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Good control for electric radiant slab

A friend has a glazed-in room with electric elements in a slab, controlled by an on-off slab setpoint control, and is experiencing constant indoor temperature variation - no surprise there, really. What's a good, relatively simple control that will vary the slab temperature based on what it figures is needed to meet the indoor temp setpoint?


  • Gordan
    Gordan Member Posts: 891
    Sorry, couldn't hear that from all the crickets. :-)

    Seems like a relatively common question to ask, but my searches and my perusal of Tekmar's offerings haven't resulted in an answer.

    There are hydronic outdoor reset controls that are designed to vary supply water temperature based on outdoor temp, and some have indoor feedback. Then there are setpoint controls with slab sensors. Then there are snowmelt controls that seem to offer slab temperature control on outdoor reset, but these are geared to temperatures for snow melting rather than heating. Given that heat output from a slab is roughly proportional to slab surface temp minus ambient temp, wouldn't one want to vary the slab temperature to try to match anticipated heat loss?

    Tekmar 509 does sense slab temperature but its control function appears to be, based on the docs, only to try to maintain the room temperature while maintaining slab temp between the preset minimum and maximum. Not necessarily to figure out the optimal slab temperature for maintaining room temperature.
  • cattledog
    cattledog Member Posts: 60
    I'm still looking


    You have presented the question/problem well. Slab temperature needs to vary with outdoor temperature, and, like you, I haven't found an automated way to do that.

    My experience is with a one zone, high mass slab and TT 110 boiler. The circa 1950 ranch house is well insulated, there is some solar gain, and I use a wood stove frequently. The winters (pacific northwest) are mild, and the slab is not insulated.

    I had poor results with a dual sensor control and there was no heat to the slab for extended periods of time, long recoveries, and overshoots. Room temperature was not very stable.

    I switched to controlling from the slab sensor only. Varying the boiler water temperature with outdoor reset does not change the slab temperature. Water temperatures to the slab are already low, and there appears to be no efficiency gains using outdoor reset on the boiler.

    I have played around with time based controls using a repeat cycle timer with the slab sensor/thermostat as a high limit, but have not found a way to automate the variation in time cycles with outdoor temperature.

    I believe I could get there with something like the time based building steam controllers from Tekmar or Heat Timer but that route appears to be over complex and expensive for my needs. I briefly looked at snow melt controllers like you suggested, but didn't find anything readily adaptable for an indoor slab.

    In my application, there is not much variation in slab temperatures needed, and I am OK with turning the set point up a few degrees when it gets cold.

  • SWEI
    SWEI Member Posts: 7,356
    proportional electric control

    Not sure if you solved this one, but Aube has several wall stats with PWM triac control that do a credible job with electric resistance heat.
  • Magnehelic
    Magnehelic Member Posts: 63
    If the Electric Heat is 208/240V

    I highly recommend this thermostat.  TL8230A1003 Honeywell LineVolt PRO Programmable Thermostat.  They are inexpensive (45-65 bucks) and I recently put two in on my electric radiant ceiling heat.  tehy work great and provide a very consistent zone temperature.  If it is 115V or low voltage this stat won't work and I have no experience to draw from for 115V or low voltage radiant.  Maybe look at and see what they use.
  • Gordan
    Gordan Member Posts: 891
    Don, thanks, but...

    Radiant ceilings will be low mass, whereas the slab is high mass and so will respond very slowly. Would you recommend that stat under those circumstances, as well?
  • Magnehelic
    Magnehelic Member Posts: 63

    I would.
  • Gordan
    Gordan Member Posts: 891
    Thanks, I checked them out and they look interesting!

    I was expecting that they would use the triac the way incandescent dimmers do, by chopping the sine waveform, but they appear to only use it as a quiet, non-mechancal relay. Probably out of concern for noise (audible or electrical) from the resistive elements? Not an issue functionally (given the flywheel effect), just my curiosity.
  • Gordan
    Gordan Member Posts: 891
    Thanks for the recommendation!

    With the Aube that SWEI mentioned, this gives me two decent options.
  • SWEI
    SWEI Member Posts: 7,356
    Aube = Honeywell

    Honeywell owns Aube - the H branded stats probably come off the same assembly line.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,217
    I would be cautious with a controller

    that turns down the juice going to the electric heating coils. I would rather alternate the coils in the floor and stage the leads with contactors.
This discussion has been closed.