# Solar Linked To Concrete Slab



## Virgil (Sep 29, 2009)

Does anyone have any real life experience with Solar Thermal Collectors linked to a Radiant Floor System {PEX} set into an insulated Concrete Slab on grade foundation ??? I am familiar with the various rigid foam board insulation techniques required for such an installation and with SolarGary Website... Also, we have used for several years DIY Thermosiphon in the past and currently a Fresh Water Drain Back DHW system....


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## SolarGary (Sep 8, 2005)

Hi,
Did you see this one: http://greengateguesthouse.blogspot.com/search/label/Solar Projects

Its a bank of solar thernal collectors directly linked to a concrete slab with pex loops in the slab.

I'm guessing this can work pretty well, but one thing to think about is that the slab is both your heat storage and the floor for the space you spend time in. So, with heat storage its nice if you can take it up to a high temperature (say 150F) to store more heat, but if the storage is also your floor, you have to keep the temperature low enough that it does not overheat the space too much.

Gary


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## Virgil (Sep 29, 2009)

Hi again Gary.. Thanks for the reply... As it seems not many people have end user experience with thermal solar arrays that are connected directly to PEX radiant heating loops that are installed in concrete slab foundations... Noted exceptions... builders Ramlow and Seton..

I find it interesting that the builders of the Green Gate House project went through all the work to install the 2 feet of recommended sand for the thermal storage than did not put the PEX tubing within it... Opting instead to place the tubing into the upper layer of the concrete negating the use of the thermal mass below... (If, I read that right),,, It appears that this project was done a couple years ago and I wonder if they have an updated report on the comfort level within the house...???

The proper balance of the amount of heat producing collector area to that of the insulated heat sink thermal mass within the house, of course, is of prime concern to retain a comfortable living environment... Cold concrete is unforgiving in a +8000 degree day heating latitude... which was the reason for my lead question... We live in the very cloudy northeast where realistically the available solar heat gain would be in the low percentage range of possible sunshine during the heating season... 

I am intrigued though, about the sand box thermal mass concept... It reminds me of the pre - plastic PEX days where heated air from collectors were forced through tons of box of rocks and tubes laid in sand boxes... Those systems required lots of cfms and electricity for the blower motors to operate... Dirt and accumulating lint in the air channels over time I think were a problem....

Back to the PEX tubing.... I can see that charging the sand box towards the end of summer in warm temperatures would have the advantage of moderating the temperature within the house as the out side temperature began to slide into the Fall season... provided that it did not over heat the house.... And, it seems that with the PEX tubing buried in a couple of feet of sand it would be well protected from damage during the rest of the concrete slab construction... Finding a leak later, I bet, though, would be a challenge.... 

I agree with the use of vertical collectors for the heating season... If we go ahead with a project in the future the collectors would be separate from the house as you and others have done... For the buried pipes and wires from the collectors to the house I believe I would place them within the light weight foam blocks that are used to make concrete foundations... ARXX, etc... I would use spray foam and ridge HD foam boards fitted to seal off the top and bottom of these blocks... Extra pipe insulation would likely be wrapped around the pipes that are placed within this chase way also.... 

I suppose ideally having both a 500 gallon water mass storage external to the house and the sand box mass storage for longer term heat retention within the slab foundation would have it's advantages... However, two layers of PEX tubing with diverting valves within the house may be necessary... The monitoring of temperatures via buried sensors through out the system I believe would be very important... This maybe getting too complex... 

Any thoughts...??? And thanks again for the feedback....


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## Virgil (Sep 29, 2009)

Clarification.... Fred Seton may not be a published author but, he has an extensive website detailing the installation of PEX radiant floor heating systems... Gary you probably knew that ... just a clarification to other readers on this site... 

http://www.radiantdesigninstitute.com 

Virgil....


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## GregYohn (Jan 24, 2013)

Here's a thing to consider. You can close the Pex tubing with shut off valves, so the thermal mass does not get hot in the Summer.

This is if course is if you do not have sensors to get the heat to the heat sink of the cement slab.


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## Virgil (Sep 29, 2009)

Thanks GregYohn for the reply....

Monitoring the various temperature points from the collectors to the thermal mass storage areas whether it is a tank of water or an insulated sand box beneath the floor of the house or the pipes in between would be a key component to controlling the distribution of the heat flow from a large collector system... The actual flow control could be a few simple manual water valves or in this age of affordable electronics a fully automated micro-controller that is linked directly to motorized water valves or solenoids which respond to the changing temperature conditions within the whole system... The possibilities are nearly endless...

In terms of remote sensors, for our small fresh water Drain Back DHW system I use #24 AWG type K thermocouple wire... A spool of this bimetal wire is reasonably priced and is very simple to use... I have used for years the thinly insulated wire which, if I recall right, is referred to as rip cord... I believe this wire will produce a detectable linear microvolt signal out to about one hundred feet in length.... A good online source for this thermocouple wire and a great source of information on that topic could be found at:
http://www.omega.com

I have several lengths of this thermocouple wire ran to the four collectors in our system... Just strip the two wire ends of the insulation and than tightly twist them together with a pair of pliers... A dub of silicon caulking could be placed over the bare wires, but I usually don't.... To prevent dissimilar metal corrosion and resultant false signal readings later, I never place the bare thermocouple wires against the surface of the copper tubing... I opt to secure the wire wrapped around a couple of screws so that the twisted ends are free standing within the collector chamber... My differential controller is than calibrated to sense the rise and fall of air temperature within the collector... I do the same thing with the thermocouple wires which are immersed in the water storage tank... 

I built my own differential temperature analog to digital controller.... This was built a few years ago.... It is a stand alone low voltage signal processor circuit which is interconnected to programmable Parallax microprocessors... This controller has the capabilities of outputting real time temperature data to my desktop computer... However, I have not had the time to finish the final link and the additional computer programming... Too many other projects on the agenda....And is beyond the scope of this website... 
Online source for the microprocessor: http://www.parallax.com
Electronic parts: http://www.mouser.com
http://www.jameco.com
http://www.digitkey.com

Last thing... I like the use of vertical collectors for winter solar gain.. A couple of years ago we added a wall mounted one on the second story of our house... We calculated the shadow cast of the roof eaves and built the collector so that during the summer the collector is mostly in full shadow and in full sun light during the colder months....from September on.... In essences, it's output is self regulating with the seasonal changes.... We valve it off from water flow during the summer months.... Three other tilted smaller collectors produce more than enough DHW during the summer months... The complete system was home built.... All four collectors are placed back online during the winter months... 

Virgil...


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## GregYohn (Jan 24, 2013)

Check out Home Power magazine. An article recently covered the subject, including nice diagrams how it's down just using solar hot water system.


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## SolarGary (Sep 8, 2005)

Virgil said:


> Hi again Gary.. Thanks for the reply... As it seems not many people have end user experience with thermal solar arrays that are connected directly to PEX radiant heating loops that are installed in concrete slab foundations... Noted exceptions... builders Ramlow and Seton..
> 
> I find it interesting that the builders of the Green Gate House project went through all the work to install the 2 feet of recommended sand for the thermal storage than did not put the PEX tubing within it... Opting instead to place the tubing into the upper layer of the concrete negating the use of the thermal mass below... (If, I read that right),,, It appears that this project was done a couple years ago and I wonder if they have an updated report on the comfort level within the house...???
> 
> ...


Hi,
I read over Bob Ramlow's article on using sand bed storage for seasonal storage of heat in Solar Today, and to me, the numbers just don't add up. I detailed why here: http://www.builditsolar.com/Projects/SolarHomes/SandBed/RamlowSandbed.htm

On the other hand, I've heard from several people who live in sandbed storage homes and they are generally happy with them. But, their expectations are not that you can heat the sand bed up in the fall, and have it take you well into the winter. They just like them as homes that use solar well and are generally comfortable to live in. 

Seasonal storage of heat is really tough, and takes an exceptionally good thermal envelope to keep the storage from getting to be the size of a football field.

I've not heard from the Green Gables guy, but I think you could probably get hold of them through the bed and breakfast they operate. If you do, please pass on what you find out.

Gary


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## Jim Bunton (Mar 16, 2004)

I have wondered if the pex is run in the concrete and the only form of heat is solar what affect the daily extreme heating and cooling cycle would have on the concrete or would the concrete hold enough heat over night to temper that affect?
Jim


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## SolarGary (Sep 8, 2005)

Jim Bunton said:


> I have wondered if the pex is run in the concrete and the only form of heat is solar what affect the daily extreme heating and cooling cycle would have on the concrete or would the concrete hold enough heat over night to temper that affect?
> Jim


Hi Jim,
I guess it depends on the ratio of collector area to concrete mass, and also on how much heat loss the building has.

A sqft of solar collector produces about 800 BTU per day on a sunny day.

A 4 inch concrete slab weighs about 40 lbs per sqft.

5 sqft of slab (200 lbs) going from 60F to 80F stores (200lb)(20F)(0.2 BTU/lb-F) = 800 BTU/day. 

So, if you have 5 sf of slab for each sqft of collector, you might expect (at most) a 20F swing in slab temperature. It would be less than this because the slab is losing some of the heat that the solar puts into it during the day to the building.

I think that if you have quite a bit of slab and not a lot of collector, just putting the solar heat right into the slab would probably work well and not result in any problems. But, if you have not so much slab and a lot of collector, I'd guess that there will be times when the slab gets pretty warm and tends to overheat the room. In conventional solar storage systems where the storage is separate from the living space, you can run the storage temp up to 150F or so without worrying about overheating the room, but these systems are also more complicated and expense -- lots of tradeoffs.

Gary


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## Virgil (Sep 29, 2009)

Hi Jim and Gary.... A general comparison of potential heat rise between a water mass and that of concrete... Giving the following guidelines and practical starting point assumptions... The specific heat of earth materials such as rock and concrete is only about 20 percent of that of water... A fact we all know... General system design indicates that 1.5 to 2 gals of clean fresh water is needed for effective solar heat transfer for each square foot of collector area...

Than starting point guidelines:...
Specific heat of water = 1 btu/lb - deg f
Specific heat of concrete = .2 btu/lb - deg f
Weight of water = 8.5 lbs/gal (231 in^3/gal)
Weight of foundation grade concrete (nominally) = 144lbs/cuft
Minimum slab concrete foundation insulation @ R20 (Perimeter & Bottom)
Minimum tank insulation @ R30
Solar gain day = 6 hours/day (avg)

Component Size & Starting Temperatures...
Water storage tank size: 500 gallons
Beginning water storage temperature: 60 F
Gross concrete foundation size 1500 sqft @ 4in overall thickness
Beginning concrete temperature: 60 F

Mass Weights:....
Volume of concrete = 1500 sqft x (4in/12in/ft) = 500 cuft
Weight/mass of concrete = 500 cuft x 144lbs/cuft = 72,000 lbs
Weight of concrete in tons = 72,000lbs / 2000lbs/ton = 36 tons (short ton)
Weight of water = 500 gals x 8.5lbs/gal = 4,250 lbs
Weight of water in tons = 4,250lbs/2000lbs/ton = 2.125 tons
Mass/weight ratio of water to concrete = concrete 36 tons / water 2.125 tons
Weight Ratio = 17 concrete : 1 Water (nominal)

Flat Plate Collector Array Size:....
Collector area = 500gal/(1.5gal/sqft of collector) = 330 sqft
4' x 8' panels = 330 sqft/32sqft/sheet = 10 panels (nom.)
Collector efficiency = 48% (or less variable)
Solar gain period 6 hrs/day (avg), Clear low humidity unobstructed sky (highly variable)
Collector Irradiance = 250btu/hr-sqft (avg) (highly variable)

Therefore, under ideal weather conditions of 40 to 50 degrees F on a bright sunny day with low relative humidity, and little wind a comparison of potential temperature rise of a tank of fresh water to that of a large concrete thermal mass floor...

Solar Heat Gain = 330 sqft collector x (250btu/hr-sqft) x 6hrs x .48 = 237,600 btu/day
Using: 237,600btu/day = (4,250lbs water) x (Temp Change) x (1btu/lb-deg F)
Temp Change = + 56 deg F
Therefore, Potential Final Tank Temperature = 60F + 56F = 116F (with no static losses)

Where solar heated water is pumped directly into the concrete floor mass for the same amount of heat gain:...

Using: 237,600 btu/day = 72,000lbs x (Temp Change) x (.2btu/lb-deg F)
Temp Change = 16.5 deg F
Therefore, The Potential Concrete Floor Temperature = 60F + 16.5F = 76.5 F
(over an unspecified time period)

The enormous weight thermal mass of 36 tons of concrete at a much lower specific value, as it appears, greatly dampens the potential of any thermal over heating of the floor...(In this very limited snapshot analysis.) This is a huge heat sink as compared to the 500 gallons of water mass at a weight of a little over 2 tons. The amount of heat lost the concrete slab foundation continually sustains while solar heat gain is not available at night or during cloudy daylight hours is a very big variable.

We live in the cold very cloudy high humidity area of the Northeast. At best our annual thermal solar gain hovers around 30 to 35 percent of potential sun shine. Therefore, for us, my concern is not floor overheating so much in this application but, rather building a collector array large enough to sustain sensible minimum solar heated comfort levels... 100% solar heating would be unrealistic.... 

From other solar thermal engineering texts sensible heat storage for concrete can also be calculated using 37.8btu/cuft-deg F... And, likewise sensible heat storage for water use 60btu/cuft-deg F.

Virgil....


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## SolarGary (Sep 8, 2005)

Hi Virgil,
The calculations seem fine to me.
It looks like you have enough slab mass compared to the collector area to keep the floor temperature within comfortable bounds.

It seems likely that in cold weather the slab won't get to the 76F in that its losing heat to the conditioned space and also some downward through the slab insulation to the dirt. So, that gives you more margin against overheting.



Just some nits:
You will get days with more than 250 BTU/sf-hr sun. Full sun is about 317. I have a good pyranometer, and find that sun on the collector can be up toward 350 BTU/sf-hr on good sunny days with a large reflective snowfield in front of the vertical collector. 

Efficiency of a plain old flat plate collector with black painted absorber for 250 BTU/hr-sf sun, 45F outside and 90F water is 53%. With 317 BTU/sf-hr sun and same other conditions, its 58%. So, you may do a bit better than 48% on good days.
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

Gary


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## Virgil (Sep 29, 2009)

Hi Gary....Thanks for the reply... As you know the above analysis was very simplistic at best... Other factors such as the efficiency of heat transfers from one type of material to another, fluid flow possibly at different rates through the floor piping, heat delivery losses from the collector site to the condition space, the air mass temperature within the living space at any giving moment, passive solar gain, did some one leave the bathroom window open all night....etc... etc..were some of the factors not considered... Though, all contribute to the static and dynamic heat lost or gain features of a structure... It is hoped that the overall hysteresis effect moderates all the pluses and minuses to achieve a comfortable living environment. 

I believe that your location in the relatively cleaner air of the plateaus of Montana provides you with much better solar access... That certainly is a very very good thing.,,,

As a casual observer for decades of the power of the sun living in the Northeast I have learned to lower, though, my expectations in that regard... We live at the edge of the foothills of the Adirondack Mountains and have been told that many lakes within that region are dead zones caused by the infiltration of air pollution from many sources.... 

We also live underneath a major air flight corridor of high flying jet liners traveling to and from Europe and the eastern seaboard. On countless occasions I have observed what would had been a perfectly 100% clear solar gain day be turned into a hazed over milky white sky caused by the spreading out of numerous high altitude vapor trails from the jet engines.

Weather wise, often is the case, our region will be affected by storms that come from both the west and also from the south along the eastern seaboard. Many times these storms will combine off the New England coast and circulate overcast cloud laded skies for days..... We have had at times weeks without any useable thermal solar gain because of these powerful sea storms... 

And lastly, a phenomena we have observed which maybe linked to global warming... It seems that Lake Ontario does not freeze over that often in the winter time anymore... We are down stream of the westerly winds which blow across this lake... Due to this we have considerable persistent cloudiness and snow fall at times not from low pressure weather fronts in the area but, from the cold winds picking up moisture from this wide unfrozen lake... 

All of these factors by some small percentage combine to have a negative impact on our potential to free useable solar gain... That's why my figures and expectations are a bit lower... 

Virgil...


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## SolarGary (Sep 8, 2005)

Hi Virgil,
I would be interesting to find out what your actual sun readings are, and how much the things you mention pull it down.

Some weather stations and airports record sun on a horizontal surface on an hourly basis. I'm not sure how you get the records, but I'd guess they are available to the public. You can probably find a wx station or airport near you.

Another option would be a pyranometer. Good ones are a couple hundred dollars and up, but this one is just a few dollars:
http://www.builditsolar.com/References/Measurements/KitPyr/KitPyr.htm

I don't use the one I tested in the link above, and if you want to give it a try, I can send it to you. Just PM me if you want to try it.

The nice thing about having your own pyranometer is that you can mount it in the plane of the collector, and directly measure the solar and reflected radiation on the collector surface rather than having to estimate from the radiation on a horizontal surface.

Gary


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## Virgil (Sep 29, 2009)

Hi Gary...
Was perusing some on the internet for the daily insolation data and there is a mind-boggling about of information from various sources... To name a few, the National Weather Service, National Solar Radiation Data Base... NOAA,, NREL.. the National Renewable Energy Lab... The State University Of New York at Albany is a primary collector point for such data... However, though this data is collected on an hourly basis from both satellite and surface monitoring stations it seems it is not directly accessible via the internet... But, the data is reconfigured into maps illustrating monthly and yearly averages... Just too much information to try to go through with limited time... 

The eases I found to comprehend quickly was at this website:
http://www.solar-atlas.blogspot.com It provided free monthly insolation maps of the conus... in a relative color coded fashion for about the last three years... Reviewing through several months of maps I could see the settle difference of more insolation impacting the earth in the upper plains states as compared to New York State and the northeast in general... If you look it over I believe you will see that also.. The average Watts/m2 for the plains state will be colored in the green to yellow areas which indicate more insolation amounts as compared to the Northeast for the same time periods which favored the lower value green to blue areas.. The actual numeric color coded scaling is giving to the right of the maps... 

Thanks for the offer on the pyranometer.... but, no need... I usually have these parts or similar ones on hand in inventory.... 

Since we built our current small DHW drain back solar system, (430 lbs water thermal mass) three to four years ago I have maintained a simple running log of how long the water pump has ran. This of course, would be a direct indicator that the system was producing useable solar heated hot water... And likewise, a back door analysis that enough insolation is impacting the collectors to sustain positive heat gain... For the first couple of years the system was on line the annual percentage of pump run time was about 38%... This is an overall average calculation...(using the KISS system). It is based on a 6 hour solar window for each day for the time period from October 2009 through December 2011... Maybe this winter I could spend some down time to run through more data averaging... The last couple of years right through the winters have been very busy for me doing various renovation and maintenance projects... And this year is looking to be the same... We added a large vertical collector to the system a couple of years ago... And for the winter months it appears to out perform the three smaller horizontal titled collectors... So this may boost the average annual number up a bit... But,, there still is nothing we can do about the extended cloudy days we experience.... 

Eventually, I will link my home-brew solar controller system to one of my computers upstairs for purpose of data logging... But,, that too is a winter time project... 

As always, thanks for the input... 

Virgil....


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## Virgil (Sep 29, 2009)

Note ... To the website moderator I do not know why the Solar Atlas website that I referenced in my above posting is not working.... It works OK under my bookmark which shows various insolation maps of the conus.... Sorry !!!!

Virgil...


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## Allen15 (Apr 18, 2013)

The link that is shown is different than the actual underlying link that is embedded.

http://www.solardatawarehouse.com/altas is the embedded link, which doesn't work, although the base URL without the atlas at the end is the company's main commercial website.

The free data maps are at http://www.solar-atlas.blogspot.com/, and that link does work (for me?)

Allen


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## Virgil (Sep 29, 2009)

Thanks Allen15 for figuring that link out for me... It looks like I didn't put the backslash at the end.... I'll see if I can edit that post.... 

Virgil...


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