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Concrete slab SWT+ FLOW rate?

Jlinde37
Jlinde37 Member Posts: 31
Hey guys, new here. I live near Buffalo NY zone 6. Last night was -10F. I've had my radiant setup in the first level of my house for about 5 years. Trying to find the optimal floor temp and flow being most effiecient and comfortable.. i know i know it really needs to be variable with ODR.

Anyways, Here's my current setup..
Long story short...2story house was build on a monolithic slab, very poor pour, no pun intended.  I over poured 4" of concrete Inside the building so no exposed edges except where the garage door was. (16'), r5 under the new  slab and r10 along edges.  1000sq ft, 1/2" pex in 5 loops roughly 200' in length. So 1ft tubing per sq.ft.  This is a dual temp setup with CI rad/fin tube on second story. 1 radiant zone, 2 CI/fin tube zones upstairs. Piped in primary/secondary with a hanfed DS coal boiler. 

For most part I've ran it at 100°F SWT. On warmer days I do get pretty bad overshoots, and on really cold days I get close to condensing boiler return temps.
The primary grundfos 15-58 circ runs constant on speed . The radiant pump is an alpha and manifold has flow meters usually 2.4GPM combined on constant speed 2 or pressure 2(Not really sure the difference they both flow about the same.)  Temps are adjusted with a honeywell am-1 mixing valve.

Last night for the heck of it I set the SWT 83° and was getting roughly 70° back for a 13° deltaT. flowing 2.4gpm. That's only 15,600btu and 15.6btu/sqft. If I'm doing the math right. The Tekmar 519 was set at 68ambient with the slab sensor reading 71 when I got up. I was actually shocked it could satisfy the tstat with these supply temps amd that BTU output. As stated, I have normally been running 100° SWT.

 Shoukd I be setting my slab temp a few degrees above my ambient set point? Or below?  Or even at? I feel if it's above it will overshoot on warmer days. Just trying to make sense of it all and i've been reading all the threads I can find. And still not convinced I know what I'm doing. Lol

So my main question is should I be running say 82 degree water or 100° water and at higher flow or lower?  In the last hour it took me to type this the floor pump has cycled 3 times, on for about 10 mins each time . Not sure if that's considered short cycling or not. Should I be aiming for a floor pump that doesn't shut off? Or a pump that only runs once per hour? 

TIA Josh

Comments

  • Hot_water_fan
    Hot_water_fan Member Posts: 1,151
    If lower temperatures work, you should use them. The slab temp needs to be higher than the ambient temp to transfer heat. 16 btu/sqft is in the ballpark.
  • hot_rod
    hot_rod Member Posts: 18,900
    You will get less temperature flywheel effect in slab radiant if you keep the SWT as close to the load as needed.

    You get about 2 btu/ sq ft per every degree difference between the slab surface and the ambient temperature around it.

    So in a room at 70F, a 78 slab surface provides about 16 btu/ sq ft output.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jlinde37
    Jlinde37 Member Posts: 31
    If my slab were 78° degrees my house would be cooking. I've never seen it above 72°. My slab temps are always with 2 degrees of the ambient temp which I normally set at 68. I will report back today as the temps are going to be in the 40s and sunny. I'm betting I have to keep my windows open or it will be too hot.  Should I be trying to control the temp with only slab temp or air temp as well or both?
  • Jlinde37
    Jlinde37 Member Posts: 31
    edited February 5
    So if it's only say 2 degrees different ambient/slab temp it's only putting about around 2000btu/Hr? That must be enough to overcome the heatloss in the room? I thought the btu output was the deltaT at the manifold x flow x 500. Or is that just the Input btu?  In my case that's roughly 16000 btu in to get 2000 out? Am I understanding this correctly?  

    I love hydronic heating but have never been formally taught, I'm a DIYer. Your responses are greatly appreciated!
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,151
    Your equation is right but the boiler wasn’t running nonstop for the hour, right? So a fraction of the 16kbtu was put into the slab. The delta T will move too - it’ll start wide then narrow as everything heats up
  • hot_rod
    hot_rod Member Posts: 18,900
    I’ve heated a handful of ICF homes, on some days they maintain temperature just from the internal gains, cooking, lights, people, appliances running, etc

    one home had dozens of ceiling can lights that also added heat to the space

    One gal in particular complained the floors never felt warm, yet the home maintained 70-72 with very low SWT

    If you are set up on outdoor reset, quite possible to see SWT around 80 or less on many days

    Your application is a dream for condensing boilers, expect 90% + operating conditions
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jlinde37
    Jlinde37 Member Posts: 31
    One more question.  The tekmar 519...slab temp set to 67, slab is at 67. Ambient set to 67. It's 71° in the room.  Why is the thermostat calling for heat for 10 mins?  Is the tstat that sensitive and see a half a degree below the slab temp and try to maintain it?  
  • Jlinde37
    Jlinde37 Member Posts: 31
    Your equation is right but the boiler wasn’t running nonstop for the hour, right? So a fraction of the 16kbtu was put into the slab. The delta T will move too - it’ll start wide then narrow as everything heats up
    Ahhh! That makes sense now! Since the circ only ran for 10 mins. Not a full hour. It only ran for 1/6 of an hour yielding roughly 2,600Btu.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,151
    Yup! The 500 in that equation is a rounded version of 8.34 lbs/gallon x 60 minutes/hour. 
  • Jlinde37
    Jlinde37 Member Posts: 31
    It's still overshooting. I don't get it. This morning it was 71 slab temp was 67 tstat was set at 68. I wouldn't think it would flywheel that much with only 80degree water entering the slab
  • PC7060
    PC7060 Member Posts: 923
    edited February 6
    A slab has a huge mass. Visualize stopping and starting a 16’ radius 8 ton flywheel. 
    nate379
  • hot_rod
    hot_rod Member Posts: 18,900
    Jlinde37 said:
    One more question.  The tekmar 519...slab temp set to 67, slab is at 67. Ambient set to 67. It's 71° in the room.  Why is the thermostat calling for heat for 10 mins?  Is the tstat that sensitive and see a half a degree below the slab temp and try to maintain it?  
    Are you running as floor min or max?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jlinde37
    Jlinde37 Member Posts: 31
    hot_rod said:
    Are you running as floor min or max?
    I don't think there is a min/max setting on the floor. I believe it's minimum.  It keeps a minimum 67 slab temp no problem but by doing so it's overshooting the ambient temp. Even with swt set at 80. The ambient temp is always above my set point unless it's really cold like single digits. The mixing valve goes as low as 70 it says but the gauge says it's 76. Maybe I'll try that. But that's still a 12 degree deltaT
  • Jlinde37
    Jlinde37 Member Posts: 31
    Ok after a few weeks of messing with temps and flow rates I still can't dial this in.  Constantly overshooting air temp set point.  It seems like the temperature overshoots more so with lower supply water temps.  This is really driving me crazy.  Today it's cool out about 35F and sunny. The slab is set at 65 and the air temp is set at 65. It's 72 in the house.  Should I try lowering the slab temp to like 63?  I also bought a neat little thermal camera cellphone attachment to confirm floor temp and water temps.  Most threads I've read suggest 72° slab set points with 68° air temp at eye level.  If I were to set my floor to 72 it would be 82 in the house on a day like today. I wonder if I'm just getting too much heat from the coal boiler itself. It's in a separate mechanical room off the first floor but I usually leave the door cracked open for the cat.
    Only other thing I think to try is lowering the slab set point some more. But my fear by doing that is if we get a really cold demanding night that it will run like crazy to heat the floor back up 3 or 4 more degrees.  Shouldnt the slab temp be the same as my air setpoint? Seems like my setup is contradictory than everything else I've read. 
  • hot_rod
    hot_rod Member Posts: 18,900
    get some get some additional data. When the thermostat is off does heat flow stop 109% from the boiler, or dies heat still transfer until the boiler fire goes down?
    are you getting solar gain? Any south facing glass?

    It should be easy to determine if residual heat from the coal boiler is finding it way into the space causing the overrun

    Feeding the floor via an outdoor reset control may help

    Got a pic of the boiler piping?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 20,840
    Also -- is the slab being heated all the time? If it's temperature is allowed to drop and then you ask it to heat itself back up -- and the air -- it's going to overshoot. Pretty inevitable.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Jlinde37
    Jlinde37 Member Posts: 31
    hot_rod said:
    get some get some additional data. When the thermostat is off does heat flow stop 109% from the boiler, or dies heat still transfer until the boiler fire goes down?
    are you getting solar gain? Any south facing glass?

    It should be easy to determine if residual heat from the coal boiler is finding it way into the space causing the overrun

    Feeding the floor via an outdoor reset control may help

    Got a pic of the boiler piping?
    It does it at night as well. So I know it's not all solar gain.  The boiler never really shuts down. It just idles along maintaining 180° So there isnt really temp swings from the boiler itself. It has a 40 gallon capacity and when the floor calls for heat it doesnt even change the boiler temp but maybe a few degrees. It's plumbed primary secondary.   The floor pump runs roughly 3 times per hour for about 6 mins each time maintining the slab to 65. 
  • Jlinde37
    Jlinde37 Member Posts: 31
    Also -- is the slab being heated all the time? If it's temperature is allowed to drop and then you ask it to heat itself back up -- and the air -- it's going to overshoot. Pretty inevitable.
    What do you mean heated all the time? I mean ya the tekmar 519 is maintaining slab temp at 65. But I wouldn't think that alone would overshoot the air set point by 5 degrees. That's my next idea. Lowering the slab temp more. But then say tomorrow when it gets really cold and then it has to raise the slab temp a bunch again.  I mean I shouldn't complain...It heats my house very well. I guess I'm just trying to fine tune it.
  • hot_rod
    hot_rod Member Posts: 18,900
    The thermostatic mix valve will drift a bit also. Has it ever been cleaned? When they get sticky they can send out wide temperatures swingsYou must have it set for the highest temperature required on design day?

    So 80% of the year it is sending SWT higher then you need. As such a slab will flywheel beyond the thermostat setting.

    Constant circulation can lessen the overshoot, or a mixing valve that can vary SWT based on loads.

    Poorly insulated slabs, and monolithic pours that you mentioned worsen the overshoot, more mass to ramp up and down.

    Why is the boiler always running 180? Is there as tankless coil?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGross
  • GroundUp
    GroundUp Member Posts: 1,545
    Turn the floor sensor off completely and run it off the air stat only. That will tell you what you need to know, but there's a really good chance that your solid fueled boiler and piping is simply radiating heat out into the living space.
  • Jlinde37
    Jlinde37 Member Posts: 31
    edited February 23
    Hot rod,  Yes I clean the screens prior to every heating season and put some faucet grease on the O rings. The boiler is always at 180 because it's a hand fed coal boiler with primary pump running 24/7.
    I do heat my DHW with a sidearm however.
     The foundation is monolithic but not heated. When I purchase the house I put 2" foam down and over-poured the SH*TTY slab "inside" the house. Its essentially an insulated slab on a slab.  As far as setting the SWT I've tried everything from 75-120°.  It
    Still overshoots ambient set point at 75swt on a 30° night... But the pump runs alot more maintaining slab set point.

    Should SWT be cooler water, say 75 on a 35°F day?
    And 100° on a say 0°F windy day?
    Should pump pump run faster when SWT is cooler or warmer? Or just set flow at .5GPM/loop and forget it. 

    Maybe I should just pony up and install a Taco I-Series mixing valve and get an outdoor sensor. It seems kinda silly using an archaic handfed boiler with outdoor reset. Still won't change my fuel consumption. I burn 40# a day no matter what it's doing outside... just bugs me everytime I walk by the thermostat and its it's 72 when I told it to be 67. Lol

  • hot_rod
    hot_rod Member Posts: 18,900
    Close to perfect high mass radiant control would have two inputs, both outdoor temperature and indoor feedback.

    ODR can make a big difference in control, once you take the time to get it dialed in. That can take an entire season. You need to be able to monitor and adjust to get full benefit.

    However just because outdoor temperature is dropping doesn’t alway mean the SWT needs to ramp up. So indoor feedback, even slab sensing can override the ODR request.

    A fellow named John hangs around check out his control at www.exqheat.com. He is an indoor reset proponent
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Jlinde37
  • Jlinde37
    Jlinde37 Member Posts: 31
    GroundUp said:
    Turn the floor sensor off completely and run it off the air stat only. That will tell you what you need to know, but there's a really good chance that your solid fueled boiler and piping is simply radiating heat out into the living space.
    Ya that's what I don't want to do is come home at night and slab is freezing and it gets cold out and I have to heat all that concrete back up.  Trying to find the happy medium. 
  • yellowdog
    yellowdog Member Posts: 68
    in my experience anything with a floor sensor overshoots the room temperature. you're going to have to pick what you want, either a constant floor temp or a constant room temp. cant have both.
    nate379
  • hot_rod
    hot_rod Member Posts: 18,900
    edited February 23
    I think that thermostat watches both temperatures when you add a sensor.
    Also, pretty sure it can be used with a not to exceed slab setting. set that to 78 and it would not go beyond even with an air temperature heat call. Try it and see. Why else would you add the floor sensor?

    Just adjust that parameter, add the sensor right at the stat, warm the sensor with your hand, observe what the stat does, it’s a 10 minute test.

    The manual tell about how to use the floor sensor and the two control options how is yours set?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jlinde37
    Jlinde37 Member Posts: 31
    Currently the stat is set floor temp at 64 with max of 74. The room air temp is set at 67. The air sensor never calls for heat. The slab never goes over 65. Just maintaining the 64 slab temp the ambient temp is almost always 70/71.  That's why I'm scratching my head. If the slabs always colder than the air temp how is it still heating the room? Wouldn't the floor need to be warmer than the air for it to actually 'heat' the room?
     If I use only air sensor only the slab temp drops during the day then when it calls for heat in the evening it runs a long time heating all that concrete back up with a wide Delta T its  pushing the boiler to or near condensing temps...especially on a day like today high of 51 and sunny and night-time low of 18. That's why I like the slab sensor as to not get "behind" in the evening. Or well atleast thats how my brains thinking. 
  • Jlinde37
    Jlinde37 Member Posts: 31

    yellowdog said:
    in my experience anything with a floor sensor overshoots the room temperature. you're going to have to pick what you want, either a constant floor temp or a constant room temp. cant have both.
    That's what I'm gathering. No matter what I do it seems to overshoot. But how would keeping slab at 64 heat a room above 70? If I set my slab to 72 it would be 80 in the house!  Many threads I've read reccomends floor temp 2 degrees above desired setpoint. Don't seem to be in this case. 
  • GroundUp
    GroundUp Member Posts: 1,545
    Jlinde37 said:


    GroundUp said:

    Turn the floor sensor off completely and run it off the air stat only. That will tell you what you need to know, but there's a really good chance that your solid fueled boiler and piping is simply radiating heat out into the living space.

    Ya that's what I don't want to do is come home at night and slab is freezing and it gets cold out and I have to heat all that concrete back up.  Trying to find the happy medium. 

    If the slab is the sole heat emitter, it would be quite impossible for the slab to go cold. If you want a 67 degree air temp, you have to set the air stat to 67 degrees and shut the floor sensor off. That's how this works.
  • hot_rod
    hot_rod Member Posts: 18,900
    Jlinde37 said:

    Currently the stat is set floor temp at 64 with max of 74. The room air temp is set at 67. The air sensor never calls for heat. The slab never goes over 65. Just maintaining the 64 slab temp the ambient temp is almost always 70/71.  That's why I'm scratching my head. If the slabs always colder than the air temp how is it still heating the room? Wouldn't the floor need to be warmer than the air for it to actually 'heat' the room?
     If I use only air sensor only the slab temp drops during the day then when it calls for heat in the evening it runs a long time heating all that concrete back up with a wide Delta T its  pushing the boiler to or near condensing temps...especially on a day like today high of 51 and sunny and night-time low of 18. That's why I like the slab sensor as to not get "behind" in the evening. Or well atleast thats how my brains thinking. 

    The ONLY way a 65 degree slab surface can warm a room to 70 is other internal gains, there is no thermodynamic magic in play here.

    Heat transfer can only occur from a warmer surface to a cooler one. 65 cannot heat to 70, there is no energy available to cause the movement of heat.

    Can you boil water on your stove at 150 degrees?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Jlinde37nate379
  • GroundUp
    GroundUp Member Posts: 1,545
    hot_rod said:

    Jlinde37 said:

    Currently the stat is set floor temp at 64 with max of 74. The room air temp is set at 67. The air sensor never calls for heat. The slab never goes over 65. Just maintaining the 64 slab temp the ambient temp is almost always 70/71.  That's why I'm scratching my head. If the slabs always colder than the air temp how is it still heating the room? Wouldn't the floor need to be warmer than the air for it to actually 'heat' the room?
     If I use only air sensor only the slab temp drops during the day then when it calls for heat in the evening it runs a long time heating all that concrete back up with a wide Delta T its  pushing the boiler to or near condensing temps...especially on a day like today high of 51 and sunny and night-time low of 18. That's why I like the slab sensor as to not get "behind" in the evening. Or well atleast thats how my brains thinking. 

    The ONLY way a 65 degree slab surface can warm a room to 70 is other internal gains, there is no thermodynamic magic in play here.

    Heat transfer can only occur from a warmer surface to a cooler one. 65 cannot heat to 70, there is no energy available to cause the movement of heat.

    Can you boil water on your stove at 150 degrees?
    Do you think there's a possibility that his slab sensor is in a poor spot and is reading lower than the rest of the slab? Or possibly a bad sensor?

    Jlinde37
  • Jlinde37
    Jlinde37 Member Posts: 31
    It's not the sensor. It's accurate withing 0.5° of both IR thermometer and thermal imaging. Also placed between pex tubing and in the middle of the slab
  • Jlinde37
    Jlinde37 Member Posts: 31
    It pretty much has to be radiating heat from the boiler itself. I'm gonna shut the the slab sensor off tonight see if temp is still higher in the morning. 
  • hot_rod
    hot_rod Member Posts: 18,900
    if you have a thermal camera this should be a simple troubleshoot?  Point it at every object within the building envelop, it should be obvious which are above ambient temperature 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream