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Radiant hydronic heating, embedded in slab, or over slab?

davorradman | Posted in Energy Efficiency and Durability on

This is perhaps a very stupid question with an obvious answer, but..

Are there any numbers of how much more efficient is it to embed pipes for hydronic radiant heating in the slab, vs laying them ON the slab?

Also, as a rule of thumb, doing it in slab should be less expensive?

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  1. Expert Member
    Dana Dorsett | | #1

    It's not measurably more efficient to put the tubing on top of the slab, but it is more responsive.

    It's usually quite a bit cheaper to install the tubing in the slab, but the thermal mass of the slab gives it a more pronounced time-lag between when the water temp rises and when the floor surface begins to rise. Unless you're using deep temperature setbacks or the slab is unusually thick the time lag is not really a problem.

  2. GBA Editor
    Martin Holladay | | #2

    Dana's right -- the options you list don't really affect efficiency. The main factor that affects efficiency is the heating appliance used to make hot water; the second most important factor is the efficiency of the pump. (Many hydronic circulators are oversized or inefficient.)

    If you want low energy bills, remember that you need a thick horizontal layer of rigid foam under your slab -- generally 4 to 6 inches, at least, depending on your climate zone -- as well as adequate vertical insulation at the slab perimeter..

  3. davorradman | | #3

    Thank you for your answer.

    I do have an additional question (they do seem to multiply exponentially as we move along with planning).

    Is it possible to integrate both radiant heating pipes AND ventilation ducts into the between floors slab, say 22cm/9in thick?

    I am from Europe, we build exclusively from brick and concrete here, which makes some thing more complicated.

    BTW, this has been an awesome source of good info. The readership here is just so well informed, comments are as interesting as the articles themselves.

  4. GBA Editor
    Martin Holladay | | #4

    Most concrete slabs (including between-floor slabs) are considerably less than 9 inches thick. Four inches is more typical. Since ventilation ducts usually have a diameter in the range of 4 inches to 6 inches, it wouldn't be easy to embed the ventilation ducts in a concrete slab.

    In a building with between-floor slabs, ducts would usually be run directly under a slab (hidden by a ceiling) or would be run through vertical chases. Ducts can also be located in soffits or thick walls.

  5. davorradman | | #5

    Hm.. As I say, I am from Europe. In Switzerland, many, if not most family homes, come with 7-9in slab. They sure like concrete there. I am from Croatia, but my father is a construction worker in Switzerland, and this is the way they build, and the way i would prefer, because it's just seems to simple and elegant.
    Also, prices being what they are here, it's actually more cost effective to embed.

    What I am curious is, theoretically, would heating pipes interfere with ventilation, because I don't know how condensation works. I suppose I'd HAVE to put HRV right from the get go, so the input air is not too cold?

  6. GBA Editor
    Martin Holladay | | #6

    I will defer to your knowledge of the situation in Switzerland. The thickness of a suspended slab depends in part on whether it is supported by a steel deck or whether it must be structural.

    One factor to consider in Switzerland: The Swiss building code mandates that every new single-family home must include a basement bomb shelter. It's possible that the thickness of the first-floor slab (the ceiling of the bomb shelter) is partly determined by this unusual requirement.

    Before designing a house with ventilation ducts that are embedded in a concrete slab, you should consult an engineer.

  7. davorradman | | #7

    Thank you for your time Martin.

    Obviously, engineer will do all of the calculations and have final say. But I do want to be well prepared for that talk.

    I'll keep investigating :)

  8. Dana1 | | #8

    Managing the longer thermal lag/delay of a 23cm thick radiant slab is difficult, especially in climates that see very large and rapid swings in temperature over the course of a day. Thermostats using PID(proportional-integral-differential) algorithms might be able to handle it, using a combination of floor thermostat + room air thermostat.

    There needs to be about 7.5cm of mineral wool or equivalent insulation between the slab and the room below do be able to control the temperature of both rooms independently.

  9. davorradman | | #9

    Thanks D Dorsett, for pointing out the thermal lag issue.

    The house would be well insulated (4in XPS under bottom slab, 20cm rock wool around the house, and 20cm of something in the roof). The climate is moderate continental. Not sure what is the USA climate zone equivalent, but it's moderate. No rapid daily swings. HRV is a likely investment as well.

    Heat source would be a Heat Pump with a buffer. Supposedly in the long run, it's financially prudent, because large part of the heat would be created using cheaper night electricity. Heating would obviously be continuously ON from October to March.

    I have not planned on separate temperature settings on the floors, but that is definitely something to consider.

    What kind of indoor temperature variance can I expect with such large thermal mass?
    Is it reasonable to expect 68-75 farenheit range to be stable?

    I know it's perhaps too much to ask, with no plans or anything, but I am just curious if it CAN be done?

    I have seen those ventilation ducts that go along the junction of the wall and ceiling, perhaps that would be a better option as far as ventilation is concerned?

  10. SwitchgrassFarmer | | #10

    Davor, I wonder if you have availability of prestressed concrete planks in Switzerland? I looked at them here, even stopped to visit with the people at the largest nearby vendor, Say-Core.

    Being hollow, prestressed planks may not have such a long thermal lag. (An issue I have experienced; I installed a slab sensor for our ground floor. Caught at the 11th hour while we still had access to a suitable location for the sensor.)

    If I recall right, one of the people at Say-Core told me that they had done a residential install, and that mechanical systems are sometimes run through the planks.

    Conceivably you could use a 6" plank, and then top it with grout with radiant piping embedded in it.

  11. davorradman | | #11

    Thank you Andrew for the suggestion. That does look interesting. Sadly, I do not have access to those concrete planks. It will have to be a traditional concrete slab.

    Though, since I am actually in Croatia, I am having difficulty finding someone to do the ventilation in slab. Heating does look likely to be embedded. From what I have gathered, savings on materials, work and house height are worth it, since nobody would want to make a slab under 5 inches anyway.
    In this, it looks that I have to subjugate to the way things are traditionally done here. Apparently they build for worst case scenario here, even though we do not live in an area that is earthquake prone :)

  12. davorradman | | #12

    If it's in slab, zoning is out of the question. That is obvious.

    But, are there ANY benefits to doing it in slab? Other than some cost cutting? I mean, I don't even know what amount of cost cutting there is.

    I will take advice from anyone more experienced.
    If you were doing it now, would you be considering in slab?

    We are talking about a well insulated 8000btu/s2, airtight house, 2200f2, heat pump.

  13. GBA Editor
    Martin Holladay | | #13

    As long as you have plenty of horizontal sub-slab insulation (at least 4 inches of rigid foam), and as long as your heating system is well designed, it's fine to put your PEX tubing in the slab. It will work.

    Since you are building in Croatia, you should strive to follow Croatian methods whenever possible (as long as these methods don't undermine common sense or building science recommendations). If you are employing Croatian contractors, you should install the tubing the way it is usually installed in Croatia.

    Just make sure that you have plenty of insulation under the tubing.

    -- Martin Holladay

  14. davorradman | | #14

    Thanks again Martin.

    It does seem to come down to common sense. No point in trying to do something one way for possibly miniscule gains if there is a non-miniscule chance of something going wrong.

    Though very rare, i did get a preliminary offer to do it in slab. As you say, i will consult with local contractors. I'm just glad to have some info so as to be able to have a proper conversation.

    Thanks again Martin.

  15. SwitchgrassFarmer | | #15

    Davor, I find your interest in an in-slab hydronic/radiant heat system intriguing.

    The heating system for our home in Pennsylvania was originally designed with in-slab radiant heating on the ground floor and "European Style" panel radiators on the first and second floors. Our extremely well regarded HVAC engineer told a quite compelling story for why those panel radiators made sense. And with a few clicks on Google you find claims that indeed something like 80% of European homes use them. I was sold.

    Turns out my wife absolutely hated the appearance of the panel radiators, so at the eleventh hour we ended up re-jiggering the design for the first floor to install a thin slab. (We retained the panel radiators on the second floor.) There were multiple collateral damage impacts with that 1st floor change, from re-specifying the floor trusses for the proper height and deflection, to adjustment of the SIP package to get the window and door heights correct, to having to figure out how to attach a thin enough engineered wood floor. Admittedly though, the 1st floor did end up darn nice from an aesthetic perspective.

    If it is indeed the case that "80 percent of European homes utilize panel radiators" (paraphrasing), perhaps individuals in your part of the world are more receptive to such appurtenances given their commonality? (This would be a great time to joke that I should have married a European woman. Didn't work, my wife is Irish.)

    BTW, I wondered about your comment on a lack of zoning capability. Our hydronic design, and I assume most others, have multiple loops that return to a central "balancing" flow adjustment manifold. In our case the flow adjustments were made one time by me using a non-contact IR temperature gun to balance the heat in the rooms. However I believe the valve bodies in those balancing manifolds can also be replaced with thermostatic or electric operators such that you have active room by room control.

  16. davorradman | | #16

    I am for now taking resources like these:
    as given. It's a well respected company with a lot of experience.

    Where I live, in Croatia, thin slab is standard. And it is done with insulation beneath the thin slab, and insulation where thin slab is in contact with the walls. So you can more precisely determine temperature from room to room.

    A "main" slab is done in one piece. So the heat spreads. How fast, and how much, I do not know.
    Other thing us the slower response time, because it takes much longer time to heat a 6 in concrete slab, and 3 inch cement thin slab.

    Though, these are not problems for me. Even now, with not very good insulation on our current home and with panel radiators with gas boiler, we keep heating on October-April.

    And yes, I think it's safe to say that panel radiators are 80%, at least where I live. People think it's cheaper. And actually it often, but not significantly so.

    But, in new built homes, underfloor heating is gaining more and more. And in some countries, it's I think dominant. I have not checked the data, but according to my father, who works as a construction worker in Switzerland, underfloor is pretty much standard.

  17. Dana1 | | #17

    "If it is indeed the case that "80 percent of European homes utilize panel radiators" (paraphrasing), perhaps individuals in your part of the world are more receptive to such appurtenances given their commonality?"

    Ugly is in the eye of the beholder, and it's a matter of what you've become accustomed to. Most people in the US wouldn't look twice at fin-tube baseboard or convectors (that took over from cast iron radiators in the 1950s), but would notice a flat-panel radiator or bulky cast iron radiator in a new home simply because it's different. Many Europeans find fin-tube baseboards odd looking. (And to be sure, it's lower comfort than flat-panel radiators, since there is very little radiated heat, almost all convection.)

    This is the same aesthetic issue some people find with ductless mini-split wall coils vs. wall /ceiling registers/diffusers.

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