluids are liquids and gasses. And they rise, not because they're hotter, but because they are lighter (less dense). And they are lighter because they expand in volume with increased kinetic energy (sensible heat).

  29. Michael Schonlau | | #29

    Thank you all for your feedback. This has been an interesting discussion. I will try to answer a few of the questions asked above:

    -We do intend to install an ERV for ventilation
    -I will ask our energy rater about post-install inspections of the HVAC system
    -I intend to choose an HVAC contractor who is willing to do the Manual J and consider it in the design of the system
    -We have debated adding tubing to some areas in the main floor; we will have stained concrete up there anyway for passive solar thermal mass; our concern was creating a floor that was thicker than other areas and how would we transition flooring in those areas? How thick should the concrete and gypcrete layers be?
    -We've looked at geothermal as a heating source for the radiant and for forced air heating in Spring /Fall and cooling in the Summer. Our city codes require a licensed steamfitter to connect a boiler, so that drives boiler costs up. We've also had people tell us that cooling with radiant doesn't work well because the concrete will "sweat". I've considered mini-split units for auxiliary heating and primary cooling, but haven't talked to any local contractors about it yet.

    Any thoughts on geothermal over boilers for the radiant heat source and auxiliary forced air heating and cooling? I'd love to eliminate as much forced air as possible. I should note that we also have passive solar mass in the main floor and a high-efficiency wood-buring fireplace - both of which could provide some backup heating. Maybe we'll find the passive solar contributes more than we think??

    If one of you were designing this heating/cooling system, what cost-effective approach would you take if hydronic radiant was the primary demand of the client?

  30. David Meiland | | #30

    Your combination of floor coverings upstairs is going to require some thought, whether or not it is heated, and none of the solutions is particularly quick and easy. You can use poured underlayment at various elevations, frame the floor at various elevations, add subfloor thickness as needed, etc., and all of this is more expensive and complicated than simply pouring gyp at one elevation and using floor finishes that work over it (or doing the same with Warmboard or another radiant subfloor). Anything can be done, it just takes time and money, and the choice often relates to the skills of the tradespeople you have available.

  31. JD | | #31

    Perhaps you've already solved your problem by now... but here's our story:
    We installed in-slab heating last fall - older 1 1/2 storey home,- 400 sq ft bsmt) - removed old broken-up concrete floor and laid out insulation/pex-pipe/etc, then hired concrete and boiler contractors. Be very careful when hiring contractors.
    Opinions were many: some had more than enough heat on main floor w just the bsmt slab heated, one wished he'd used only a single length of pexpipe in between joists, most suggested a double length - in various patterns.
    We decided to use only bsmt slab heating first, then add if necessary. After many delays boiler was finally installed mid Dec! Despite "good" reputations I believe both contractors sent inexperienced subs to do the work; neither showed up to supervise, review or address complaints. You just never know. I think we ended up w a "new" boiler that is 6-7 years old and probably an expired warranty!

    First winter: the bsmt was nice and toasty, but main floor was rather chilly, esp on the feet. Floors are mostly hardwood 1 1/2" thick. We didn't factor in any hydronic heating for the upstairs ... "warm air rises". But we bought an electric cast iron radiant heater for the bedroom - set at 2-3 (max 6) - it's great! We plan to connect upper zone (single length pexpipe between joists) for our 2nd winter.
    We'd like to install an electric European towel warmer in the bathroom. When power goes off: woodstove. In future we plan to install a tracking solar panel for domestic HW and/or upper zone.

    Ideas: install the pexpipe heating in stages. You may need to do only certain rooms on main floor. I agree with other comments that you hire someone reputable to do heat req't/loss calculations. I understand it to be 5 BTU/hr per cu ft. but there seems to be numerous ways of doing it.
    Geothermal is very expensive up front but I know a couple of people who are happy with it - I wonder though, about future maintenance. I think solar panels are a good bet.
    Good luck and keep us posted....

    .

  32. User avatar GBA Editor
    Martin Holladay | | #32

    JD,
    I've never heard of anyone successfully putting a solar thermal collector on a tracker; there are obviously problems with flexible plumbing connections. Trackers are routinely used for photovoltaic arrays, however.

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