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Author Topic: Concrete Slab Inlet and outlet temps  (Read 333 times)

slimchance

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Concrete Slab Inlet and outlet temps
« on: November 29, 2018, 07:35:51 AM »

Anybody out there running solely a heated slab, I have a question.  What kind of temperature differentials are you seeing in your system between the water temps you have entering the slab and the temps you have exiting the slab.  I know that there are so many variables here, but I am really curious what others are experiencing.  I heat a 2400 sq ft. slab, 8 circuits, all are the same "zone", however I do control water flow differently in some of the circuits.  I am running a standard wall mounted digital thermostat (air).  After some trial and error, I have landed on 120 degree water going in the slab, and almost always, I have 79-81 degree water coming out of the slab.
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hoardac

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Re: Concrete Slab Inlet and outlet temps
« Reply #1 on: November 29, 2018, 03:32:05 PM »

I run mine way hotter. I know your not supposed to but I do and never had any problems in 10 years, the literature I read did not seem to concerned with temps under 300. I run right out of the boiler to hot water tank then to floors same setup only 6 flow zones adjust those as needed. I run the system tied into upper floor thermostat I put in this year so I have let it run and adjusted the flow to keep it a stable temp works great. Before that I let it run and just adjusted the flow as needed some days the cellar got pretty hot, tied into the thermostat is much better.

http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/28/057/28057515.pdf
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slimchance

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Re: Concrete Slab Inlet and outlet temps
« Reply #2 on: November 30, 2018, 06:36:32 AM »

That is funny to hear that you are running full-heated, not tempered down water into your floor.  I did that exact thing for 2 seasons until I had more and more people tell me that I was stupid for doing that because of the harm that will be caused to the floor.  Well, to say the least, I was a doubter, but I'm a great listener.  I still wanted to see some evidence of the claims of damage.  Keep in mind that where I live in southeast Kansas, there seems to be ABSOLUTELY NO professionals on heated concrete floors, so I have never been able to verify these claims of floor damage.  All that being said, I had no problems with moving forward, creating a primary and secondary loop, adding an inside pump and a mixing valve.  I got all of that done in a weekend.  It still bugs me that I have never heard of a single case of legit concrete damage from running 150-170 degree water through the floor.  Anybody out there want to share their first hand experiences? 
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wreckit87

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Re: Concrete Slab Inlet and outlet temps
« Reply #3 on: December 01, 2018, 09:02:07 AM »

What sort of trial and error landed you at 120? Is this slab insulated both underneath and around the perimeter? 40 degrees is a huge delta T for a slab that's been running. Not necessarily for concrete longevity, but you want to use the lowest water temp you can get away with both for efficiency and comfort. I play with mine a lot, and it's currently set at 95 degrees with a 72 degree return. Every one of my full installs ( I own and operate a hydronic radiant heating company) are set to 100 degrees off the bat and adjusted from there if necessary.

As for the slab damage, I don't have any catastrophic photos but I can assure you that anything over 140 WILL ruin a slab's heat transfer. The concrete around the tubing turns to powder and essentially you'd need to keep raising the water temp until it no longer keeps up because the tubing doesn't give up the heat to the concrete. Bad idea.
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slimchance

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Re: Concrete Slab Inlet and outlet temps
« Reply #4 on: December 04, 2018, 11:52:58 AM »

What sort of trial and error landed you at 120? Is this slab insulated both underneath and around the perimeter? 40 degrees is a huge delta T for a slab that's been running. Not necessarily for concrete longevity, but you want to use the lowest water temp you can get away with both for efficiency and comfort. I play with mine a lot, and it's currently set at 95 degrees with a 72 degree return. Every one of my full installs ( I own and operate a hydronic radiant heating company) are set to 100 degrees off the bat and adjusted from there if necessary.

As for the slab damage, I don't have any catastrophic photos but I can assure you that anything over 140 WILL ruin a slab's heat transfer. The concrete around the tubing turns to powder and essentially you'd need to keep raising the water temp until it no longer keeps up because the tubing doesn't give up the heat to the concrete. Bad idea.


wreckit87, that is the most information on the subject that I have heard yet, and I thank you for that!  My building is 60x40, I have a 1" owens-corning pink board + plastic under the slab and I'm not really sure what is on the sides if anything.  My building is completely spray-foam insulated, top and sides for a very tight building.  I have not played with lowering the input temps much at all.  Not being a professional myself, I guess that I figured by lowering the input temps, my output temps would only lower as well, therefore not producing the heat as needed to keep up with the demand.  I am beginning to see the concept of the lower temps having some benefit.  I'm curious, what type of room temperature are you set at with your 95 degree in and 72 degree out?  Another characteristic of my building is that the thermostat is in the living side of the building, set at 72 degrees, basic air thermostat only, and from what I am seeing, when it is calling for heat, the secondary pump will run for around 1.5-2.0 hours to regain the set-point temperature.  Then, I have seen it up to 5 hours before it calls for the heat cycle to start again.  Is there a "normal" time frame from the time the thermostat turns the pump on to the time the thermostat turns the pump off???  Also, it has not been super cold here yet.   Does any of this sound abnormal or crazy??  I appreciate your input....thanks!
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wreckit87

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Re: Concrete Slab Inlet and outlet temps
« Reply #5 on: December 04, 2018, 03:50:31 PM »

Sure, I'm always happy to share what little I know! A 5" slab will have an R-Value of about 1, so depending on the depth of your tubing there is always the percentage of heat that will go down through the insulation instead of up through the slab. Edge/perimeter insulation is key to a proper radiant slab as it's always adjacent to frozen ground versus the 50+ degree dirt in the middle of the slab and would explain your large delta T if you have little or none in place. When performing a heat loss calculation, I'd be lying if I said an uninsulated wall was worse than an uninsulated perimeter when using a radiant slab- the majority of your heat loss is likely going outward. In a perfect world, the slab would circulate constantly and adjust water temp to maintain air temp rather than cycle the pump with too-warm water. I'm afraid you may have cold spots along the walls if you drop the supply temp too far, but it will run longer and therefore keep the slab temp more even. My current setup with 95/72 keeps the building at 60, but I can achieve 70 with the same 95 degree water as it just tightens the delta to around 95/80. 1.5-2 hours on and 5 hours off is a pretty big swing, and I think you could benefit from lower supply temperatures to even that out to maybe 3.5 on and 3.5 off. Those cycle times will likely change when it gets cold out, and at that point maybe 120 will be the answer for you, but in this warmer weather it sounds like you could get away with lower. Maybe try 110 and see how it goes if you'd like to play with it, and you may find it can stay lower yet especially in the warmer times
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slimchance

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Re: Concrete Slab Inlet and outlet temps
« Reply #6 on: December 05, 2018, 07:21:12 AM »

wreckit87, I did a little playing last night.  I discovered something interesting that I have no idea what it means.  I ran my input water down to 100 degrees (from 120 degrees), and my output water temp only changed by about 1 degree at the most, looks like it is coming out at about 79 degrees.  I'm trying to watch and record to on/off times and watching my thermostat like a hawk, but that job is about impossible unless you have nothing else to do while at home.  I'm just would have thought that dropping my input water 20 degrees would have affected the output water temps more.  Would you suggest I drop some more to find some sort of "sweet spot".  I think understand what you are saying about the ideal "continuous ultimate-temp water flow", versus trying to "charge" the slab with hotter water and expecting it to radiate upwards to satisfy the thermostat in great, wider temp swings.  Am I catching on?
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wreckit87

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Re: Concrete Slab Inlet and outlet temps
« Reply #7 on: December 05, 2018, 04:21:48 PM »

Now you're catching on! The return temp is basically a reflection of slab temp, so running 5 hours at 100/79 keeps the slab at 74-78 degrees to maintain 70 degree air while the 120/80 swing you had before keeps the slab at 65-85 to achieve the same air temp with less comfort. I'd leave it at 100 for a few days and keep an eye on cycle times and return temps. Depending on the point in the cycle where you lowered SWT, the return temp may drop a little lower yet especially at the beginning of a cycle
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slimchance

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Re: Concrete Slab Inlet and outlet temps
« Reply #8 on: December 10, 2018, 10:21:05 AM »

Ok, I am totally a believer in the lower input temperatures sent into the floor!  It really works.  I have went all weekend running a 100 degree SWT, and at the beginning of a cycle, the RWT is right around 77-78 degrees.  It takes quite awhile, maybe 30-45 minutes, but the RWT will rise to about 80 to maybe 81 degrees and sit there for some time before the wall thermostat is satisfied at a 72 degree set point.   I will continue to play with the Supply water temp, running it down slightly more to see what it will do with the return temps.  I need to purchase an infra-red thermometer to check floor surface temps. 

The biggest problem I am facing now is the fact that once my wall thermostat is satisfied, and shuts the circulating pump off, my living space is so tight, that such a long period of time may pass before the wall thermostat calls for heat again, and in the mean time, I am loosing my fire in the OWB.  Hardy H2, I have removed both screws from the blower flap, I have turned the aquastat up to 175 set point.  In the past couple of weeks, I have come home from work to absolutely no fire in the box, having to unload and re-start a fire.  This is VERY frustrating.  I am simply not getting enough burn cycles during the middle of the day, allowing the fire to burn out.  Some may say this is a good problem, but really, it sucks.  This is an issue that I do not know how to overcome.  Maybe it will be better since I have lowered the SWT, creating a more constant cooler RWT to the boiler, therefore creating a few more burn cycles throughout the day.  Yesterday, being Sunday, I found the Wall Thermostat at 72 degrees, pump off, and about 4 hours later, it was 73 degrees in the house.  I ended up opening windows to get some cool air in the house (yesterdays high was about 39 or 40).  This is my year #2 and I'm still learning as I go!
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wreckit87

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Re: Concrete Slab Inlet and outlet temps
« Reply #9 on: December 10, 2018, 05:57:56 PM »

Glad it's working out! Sounds like you might be able to benefit from an even lower SWT to keep a little more constant draw on the boiler. I'm not familiar with the Hardy stoves but what is your differential set to? A tighter differential will help keep a fire going, or you could add a timer in the wiring like Heatmaster does to make the stove cycle for a couple minutes once per hour of inactivity or whatever it takes?
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slimchance

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Re: Concrete Slab Inlet and outlet temps
« Reply #10 on: Today at 11:06:53 AM »

OK, I'm going to try to lower the SWT a little more still.  When you say differential, are you talking about the aquastat on/off differential?  If so, I did not know that was an adjustment that I could make.  I know I can change the set point temp, but I did not know I can adjust the differential...I will look at that tonight.  Yesterday for an example, I checked/loaded stove at lunch, about 12:30, looked at it when I got home from work, about 5:30, it it honestly looked like it had not even performed a heat cycle in that amount of time.  It blows my mind that just the constant circulation of water itself doesn't drop water temp enough to cycle at least once in that time period....hence your suggestion of tightening up the aquastat differential.....   right?   
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wreckit87

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Re: Concrete Slab Inlet and outlet temps
« Reply #11 on: Today at 06:00:58 PM »

Again I'm not familiar with Hardy or what electronics they use, but yes the differential is the "cut in" and "cut out" temps of the aquastsat. Generally speaking, most Honeywell aquastats are factory set with a 10 degree differential. So if you set it to 180, it'll open the draft at 170 and close it at 180. If you tighten that to 5, it'll only have to drop 5 degrees before it cycles again. Longer cycles are generally better for the stove in terms of creosote and whatnot but if you're having trouble keeping it going, that will essentially half your time between cycles
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Re: Concrete Slab Inlet and outlet temps
« Reply #12 on: Today at 07:19:07 PM »

To change the differential, remove the cover from the aquastat and you should find a white wheel on the left side with numbers on it. (Assuming you have a Honeywell aquastat)
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