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Controlling Radiant with a forced-air system

myqhenryt
myqhenryt Member Posts: 28
We have a single-story ranch which is currently heated via a hydronic heat coil (via an oil fired boiler) in a Lennox air-handler. There are two zones through the use of Lennox's zoning system using Honeywell's th8320wf1029 thermostat. A couple years back we had added on to the house and had Warmboard installed for approx 500sqft over an unheated crawl space. While the piping is in place with a manifold, we had never tied the Warmboard into our boiler. We're now in the midst of having a Lochinvar Knight boiler installed and we'd like to get the radiant setup.

The concern we have is how we control the radiant. We're basically looking to keep the floors warm when it's pretty cold out. When it hits 20f outside, the floors feel fairly cold. I'm not aware of how we could control the radiant with our existing honeywell thermostat (in that area).

Should we have a separate thermostat installed in that area for the radiant? Or there better (simpler) options given that we're only looking to keep the floors warm and not heat that space completely via radiant? Is it possible to have a setup where the radiant only comes on when it is a certain temp outside?

Thanks!

Comments

  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    Assuming the radiant area is included with the rest of whatever hydroair zone, my initial thought is to use a seperate thermostat in that space with a slab sensor. Uponor 511 would the ticket. Assuming the finish floor is already down, and you can't embed the floor sensor (sold seperately) your next option is to fasten it to the bottom of the WB, and sandwich it under a piece of rigid Styrofoam. Do not penetrate the WB more than 1/2" when screwing the foam up!

    you can then set the thermostat to maintain floor temp, regardless of air temp. A little playing around with settings will be needed, bc eventually the room may over heat, especially when the hydro kicks in. Any way to access the ducts that supply that room?
  • myqhenryt
    myqhenryt Member Posts: 28
    Yup, the radiant area is currently heated via hydroair and the floor is finished. We do have carpet in one room which might be easy to slide a sensor under.

    We do have access to the ducts over the area.
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    Is the radiant area one of the 2 zones of hydro, or are there more non-radiant rooms in that zone.

    Well, the slab sensor typically is sent down a sleeve that's embedded in the radiant panel (WB/poured slab/other). The sleeve goes into and up the wall to the thermostat. Or someplace else accessible. The sensor is snaked into the sleeve. 10' lead length. I can't picture a sensor under a carpet and it's wire lead going.........where, on its way to the thermostat.
  • myqhenryt
    myqhenryt Member Posts: 28
    The radiant area is in one of those two zones and there are other non-radiant rooms in that zone. The original plan was to install JoistTrak for those other areas, but budget prevented us from doing so.

    Attached is a floor plan of the relevant zone generated from LoopCAD --hopefully it uploaded via my phone correctly. The foyer, laundry, dining, powder, and TV rooms have Warmboard.
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    So it is that you cannot shut that zone off and use purely radiant or even hydro as a second stage- because some areas that are not WB would receive no heat.
  • myqhenryt
    myqhenryt Member Posts: 28
    Unfortunately, it doesn't look like we can shut off that zone.

    Does it make sense having a thermostat hidden in one of the closets (above the radiant) or even behind the laundry sink that controls the radiant? Maybe even have a floor sensor epoxy'd to the top of the tile floor in one of those locations?
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    Ok. Just checking if you can make a clean seperation of radiant/hydro and control the hydro as supplement or even cooling only. No, so it's gonna be floor conditioning.

    Absolutely. If you're running slab sensing only, the stat location won't matter. I've stuck them in laundries or pantries. Sure, you can epoxy the sensor to the surface, it ought to be generally in an active radiant area, and not in a real confined space for heat to get bottled up. I'd still cover the rest of the sensing bulb with rigid foam to get a more accurate sense. I'm assuming you're talking about an exposed sensor and wire running on floor to the wall, up to the stat.

    Viega also makes a slab sensing stat that's simpler to operate, costs less, and the sensor is smaller. I've been using them lately. Again, the sensor is sold separately. You'll need a minimum of 4 conductor 18 gauge wire with either stat running back to equipment.