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Radiant Heat tube depth in slab vs heat cost

Radiant Heat

Greetings All

I'm a couple of days away from laying a 6" slab w/ radiant heat in my new detached garage.

Industry radiant heat practice is 2" from top, based on my limited research.

here a couple of good sites that I read

http://www.inspect-ny.com/heat/Radiant-Slab-Heat-Mistakes.htm

http://findarticles.com/p/articles/mi_m0BPR/is_6_20/ai_102862289/


current issue is that contractor wanted to install heating tubes on mesh and then install 1" high x 5 ' long spacers/chairs on the 2" insulation foam while the concrete is being poured.

I resolved issue, I think, by having him install 3" high chairs under the mesh and tubes. Tubes should be 1/2" dia ID.

So.. 3" + 3/4" OD tubes
6" - 3 3/4" = 2 1/4" below slab surface [2" below is industry practice]



I can't find a chart or an educated opinion on what % difference in heating costs between 2 1/4" below and 5" below surface. From the above sites it seems like it would be substantinal? I roughly figure 2-3x more at 5" below that 2 1/4' below based on reading those sites. It's just my wild guess.

This is more than just searching for abstract thought. I have a relationship with the contractor "off work site". We didn't know each other very well before this garage came on steam. I gave him a heads up and history of my act so he had some idea of the kind of guy he was dealing with. "some idea" = warning

needless to say his 5" from slab surface and my 2 1/4" from surface caused some friction etc

so now that we're friends again [old build history is again again again etc] and tubes will be placed contrary to his routine I'm looking for hard info on this. My heating costs 2 1/4" vs 5" below?

I can't find anything on google

If you can offer an educated opinion or a lead that would be great

thx
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Ronin LB
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Old 09-14-2009, 07:10 AM
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You have a boiler in the garage? Assume hydronic? or not

Why not space heaters? Lot less expensive. What's the goal here, keep it above freezing in the garage all winter or just warm it up when you work in the garage? For the latter, space heaters as it will take hours and $$$ for the slab to heat and warm the garage.

Why 6" slab? Heavy trucks in garage? Maybe you feel that you need the mass to keep it warm?

Mesh should be in the lower 1/3 of the slab, not in the middle or upper, regardless of where the tubes are. The mesh is to help with tensile stresses, which are the bottom half of the slab.
Old 09-14-2009, 10:54 AM
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This is the first i've heard of depth variances effecting heat transfer. I was under the impression that the whole slab just absorbed the heat and radiated it up. Very interesting.
My concrete contractor just hooked the rebar mesh with a rake and picked it up as they poured. My system has performed very well so far. (2 years)
Here's the company i bought my system from www.radiantcompany.com
Maybe they could steer you in the right direction.
Wish i could be of more help, but i will tell you, you'll LOVE radiant heat.
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Old 09-14-2009, 02:03 PM
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[QUOTE=tcar;4897176]

You have a boiler in the garage? Assume hydronic?
------- seperate boiler for the antifreeze and hot water for the slop sink

Why not space heaters? Lot less expensive.
[b]------ I've been to too many electric fires caused by them and I'm spooked about the gas ones. The CO meter takes a memory from any low meter fumes and it becomes the new ground zero. It's one large room garage, 25' x 33' so it's not like I can mount the CO meter in another room like it's supposed to. Many people leave it in the boiler room which is a no no.

What's the goal here, keep it above freezing in the garage all winter or just warm it up when you work in the garage?
-------- Maintain 55F all winter and warm it up when necessary.

For the latter, space heaters as it will take hours and $$$ for the slab to heat and warm the garage.
-------- My bud lives up state NY downwind of "Lake Effects" winters. He has snow on the ground 7 months a year. He leaves his radiant heat on in his detached huge garage and cost is minimal at 55F. Although I see your point.

Why 6" slab? Heavy trucks in garage? Maybe you feel that you need the mass to keep it warm?
-------- No heavy trucks and the mass for heat is a non issue. Actually the mass is in my head. It's pretty thick when I'm into overkill. Actually this is my last hurrah at my age.

Mesh should be in the lower 1/3 of the slab, not in the middle or upper, regardless of where the tubes are.
------ Seems the routine is to attach the tubes to the mesh. It has to be attached to something.


The mesh is to help with tensile stresses, which are the bottom half of the slab.
Yep. The issue is tube placement for a good system. I read the above sites I posted and if tube placement isn't correct issues arrise. I hope the 1" cuts and all cuts in a square will prevent cracks.


thx for response
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Old 09-14-2009, 03:12 PM
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[QUOTE=pete3799;4897615]

This is the first i've heard of depth variances effecting heat transfer. I was under the impression that the whole slab just absorbed the heat and radiated it up. Very interesting.
it was news to me also. I know a few people who put radiant floor heat in either a few rooms or the whole house and didn't like it. Seems I now know thw reason why.

My concrete contractor just hooked the rebar mesh with a rake and picked it up as they poured.
------ The original plan was to do same as you and let it fall on these 2.5' long x 1" high chairs/wire spacers to keep it off the very bottom. The concrete guys just finished installing 3" high risers. Do you know how high your tubes are into the slab? I guess you read the site info?


Here's the company i bought my system from www.radiantcompany.com
Maybe they could steer you in the right direction.
thx

Wish i could be of more help, but i will tell you, you'll LOVE radiant heat.
Thx for encouragement. Right now I need it. I'm not sure if it's more work for a contractor to deal with a 911 weekend wrench or for the 911 guy to deal with a contractor?
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Old 09-14-2009, 03:25 PM
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I'm not sure where my pex tubing ended up at.
It may have settled to the bottom for all i know.
It started out like this
Don't forget to run some insulation around the perimeter of the floor so that the footings don't wick the heat away from the floor.
I think there's a picture of it on the above web site.
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Old 09-14-2009, 05:41 PM
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Quote:
Originally Posted by RoninLB View Post
I hope the 1" cuts and all cuts in a square will prevent cracks.[/b]
thx for response
The control joints (1" cuts?) don't prevent cracks, they just control where they occur - on the cuts, hopefully - so they aren't just random.
Old 09-15-2009, 12:06 PM
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2" is a minimum depth, that is so you make sure you do not cut the tube when you cut the control joints, or if you want to fasten down walls with hilti nails. The idea is that the whole slab is heated, so if the tubes are in the middle to slightly below it would be best, but doesn't really make much difference.
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Old 09-22-2009, 06:44 AM
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I did my barn floor with radiant heat. Tubes are on top of the insulation then covered with 8 inches of sand then the 4 inches of concrete. The power company made me do it this way in order to qualify for off peak rates. I do not have a secondary heat source so if power goes out the heat sink will keep the place above freezing for some time (days).
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Old 09-22-2009, 07:32 AM
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Greetings guys

thx for helping out here.

I've been waiting for some hard info and pics before I got this thread going again. So just to warm this up...

The slab pour turned me into a nut case. The docile bear became uncaged when their routine method of getting the tubes within 2" of the surface was a lousy act by its nature. I wild guess is that only about 25% of the 25' x 33' slab had tubes that high. About 50% of the tubes are buried within 1-2" from the bottom. Not to belabor the point I'll simply state that I don't think it's an efficient way to install radiant tubes. The slab depth is from 6-6.5"

Anyway, I ordered a "secondary" gas meter to only monitor gas consumption to the garage boiler. My limited experience about this stuff speculates that it may cost me more to heat the garage than to heat my house? The details why and the cartoon between me and the 8 guys doing the slab pour will be left for the near future. Hopefully the radiant system works out as there is no re-do. The gas meter hard info will eliminate any speculation from me about this.

My attitude is based on the info I was able to dig up that the resistance to the heat energy flow increases exponentially as the depth from the surface increases. Going from 2" from the surface to 4" from the surface is a greater multiple of resistance than twice. Also the common term "heat rises" doesn't apply to energy flowing through concrete. I hope I'm wrong?
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Old 09-23-2009, 12:06 AM
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Ronin, you're right. Depth is an issue. Heat is lost on all sides, the heat flux at the surface depends on depth.

The thermal conduction resistance for a long, isothermal tube (the heating element) can be expressed at 1/Sk, where k is the thermal conductivity of the concrete and S is known as the shape factor.

For a tube radiant heating system buried in concrete, S=(2*pi*L)/(cosh^-1)(2z/D) where L is the length of tube, z is the depth, D is the tube diameter, and cosh^-1 is the inverse hyperbolic cosine func. If the depth z is greater that 3D/2 (should be...) then that can be reduced to a easier to work with form of (2*pi*L)/(ln(4z/D) - 2 pi * length divided by the natural log of (4*depth / diameter of tube)....

With that out of the way, you can see how diameter effects the thermal resistance network, R(t,cond)=1/Sk

And the ultimate heat flux equaling q = Sk(delta T) .. the delta T being the temp difference between the surface of the heating pipe and the top surface of the slab. This, ultimately, would be expressed in watts/square meter.


Whew. I think I got all that right, I may have misquoted a figure somewhere in that but the idea is yes, the depth of the pipe has a relationship on the heat flux through the top surface of the slab, the deeper you go the worse it gets obviously.. and if I was even more bored than I was I'd plot that out non-dimensionally so we could see the relationship more clearly..
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Old 09-23-2009, 12:54 AM
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Is there any difference in efficiency once the slab is up to temp? I keep the barn at 65 degrees for the 6 month heating season (Oct-Mar) for less than $700. This type of heat, at least in my case, is slow to respond in either direction.
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Old 09-23-2009, 05:35 AM
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More importantly is mesh location. Mesh should be min. centered in slab, and optimally higher towards the load bearing surface.
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Old 09-23-2009, 05:43 AM
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Sorry to hear about the knuckledraggers "doing it their way".
You can check the floor with an infrared thermometer to see if the heat is consistant, and adding foam insulation around the outsides down to the frostline might help keep the heat in.
Old 09-24-2009, 08:35 PM
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