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How controllable IS hydronic radiant floor heating zonally?

Mark_Nagel | Posted in General Questions on

I am now looking to assess the possible value (comfort return on investment) of including hydronic radiant floor heating in my build, a build which will be based on a floating raft slab (fully insulated) in zone 4C.

I’ve read of folks installing lines but never deploying the system (the supporting).  I am thinking of perhaps hedging and at a minimum  instal lines: if not me then perhaps a future owner can install the remaining bits.  NOTE: I looked at Legalett’s radiant heating using air solution and have concerns about the pipe sizing displacing too much concrete (lose mass) as well as it’s overall functioning; I like the idea of not having a fluid, but that doesn’t mean air is better.  With this all said…

Do multiple zones actually make much difference?  Are they really all that controllable?

Floor layout is basically a rectangle (51′ x 38′) with one long side/half bedrooms and bathrooms and the other long side/half living, dinning and kitchen rooms.  On one end lies mud, mechanical and laundry rooms.

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  1. Expert Member
    NICK KEENAN | | #1

    I don't get what you're asking. Do thermostats and zone valves work? Yeah, they work great. Most hydronic systems can keep the temperature within a degree or two of the set point, at the thermostat.

    In a well-designed system, the capacity of the emitters in each room are sized to the heat loss for that room, and rooms with similar heat demand characteristics are grouped together into zones, so that if the thermostat is kept near its set point so is the rest of the zone. This is true of any heating system. But like any system, hydronic systems can be poorly designed. It seems that hydronic systems are somewhat easier to mis-design than other systems.

    Thirty years ago, when in-floor radiant started getting big, it was believed that "thermal mass" was needed in order for radiant floor heat to work. Systems were built that took a long time to heat up, and they were unresponsive and prone to over-shooting. These systems were mis-designed because there was a fundamental misunderstanding of the science and how they were working. One of the keys to a radiant floor is having the temperature consistent across the floor, and for that you need a subfloor with high thermal conductivity. Concrete is often used because it is highly conductive. However, the less heat capacity your floor has, the faster it reacts to changes in heat input and the more responsive and comfortable your system is. Modern systems like WarmBoard market themselves as "highly responsive" which is the characteristic you want.

    You're wasting money putting tubing in your floor if you haven't engineered the system. The odds that it's going to just work are close to zero. Also, one of the big advantages of hydronic heat is that you don't have to devote space to ductwork. If you're putting in ductwork anyway you're negating that advantage.

  2. Mark_Nagel | | #2

    Thanks for the input. I'm NOT saying that I'm not going to put any design into things: that's why I'm here asking :-)

    I don't expect such a system to carry any major heating load up front, which is why I'm considering the option of excluding the mechanicals. I've got a tiny bit of knowledge as pertains to this stuff (having researched many years ago- I took a course in solar water heating, which actually got me hands-on experience; I understand that tech changes; the WarmBoard shows that!).

    I don't require "highly responsive" so much as I require predictability (with the properly designed system). And I require pretty reliable things seeing as I'm out in the country. I heat solely with wood (been nice and comfortable on many occasions when the electrical mains have been down - if down for any significant time I fire up a small diesel generator to ensure that I have minimal power- needed for my electric fence, Internet and such): no dials to control heat, no ducting to change heat patterns in various rooms. The aim is to mitigate a future risk of it becoming illegal to burn wood (I spend a lot of time tending to my woodlot; it's kind of a hobby for me). I don't expect that radiant floor heating would be but a partial contributor to meeting heating loads.

    I have a couple years to do my design. I won't be slamming things together.

  3. Expert Member
    NICK KEENAN | | #3

    >I don't require "highly responsive" so much as I require predictability

    If it's not highly responsive, it's not predictable. If it takes half an hour for a change in the thermostat to affect the heat output there's no predicting what's going to happen.

  4. billfrombirchwood | | #4

    John siegenthaler has a number of videos on you tube that describe hydronic heating and how to make it reliable with renewable sources. You can configure dual fuel systems such as a wood burning boiler and a heat pump. Check the coffee with caleffi series on you tube.

    His book is also available on Amazon which takes you through the sizing calculations. That will walk you through the tube layout, and runs back to a manifold. He also shows how to lift the PEX tube so it is located in the mid point of concrete.

    Zoning works quite reliable, I have zones for each bedroom, bathroom. And then one for a great room. Typically the bathroom are kept at 70 and the bedrooms at 65. Water temperature is important, typically you want to keep it below 100F.

    In terms of maintainability, You may need to replace a zone valve or a pump once every 10 years, so purchase a few spares. Install isolation valves so you can quickly replace failed components.

  5. Mark_Nagel | | #5

    William, yes, that's pretty much what I was fishing for, the efficacy of zones. I was, crudely, attempting to ask how well zones are able to delineate (not how fast they could respond).

    My floor plan, unfortunately, has space for a wood stove on the other end of the building, away from the logical place to locate the valves and such.

    When I attended a hands-on class for installing a solar water heater system (man was the roof pitch steep!) one of the presenters in the class had a wood cook stove that he used for cooking, of course, heating and hot water (in the winter- in the summer he'd switch to heating via roof-top flat plate).

    What is your climate zone? Is the radiant heating primary or secondary heat?

    I'll look into the resources you suggested, thanks!

    1. Expert Member
      NICK KEENAN | | #6

      Are you trying to keep different parts of the house at different temperatures? The reason I ask is usually the way zones are used is to keep the whole house at the same temperature -- no hot spots or cold spots. They are very good at that.

      1. Mark_Nagel | | #8

        Appreciate the comments and questions!

        After reading a bunch from Siegenthaler, thanks William!, I am getting a better handle on what's possible.

        Initially I wasn't going to go with in-slab radiant floor heating, but was worried that the time to do it was when the slab was poured and I didn't want to look back with regrets. That caused me to waffle on the idea (which is what started this thread).

        I ran across an article by Siegenthaler in which he noted the difficulty of getting in-slab radiant floor heating to work in highly energy efficient homes. He said that in order to get the floor to provide the desired "warm" feel (i.e. walking barefoot on it) that you'd likely be cooking your house.

        As you, DCContrarian, noted, there's a lot to getting things right. Key is what your actual heating load is. Right now I can only go by numbers cranked out from a Manual J on my initial design aims. I came up with just shy of 17k btu/hr for heating (cooling is almost irrelevant here). When the plans start to produce more concrete build info I'll expect that that number will be changed; likely it'll be more, though I'd hope that things aren't too far off this. So...

        While reading Siegenthaler's articles I kept in mind your comments about being able to have rapid response. At issue isn't that I want to run around changing temps, but that I want the system to react pretty much on its own w/o significant lag (for on large zone at least- the living space). I'm now leaning toward radiant wall heaters. I briefly looked at radiant ceiling panels but am not convinced that it would really get me much; I am, however, until I start nailing things down, always open for changing my mind if given compelling reasons.

        Siegenthaler is a fan of home-run runs. I can see this as quite viable in my build as I'll be having a dropped ceiling and a main chase which will make routing pretty easy (limited drilling- also easier to maintain pipe insulation/wrap).

        To come back around to your question as to whether I'm looking to keep different parts of the house at different temperatures... I can now state that that's the case: initially I thought: "Why not just have everything the same?"; then, however, I ran across Siegenthaler's article I mentioned above. Operationally the house is two parts, a North part that contains sleeping quarters and bathrooms, and a South part which contains the living spaces (living room, dinning room and kitchen) and is one big open space. I've tentatively mapped out 6 zones, splitting each bathroom separately as one is the guest and it would likely be used on a more regular basis as it's more accessible from the living space(s). I can see the bedroom zones and the bathroom zones being mostly variable and on-demand, whereas the living space would want to be kept more or less steady (though, as it's south-facing, can be significantly influenced by solar, in which case quicker reaction times here would also be important).

        Electricity here in "cheap." I see increasing pressure on pushing out fossil fuel sources: I currently have propane for cook stove/oven and clothes dryer (also for forced air furnace but that hasn't been used since we have been heating with our wood stove- almost 10 years). I have found electric boilers in the 20k BTU range. This might be what I end up going with. It would be great if I could leverage such a boiler to also heat for potable water: not sure if I can (assume it would require a heat exchanger in an open tank). I suppose that if I could do the reverse, use a domestic hot water heater to provide potable water AND heated water (via exchanger) to the radiant heating devices. that I'd be OK with that too.

        1. user-2890856 | | #15

          Radiant ceilings are GREAT alternatives in high performance homes . You can read what Siggy thinks about those also .

    2. billfrombirchwood | | #7

      Climate zone 6 Minneapolis, -13f to 90 f. Infloor radiant heat with taco zone valve controllers. Heating is done through a Taco XPB connected to a hot water tank. The XPB is on ODTR. This keeps a very even temperature as it varies throughout the season.

      1. Mark_Nagel | | #9

        William, thanks for the info.

        At this point I'm thinking of an Alpha circulator pump and zone valves. I've thought about a lot of things and changed my mind lots of time so this also is likely to change; but, for now, I'm kind of feeling like this is something worth doing a little more serious research on.

        1. Expert Member
          AKOS TOTH | | #11

          I would read through here:

          Even if you take out the wood stove and the buffer tank, a lot can be said for the simple setup he choose.

          Any time you say zone, you are adding around $500 to your materials, so make sure there is a good reason for it.

        2. user-2890856 | | #14

          A most overlooked part of system design is the DESIGNED for DELTA T . Loads will change , water temps will change , flow rates will change , pressure requirements will change , the most important thing to keep constant to maximize performance of the entire system is Delta T for the largest percentage of the season . Don't disable your system using a pressure circulator , although it says Delta P circ it maintains a set pressure as zones open and close . You'll size this circ for the design day which hardly happens and your entire system will not be what you designed it to be .

          I do not work for , nor am I compensated by any manufacturer , I design , maintain , and FIX poorly designed hydronic systems with a variety of issues . Pressure circs are well suited to panel rad and constant circulation systems , that being said , any system will work in a superior manner using a Delta T circulator .

          Taco VT2218

  6. gusfhb | | #10

    There is zoning and there is balancing.
    In my last house the radiant system had but 3 zones, but every loop had a valve which could adjust that loops output.
    You start with all wide open, assuming they are of equal lengths. If you end up with a cool area, you throttle back the zone closer to the thermostat until it is balanced. If you cannot balance it you did not put enough tube in the floor.

    1. Expert Member
      NICK KEENAN | | #12

      For balancing to work the system has to have been fundamentally properly designed, the emitters are sized to the rooms they're in, but the flow isn't quite what you assumed. Then you can tweak the flow. If the emitter is too small neither zoning nor balancing is going to fix it, the only thing that might work is making the water hotter. This is a recurring theme with radiant heat, if it's not designed right you have limited options to fix it.

      Zoning is good for when different parts of the house have varying heating loads. For instance, a house that has a lot of south-facing windows. On sunny days the south side of the house roasts and the north side freezes. At night it's reversed. No amount of balancing will fix that.

  7. user-2890856 | | #13

    Install the tubing as if each room is it's own zone , if you have a couple baths that are adjoining you could use one loop .

    For rooms to be on the same zone and do this well , 3 criteria MUST be met .

    1 . Must have similar BTUh/sf output requirements
    2. Must have similar finish floor R value
    3. Similar use pattern .

    The number of loops is the number of loops , that won't change (maybe) . Pull them to a manifold location and make the decision when the time is to make it . Don't hamper what could be a great system by being lazy . We all know what happens when you do that , you end up defending systems that are good because early adopters sucked .

    Like mentioned above about water temps and balancing , this is also addressed by making each room as if it will be it's own zone . Your SWT will be determined by the hottest water requirement when a proper room x room heat loss is performed . Use that as your design SWT for the whole building , rooms that require lower SWTs will not be harmed or overheated . When the floor reaches the temp required , the thermostat will know that the air is at setpoint and turn off flow to that zone .

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