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Slab Thermostat Recommendations

Chevra Member Posts: 8

I didn't get many replies in the thermostat section so lets try here.

Running a Navien condensing boiler for the heated floor in my shop. Single zone and currently using air temp thermostat control. I would like to switch to a thermostat that can control by slab temp and air temp. It felt like on certain days when it was warm outside the slab would cool down to much, then the boiler runs way more to catch up once the sun goes down. (I have ODR sensor hooked up now as well)

What are some recommended thermostats to go with? I don't want anything fancy, nothing programmable or anything of that sort. Basic but reliable. Will entertain any other ideas to increase efficiency to. I have the tekmar 519 on my radar so far.


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,693
    edited October 14
    There are some sophisticated systems that have algorithms that calculate slab temperature, room temperature and outdoor temperature in order to maximize the savings on major projects. But for a little garage project, I don't see the cost of such an expensive control system being a viable expenditure.

    Does the boiler have a place to wire up a second temperature sensor? Remember you are probably using an on off thermostat for the air temperature, You are probably looking for an economical floor sensor that has an on off feature also. If you put them in series, then both need to be calling in order to get the burner to run. If you wire them parallel then either one will operate the burner and that may cause overheating if the slab stays warm and the afternoon sun also heats the air in the room.

    I have not had any luck with that way of doing things. Pick one or the other but not both.

    Bottom line. Is the room comfortable during the extended run time after the sun goes down? The extended run time is really not a problem unless the room temperature actually drops to an uncomfortable level at some point in the day. If you are always comfortable, then the air thermostat is doing its job.

    @hot_rod may know of something that I am not familiar with.

    Look at it this way... if it is off for 4 hours... then it runs for 4 hours and you are comfortable, that is tha same as running 15 minutes on and 15 minutes off fir 8 hours in a row. the system still runs for the same amount of time.

    Same difference on modulating systems. If it is off for 4 hours and it runs at 80% input for 3 hours the ramps down to 40% once it catches up, that is the same as running at 40% input for 6 hours. Your building is not using more energy than it needs. It uses exactly what it needs when it needs it.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Chevra
    Chevra Member Posts: 8
    I definitely see what you are saying. I am probably overthinking the situation a bit. The shop did always stay comfortable and I never had an issue hitting setpoint. I also had the system installed towards the end of the winter so I don't have a lot of test time yet. Hooking up the ODR made a difference since it now adjusts the boilers supply setpoint based on outdoor temps. I played around with pump speeds as well and it didn't make much difference to my return temps. The boiler has the option of hooking up supply and return temp sensors directly on the zone manifolds. Maybe this is something that would benefit me more, but also not sure what it does to the boilers algorithm in the end.

    As far as the thermostat goes. I wanted a single thermostat that monitored both air temp and slab temp, not two separate thermostats. I don't know exactly how those thermostats work though. Do you set it to either work off air or slab? Or is it smart enough to pick between the two and do what is best?

    I am probably obsessing a bit but saving propane is the end goal. The on/off air temp thermostat is working fine. I should give it some more test time before I start buying to many things but I am also open to other peoples recommendations and experiences. There are so many variables to play around with now in search of "maximum efficiency" but it is kind of a fun game. (Such as does a person run the ceiling fans or not?)

    Let me know what you guys think, always excited to learn. Also the shop is 30x50 if that makes any difference to anyone.
  • GroundUp
    GroundUp Member Posts: 1,737
    Floor temp is directly relative to air temp, so there's not much to gain here. If the slab is going to maintain a warmer temp than it is currently, the air temp will also be higher during that period. Perhaps your SWT to the slab is too high, causing this flywheeling? With outdoor temps like we're having now, a water temp only 10-15* above desired air temp is pretty typical. I'm running my SWT at 74* right now for a 68* air temp, and on a design day of -30F it's only 95* so the ODR curve is short.

    With that said, I've had the best luck using the Honeywell T6 Pro thermostats in applications where a floor sensor is needed. Those applications are typically a space with both radiant and forced air, so that the floor doesn't get cold due to the forced air maintaining ambient air temp. I find no value in a slab sensor without a secondary heat source, personally.
  • hot_rod
    hot_rod Member Posts: 20,725
    The best way to save fuel is make sure the building is well insulated and tight. I’ve found the toughest detail is weatherstrip around any overhead door. That is where you can get a lot of cold air entering and make the shop uncomfortable

    Tekmar and HBX have dual control radiant tstats. Basically you set the slab to never exceed, or go below a temperature, the air sensor still controls. It is more of a comfort stat for bathrooms to always maintain a floor temperature

    Running the SWT as low as possible, use step firing if you boiler has it will keep the boiler running as efficient as possible.

    The building construction dictates the heat loss, and the cost to heat it.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Big Ed_4
    Big Ed_4 Member Posts: 2,523
    I like Tekmar they do it right ....
    I have enough experience to know , that I dont know it all
  • Chevra
    Chevra Member Posts: 8
    Appreciate all the comments. I have been focusing on sealing up the building better before the weather takes a turn here. Replaced door seals and sealed up around some piping going into the building.

    One thing I forgot to mention is that I do have a wood stove in the shop as well. With that being said, it is not used that often. It is only for special occasions when we have friends over or if the wife wants to sit and watch me work on something. Therefore I don't think it is enough of a reason for a slab sensor yet.

    I think I will hold off on buying a slab sensor thermostat for now. Instead I will focus on dialing in my ODR curve. I can't remember off hand what SWT I landed on last winter. I will revisit the slab sensor possibility later on.
  • hot_rod
    hot_rod Member Posts: 20,725
    It can be tricky to maintain a comfortable room temperature with just a slab sensor. Your body feels the ambient temperature in the room, so you need to have that input to he temperature control
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream