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Confused about slab sensors

Here's an option for a sensor well. The conduit goes up to a box where a t-stat can be mounted. With the exception of the copper stub, using electrical conduit has kept the inspectors happy. I needed to slightly ream the conduit bend to accept the stub

Best Regards



  • Ken
    Ken Member Posts: 26
    When to use a slab sensor?

    I am confused about slab sensors. I had the impression that they could help control in a room were solar gain was a problem, but I don't really understand how.

    Now the post from Rick entitled hydronic floors appears to suggest that they could be used instead of a thermostat. It appears to me that he wants to set the slab temp at 85 and leave it there. This would not lead to a comfortable room temp over much of a shift in outside temp.

    When would a slab sensor be an asset to help control the comfort level and how are they coordinated with the thermostat.

    Ken Caverly

  • Boonierat
    Boonierat Member Posts: 58
    When to use a slab sensor


    One of the applications I recommend slab sensors frequently is when I need to 'lie to the thermostat'; That is to say when I have a room that gets a lot of solar gain I use the sensor to 'overide' the thermostats direction to shut down the zone when solar gain comes into play. Keeping the floor at 83/84/85 degrees on a continuum is much smoother that trying to recover from a zone shut-down in the afternoon. it's not the only use but one that is popular in my neck of the woods.
    Stay loose; stay well,...................Nels
  • hot_rod
    hot_rod Member Posts: 21,818
    Using a slab only

    sensor can be done if the load is consistent. A dual air sensor/ slab sensor is a better approch. Yeah, there are times when maintaining a slab ay say 80- 84 will overheat the space. Just depends on how much heat the "space" is losing.

    I use a setpoint, slab only, control on my bathroom floors. I've found with the door left open (when unoccupied :) the space does not overheat. I do set it back in the summer months to 78 or so to prevent adding too much additional load to the ac.

    You need to play around and find the best control, or combinations of, for your specfic application. The Tekmar/ Wirsbo 500 series dual air/ slab stats have lots of features to customize them to your application, including set back and wake schedules. Carefully programed you could "pulse" heat to an area and customize the control darn near perfect. Still as the load changes based on outdoor temperatures, so does the floor output.

    It always comes down to how much $$ you are willing to spend to get it near perfect.

    ME has some good ideas for daylight or solar gain responsive controls. If only he had more free time to perfect this weather, sunlight, building & occupant load sensitive thermostat :)

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ken
    Ken Member Posts: 26

    So you are telling me that you use the slab sensor to override the thermostat to keep the heat on even though the thermostat is reading that it is warm enough. This creates a little extra heat at the time but moves you comfortably into the period when the sun goes down rather than having a period of chill.

    I had thought the opposite. That the slab sensor somehow could react to the solar gain and shut things down before it got too warm. No wonder I couldn't follow the reasoning process thru.

    Now I am back to asking what type of controls, if any, can anticipate or quickly recognise the solar gain problem and respond to it.

    Our house has had some problems with this solar gain and I want to try to reduce this if possible when the floor is torn up and redone. The new layout should require a much lower supply temperature and I am wondering how much of a difference that will make. Current design degree temp for that room is 135 degrees.

    We are not on an outdoor reset but expect to be with the changes. Most of the winter runs around the freezing mark but the design temp is minus 15 F.

    Are these slab thermometers only used in certain rooms such as bathrooms or sunrooms most of the time. Wouldn't most other rooms be fine with just a thermostat.

    I hope I haven't asked more questions than allowed. I have done lots of reading but these sensors have always remained a puzzle.

    Ken Caverly
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,479

    I know of a job where a slab/space tstat would of been better. I did a radiant floor in a fancy bathroom. (green marble imported from Italy dahling) When it was done and operating it would warm up and shut down on the air sensor because of the shower warming up the space. The person taking the second shower would come in and say hey I thought this floor was supposed to be warm. They would call me and I'd show up with my IR thermometer and show them it was indeed working but..... If I had a slab sensor I could have kept a bottom limit on the floor temps to keep their feet feeling the comfort. WW

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  • Bill NTSG
    Bill NTSG Member Posts: 321
    To add to

    your confusion, another application could be a type of high limit use of the floor sensor. To protect your feet from too high a temp. and to protect a "fancy" wood floor. Right now my floors are on a room temp t-stat and if I use the wood stove or have a warm day, I have a cold floor in the bath room. Next step I will install a floor sensor like Hot Rod suggest to keep mama's bath room floor warm. I have not used one but that wirsbo t-stat seems interesting
  • Ken
    Ken Member Posts: 26
    Fireplace fix

    Now I can see an application which fits the bill. My wife does like the fireplace for a little extra warmth in the evening or reading in the afternoon. I start working at the kitchen table and start to feel the chill. Keeping the floors on thru the slab sensor would solve this.

    Am I looking at alot of extra cost for a slab sensor system to be added at time of installation. Is it suitable for everyone or only the rich.
  • hot_rod
    hot_rod Member Posts: 21,818
    At this point

    all you really need is a condiut to slip the sensor into. 1/2" EMT electrical condiut or even pex with a copper stub off would work. Bring the condiut out to a point where you could slide the sensor into it later.

    Any setpoint control could be used for the control. Honeywell, Penn, Ranco, Heatimer, tekmar, Johnson Controls, literaly dozens of brands out there.

    These are two of my favorite, the RTI is real user friendly with a large digital readout and easy adjustments. The Goldline is less money but lower tech. Around 80 bucks at Graingers, including sensor. Both have adjustable differentials. These probably would mount near the boiler or manifold. Not a very attractive wall mount control, in my opinion.

    For a sensor socket, cap a piece of copper or EMT and embed it (with reporters :) in the slab. Or fasten to the bottom of the floor with clips or a chunck of heat transfer fin.

    Or pop for the Wirsbo brand dual stat that wraps all the high, low slab limits into a nice wall stat with air temperature sensing also. Probably worth the extra bucks to have all the features in one box, instead of trying to make two different stats communicate.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    Use the sun/Fight the sun

    The function of the slab sensor can be BOTH as a limiting device AND a minimizing device in the SAME application...

    How much of each depends on the weather and the design of the space.

    An intentional passive solar design will incorporate an extremely massive panel (generally the floor), usually in a darker color and most definitely hard surfaced (tile, natural stone, concrete, etc.) and few if any rugs. It is designed to capture as much energy from the sun (PURE RADIATION by the way) as possible. The remaining surfaces in the room are generally very light in color.

    As the sun shines through the glass of such a space it turns ALL of the surfaces into "radiators." These surfaces in turn heat the air primarily through natural convection. By having the reservoir (the floor) dark in color and the remaining surfaces light in color, attempt is made to reflect as much radiant energy as possible to the reservoir where it is later liberated to the space when the sun is not shining.

    For everything to work properly, the air temperature in such a space is INTENDED to fluctuate much more than in a conventional space. Excess heat can be provided more deeply into the structure through many means--natural convection; forced convection or even a "reversal" of the slab hydronic system.

    A proportional two-way slab-temperature sensing device like the Danfoss FHV-R will do two distinct things based primarily on the sun. As solar gain increases, it will decrease the flow of heated water proportionally. While the slab my go above setpoint, it will do so because of solar gain, not via hydronic input. When solar gain is insufficient or at night, it will prevent the slab from dropping too far in temperature thus helping the "flywheel action" to be maintained.

    IMO setback-type TRV-equipped panel radiators are an extremely useful adjunct in such a space as they allow a relatively quick and easy way to keep the space more consistent in temperature than nearly any other means.

    Unintended solar spaces though are MUCH different. Since the floor has MUCH less mass than in an intentional space, there is little reservoir for the excess solar radiation and daytime gain can go into "hyperdrive" so to speak--the so-called "greenhouse effect". Since there is no large "flywheel panel" temperature fluctuation can become extreme and a combined air/slab temperature controller (like mentioned by Hotrod) is likely a reasonable "solution." IMO, however, I believe it more effective in such a space to control the "problem" itself--the sun. Operable (and consistently operated) window coverings are a good solution. Motorized drapes/shades and freon-operated "mini-blinds" are both effective but quite expensive if you want to automate the process somewhat. While I have not seen them, I believe that proportional drape/shade controllers also exist, i.e. as the solar gain increases they close more fully.

  • Steve Eayrs
    Steve Eayrs Member Posts: 424

    I haven't tried it, but controls like the goldline have a NO or a NC option. Seems to me if you had the sensor somewhere where the sun was shining, you could shorten the heating cycle or lengthen it, your option.
    I don't think you would want a very big differencial setting though, since it may not kick back on soon enough.
    Anyone try this? Its not as fancy and nice as some of the above, but wouldn't cost much.
  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    Another,completely different option

    I have seen outdoor sensors used to help modulate systems according to solar load. The outdoor reset sensor is placed on the south wall in the sunlight and controls the reset curve for rooms on the south side. Was recently at a public housing facility for the elderly where it appears the only temp control for the 9 story building was this type of control scheme. The supply water temp to each side of the building was controled by an outdoor sensor placed on that side of the structure. Makes a lot of sense, especially with structures that don't have much mass so internal solar gains impact temperatures very quickly.

  • bruce pirger
    bruce pirger Member Posts: 111
    Indoor Air Feedback

    The Tekmar 363 can look at the outdoor temperature (reset) and also the indoor air temp, as well as the slab temperature. In fact, it can look at numerous air temps around the building. The idea is to throttle back on the heating based on the indoor temp...for example, a wood stove (that's my application). And with the slab sensor, a minimum (and max) can be maintained in the slab.
  • Ken
    Ken Member Posts: 26
    No such thing as a dumb question.

    I thought I must have missed the solution to controlling solar gain and overshooting problems in the reading. Now I realize that there isn't one simple solution.

    I have really come to believe that designing good radiant heating systems is both an art and a science combined. To entrust this important step to anybody other than a highly experienced and qualified professional is making a huge mistake.

    How can radiant heating be so highly regarded by the consumer when so many jobs are being designed by people without the skills to do it right.

    Here's a short story I just heard. A couple had radiant installed in thier new house using a well thought of installer. They wanted the bedroom cooler than the ensuite so the installer put it on the same loop but after it went thru the ensuite. They are now finding the bedroom too warm. They are going to tear up the tile floors in both rooms and replace them with electric heating panels over a wood floor to give them seperate controls. Now they don't want to say anything to the plumber or the building contractor because they don't want them to think they are unhappy with the house.

    Is it possible stuff like this happens alot but people paid extra to get radiant and now they don't want to admit they have some dissapointments.

    Ken Caverly

This discussion has been closed.