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Community and Q&A

Wall-Mounted Thermostat to Control Minisplits and Radiant In-Floor Heat

james_talmage | Posted in General Questions on

Zone 5. Ductless mini-splits for cooling and shoulder season heat, and hydronic radiant in-floor heat for winter.

I’d like a single wall mounted thermostat for each zone that controls both systems. While I see some mini-split manufacturers offer dry-contact input modules to call for heating / cooling, by understanding is that sacrifices the energy efficiencies and comfort that come from modulating output, and will lead to excessive cycling.

Poking around the Flair Puck website, I do see they mention Hydronic heat in a PDF, but no clear, publicly-available instructions on how it wires up. Air Patrol also seems promising.

Anyone have actual experience trying something like this?

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  1. Charlie Sullivan | | #1

    What to use partly depends on how you want it to operate. Do you want the minisplits to keep running all winter, and supplement that with hydronic heat? Or so you want to simply shut them down at some point and completely switch over?

  2. Jamie B | | #2

    I'm also curious about this set up.

    In a room-to-room zoning situation, with a minisplit head and hydronic heating with an ambient air T-stat, will it present overshooting problems in the shoulder seasons when two independent T-stats are operating in the same space?


    1. Expert Member
      DCContrarian | | #3

      He's not proposing two thermostats, he's proposing one thermostat that controls both systems.

  3. Expert Member
    DCContrarian | | #4

    It's a common thing to have one piece of equipment that you want to run when another runs. For example, a humidifier that turns on when a furnace turns on. A current-sensing relay is a non-intrusive device that clamps over a wire, senses when that wire is carrying electricity, and turns on a relay. Here's an example:

    The only question would be where in the minisplit to put it. I would think either on the compressor or the interior fan. You'd also have to have a way of making it run only when the mini-split is in heating mode. It could be as simple as a switch that you turn on and off.

    1. Charlie Sullivan | | #7

      This could work well if the plan is to run both, rather than discontinue the minisplit at low temperatures. With an adjustable threshold, you could put it on the compressor and only kick in if the current is high enough to indicate running at near full power, not when it's modulating at a lower level. And/or combine it with an outdoor reset control for the boiler which would adjust the water temperature and could be set up to avoid having the boiler run at all above some threshold outdoor temperature.

      You might even be able to control the hydronic heat purely based on outdoor temperature, set up provide not-quite-adequate heat, with the minisplit modulating to fill in the gap and regulate temperature, and skip the current sensing relay.

  4. james_talmage | | #5

    To be clear, what I'm trying specifically to avoid is multiple thermostats. I'd like one device per zone to control both heating and cooling. And, for heating, it should intelligently choose when to cross-over from heat-pump to hydronic.

    1. Charlie Sullivan | | #6

      Perhaps in the future there will be a thermostat that can figure out the characteristics of each piece of equipment, fuel and electricity prices, and decide for you when to use only one, when to use both, and when to only use one, but I think that for now, it's necessary to make a plan for what you want it to do before implementing the controls to make it do that. To start, we'd need to know why you want to have both systems, vs. just heating with the minisplits. Are you sizing the minisplits such that they can't keep up in the coldest weather? Or are you thinking it will be cheaper to run the boiler than the minisplits in cold weather when the COP gets low? Or are you thinking that you'll get better comfort from the floor heat? If that last one, from the floor heat alone, for from the combination?

    2. Expert Member
      DCContrarian | | #8

      Is there an off-the-shelf product that does what you want? Not that I'm aware of. Could you put together something that did it? Possibly, although I realize it may not be your cup of tea.

      I want to dig in a bit into what it means to "intelligently choose." Let me throw out three possible interpretations:
      1. The heat pump is no longer keeping up and maintaining inside temperature. This is the way that two-stage thermostats work, they essentially have two set points a few degrees away and each activates a separate piece of equipment. This is how backup heat for heat pumps is often controlled. So say you have a two-stage thermostat that has a set point of 72F for the heat pump and 70F for the radiant, the radiant will kick on whenever the heat pump can't maintain 72F.

      2. The outdoor temperature hits a certain point. This is simple. You have an outdoor thermostat that switches between systems at a set outdoor temperature.

      3. The heat pump is working too hard. You put a current-sensing relay on the heat pump that senses when its efficiency is dropping. For instance, you put the sensor on the auxiliary heat coil.

      I kind of like #2.

  5. Kevin J | | #9

    Sensibo works pretty well for what it is- an IR device controlled by iPhone with it's own sensing capabilities. You can tie it together using IFTTT and ecobee as the controller for the radiant. But not likely on a per zone basis.

    I gave the Flair puck about 30 seconds of my time before boxing it up and sending it back. It felt like a cheap toy, the interface was horrific and the app felt like a crappy port a mobile website.

  6. William Hullsiek | | #10

    Check out HBX controls, Zon 0550 and THM 0500. They have been answering my pre sales questions.

  7. Trevor Lambert | | #11

    I can't think of a good reason to have both in floor radiant and mini splits, and this is coming from someone who has both. If you're in the pre-build phase, I urge you to ditch the in floor heat. It's a waste of money. My house was designed to have in floor heat alone. Once I realised that (a)cooling is required and (b)even for our low heat load, a heat pump is money saving, we got mini splits. The in floor heating has been dormant ever since.

    1. Mark Nagel | | #14

      Trevor, what zone are you in? What's your flooring?

      I'm trying to understand what I might possibly include in my build (I want to understand options before I consult with people who many not understand the options or pretend that they do- can't tell you how many "professionals" I've run across who knew less than I knew about the subject they were paid to be experts on [not a knock on professionals, not at all, just a statement that I like to have my ducks in a row such that I don't run across a poser professional]).

      I use some Johnson Controls temperature controllers for various things. They work great to control temperature set points (heating or cooling) and you can define a time delay for cycling. The ones I have are 120v but pretty sure there are 240v ones available (from other manufactures if not also from Johnson Controls): some of these units can also drive 24v devices.

  8. W Ramsay | | #12

    I have several spaces w/ both in-floor and mini-splits including; Garage, Wood Shop, Photo Studio, and Conservatory. In all of these the in-floor and having the floor warm makes the room much more comfortable to work in, provides a more pleasant heat and provides heat without air movement which is extremely important in the woodshop and photo studio.

    Mini-splits serve two purposes; Cooling during the summer and quicker warmup during winter. When not in use for a period I'll keep my studio at 50°f. Using only the in-floor to bring it up to temp can take a day or two but the mini-splits can do it in about an hour.

    The in-floor in the conservatory works great for spring and fall but cannot by itself keep it warm during much of the winter. And for the most part we don't use it during these times. However, there are occasions when we do want to use it and so having heat capabilities in the mini-split is very beneficial. Similarly, we actually rarely use the mini-split for cooling the conservatory. Usually, if it's too hot w/ just windows and vents open then we don't use it as we'd rather not waste the energy but on occasion we do want to be able to use it and so having the mini-split for that is very welcome.

  9. Expert Member
    Akos | | #13

    The mitsubishi relay modules are actually pretty good. It provides two stages of control, first stage is a fully modulating stage with a setpoint target you set on the module (this should be around your actual target) and the 2nd stage where it goes full tilt.

    You can use a standard 2 stage thermostat to drive this (ie Tekmar 521).

    For example:
    -cooling both stages to mini split module
    -heating 1st stage to your floor heat
    -heating 2nd stage to 1st stage heat on mini split

    This would let you run the floor heat most of the time and the mini split when there are big changes in room temperature.

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