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Adding an in-floor radiant heat circuit to my house

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bmaclean
bmaclean Member Posts: 11
Hi,
I currently have a Viessmann Vitola 200 with a Vitotronic 200 control with an outdoor reset. It also connected to a Viessmann DHW tank. Currently it only has one circuit on it for the upper two floors of the house - all large old cast iron rads. We are currently renovating our basement which includes adding in-floor radiant heat. There appears to be two ways to control this new in-floor circuit.
The first is a room thermostat connected to the pump driving this circuit. There is a mixing valve which is set to a static temperature/setting. The Vitotronic control basically knows nothing about this heating circuit.
The second method is to add the Viessmann mixing valve actuator accessory kit which includes a supply temp sensor, and a motor that controls the mixing valve. The Vitotronic completely controls this circuit. Note that no room thermostat is used.
Clearly the second method is better as there is only a single controller (the first method could potentially have the two controllers working against each other in some cases), but it is also quite a bit more expensive. One thing I do not understand is how much better. Comfort, efficiency, etc?
My heating contractor recommends the first method as it is cheaper and he believes the performance difference is negligible.
What do you think?
Thanks.

Comments

  • Ironman
    Ironman Member Posts: 7,399
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    Is this in slab or an over-pour on slab? If so, then you DON'T wanna use a (dumb) thermostatic mixing valve. You need a smart control like the Viessman mixing box that operates off of out door reset.

    The reason being that a slab is high mass and it will continue to give off heat for hours after the thermostat is satisfied (the flywheel effect). A smart control, when properly installed and setup, will modulate the water temp to exactly match the load preventing the flywheel effect which would over-heat the room. A thermostat is not needed with this type of control.

    If your contractor does not understand this, then radiant is beyond his capability.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    SWEISolid_Fuel_ManPaul Pollets
  • bmaclean
    bmaclean Member Posts: 11
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    This will be in slab (base of gravel, then 2" of polystyrene, then radiant tubing, then 4" of cement).
  • Ironman
    Ironman Member Posts: 7,399
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    Then you want the smart control.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • bmaclean
    bmaclean Member Posts: 11
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    Thanks - this is the same problem we resolved when we had the new Viessmann boiler installed with outdoor reset.

    Not interested in going backwards.
  • Ironman
    Ironman Member Posts: 7,399
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    The Viessmann mixing control will allow a second heating curve with its own ODR parameters to be setup. The water temps this circuit will be substantially lower than those for the cast iron rads and the curve should be setup based upon ACCURATE DESIGN calculations. I seriously doubt that your heating contractor has or can do this based upon what you've related.

    Did you or the contractor provide any design criteria for the radiant floor? Is so, what was it based upon? If not, then you need to hit the brakes BEFORE the floor is poured and get someone who is capable to design it. Proper radiant performance comes from proper design. I've never known of a radiant floor that performed properly that wasn't designe properly; and every floor that I've known of that didn't perform properly was not designed properly, if at all.

    There's a WHOLE lot more to this than just laying pipe.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    SWEI
  • bmaclean
    bmaclean Member Posts: 11
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    Thanks again. Not to worry - that was just the first quote. We have two other contractors who are coming in to quote on the job.
    I just like to be an informed customer which I asked the question on this forum.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    It's installs like the one your contractor described which give radiant floors a bad rap. A simple thermostatic mixing valves is good for small bathrooms and the like.

    High-mass emitters like slabs are not fin-tube and therefore can't be controlled like fin-tube like the majority of contractors are used to.

    Taylor
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    Ironman
  • bmaclean
    bmaclean Member Posts: 11
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    All,

    We have finally finished the install of our in-floor heating circuit in the basement. As suggested, we went with a different contractor who recommended an actuator driven mixing valve (he chose a 3-way mixing valve). The other thing he did install was a thermostat connected to a temp sensor installed in the concrete slab. This thermostat can stop the pump if the temp exceeds some value. The contractor has not yet come back for the visit where he explains how all this works so I figured I would ask the wall to educate myself better before asking him the same questions.

    I suspect this should be setup like the upstairs radiator circuit - the pump should always be running and the water temp is driven by the outdoor reset curve. Note that we actually do have a thermostat upstairs that can also turn off the pump when the upstairs gets overheated but this rarely happens (party, etc.) as I set the temp high.

    For the slab temp driven thermostat in the basement, I cannot get my head around what the max temp should be as the slab temp is probably closely related to supply water temp which goes up and down with the outside temp. This is not really like the upstairs thermostat which measures the temp of the air which takes into account the insulation of the house, etc. The slab is too closely linked to the supply water temp to be useful.

    Is there some advantage to the slab temp driven thermostat that I am missing? Or, what have I got all wrong?

    Thanks.
  • Jamie Hall
    Jamie Hall Member Posts: 23,661
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    I'm glad there is an in-slab thermostat in there. That should drive the mixing valve, in conjunction with an outdoor reset, so that the slab is held at a constant temperature which is just warm enough to offset the heat loss from the space. Keep in mind that a slab responds very very slowly -- it can take several days to change even a few degrees -- so you want it to be holding a temperature, not trying to move up and down. And the slab thermostat will do just that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • bmaclean
    bmaclean Member Posts: 11
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    Hi Jamie,
    There is a water supply temp sensor (right on one of the pipes) which along with the outdoor reset drives the mixing valve. The in-slab thermostat only can turn the pump off.
    Thanks.
  • Jamie Hall
    Jamie Hall Member Posts: 23,661
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    OK. That works... Maybe you don't need it -- but it's a lit easier to have it and not need it than the other way around!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,491
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    Yet another option is to have indoor and outdoor sensors. Indoor sensors are helpful if you expect to see large load changes inside the room, like frequent parties :)

    Just because outdoor temperature is dropping doesn't always mead the slab needs warmer supply, and that is what the outdoor reset controls.

    Indoor room temperature give you an even finer control over the rooms comfort.

    A slab is like a large flywheel it can be slow to ramp up or down.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream