Radiant slab heating. Heat loss to soil, insulation, efficiency. Probably over thinking.

[fastline12]

Trying to design the tubing network for a slab on grade. The way I am calculating things is heat loss for all walls and roof, excluding the floor. Then I calculate the BTUs required for the space and start getting a general idea of how much water and dT I will need.

It looks like my worst case heat loss is 85,000btu, and mean will be about 40,000btu. The slab is 6000sf. I am new to calculating for radiant but the way I am looking at this is tubing ends up at he bottom of the slab at a certain temp and the soil at a certain temp. For this example, I will go with 80F target slab temp, soil temp is 50F, target indoor temp is 60F. I am trying to determine slab insulation. I do not want to over size that due to costs.

I am considering the U value of the foam BUT what I am trying to account for is the heating of the soil under the insulation. Once heat starts moving to the soil, the soil temp will rise, this reducing the btu losses? Or should I just calculate as an infinite heat sink at 50F?

I am considering the btu gain in the space by simply comparing indoor air temps vs slab temp... IE, slab at 80F, indoor at 60F, 6000sf*20F = 120k btu?

The things that stand out to me is the edges of the concrete should be insulated as there is good qty of heat loss there. As well, it seem that the hotter I push the concrete temp, the more btu loss I will have to earth so I need to balance the slab temp with demand.

As well, I am trying to determine how the flywheel effect will work for overnight swings and how to handle concerns of overheating in the space? IE, heat loss will vary through the day. Will a radiant slab be responsive enough to modulate the btu output to demand?

Heating degree days in my area are about 4700. I am considering 1" of XPS insulation, plus 2ft vertical perimeter insulation. ?


I realize there will be some variables in water temps and slab temp but I am trying to learn what standards to shoot for to get good efficiency. What kind of dTs do I need through the tubing?

 

[racookpe1978]

80 deg F is very, very warm for floors.

Work backwards a little bit. Indoor air temperature (mid-south, 1000-1200 ft elevation, modest humidity in the GA foothills fore example). Winter indoor temperature is 67-69 degrees, many people feel too hot at 70 degrees. Summer A/C air temperature is warmer at 72 degrees. Few people want it hotter than 73-74 degrees, and few want it as cool as 68 deg F. (Add a lot of traffic in and out, and 67 deg is about the lowest common area office thermostats can be set without "wars" going on in some cubicles and sweaters being thrown on other shoulders. ASHRAE of course has official guidelines.)

SO, if the slab floor is 80, then the air right above the floor is coolest as air circulates naturally or if a door opens, but the people are "feeling" the air a bit further up (heat rises, their arms, neck, and head are in the warmer air up higher. If the air is stagnant (no fans in the ceiling) then the upper room air will be too high - you are putting too much heat in the room.

The room (its occupants) will be above an insulator, right? Carpet or rugs, wood flooring or tile, right? That will affect radiant heat out of the floor and will reduce slab-to-covering-to-air total insulation and the covering-to-air convective heat transfer coefficient.

What floor, average, and upper room air temperature are you working towards?

Ultimately, you need to put enough energy into the slab heating coils as is lost by:
slab to ground through the perimeter,
slab to ground underneath the slab,
slab to room floor to room air to room-through-4x walls-to-outside air,
slab to room floor to room air to upper room air-through-roof-to-outside air,

 

[hydtools]

The tubing should be near the top of the slab, not the bottom. That puts more concrete between the heat and soil.

Ted

 

[fastline12]

Thank you. I should clarify. The slab will be exposed, no floor coverings. The space is a shop area. I realize there is benefit to tubing being at the top but this will be a compromise as there will be some drilling into the concrete so tubing protection is being considered. All tubes will be installed with a wire on them to locate the tubing for any deep drilling.

I am trying to better estimate practical thermal transfer to the slab from the tubing. IE, inlet water temp is X to target a slab temp of Y. I know the thickness of the tubing will limit some of the transfer so there needs to be enough length to transfer the heat.

Am I right to exclude the floor in the above grade heat loss calcs and rather calculate the slab as a radiator with btu outputs on top and bottom? Basically I am assuming a certain slab temp, then using that temp to determine the heat loss through the slab insulation, then considering indoor air temp compared to slab temp for btu input into the space?


Just as a general thumb rule, I am considering 1/2" pex with approx flows of .25gpm/100ft. with my above calcs, I would need nearly 8000ft of tubing! in the designed slab, even if I put tubing on 12" centers, that is only 6000ft of tubing. In looking at this, it seems I will need bigger tubing to get a reasonable flow rate. I would have to run higher temps and try to achieve probably ridiculous dT targets across the tubing to get there.

 

[hydtools]

This may be useful.

Ted

 

[fastline12]

Cannot open for some reason. Can you confirm link at your end?

 

[racookpe1978]

Radiator? certainly.
But also (in parallel with the radiation heat loss to the air, room ceiling, and walls - each with a different form factor) you must include convection losses from the hot slab to the cool air above the slab.

No evaporation losses obviously .

Conductive losses only below down to the cooler ground below.

 

[hydtools]

http://www.infloor.com/wp-content/u...tallation-small-infloor-sales-and-service.pdf

Try this link.

Ted

 

[dgallup]

Isn't it common practice to put foam insulation between the earth and the slab?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[moltenmetal]

Typical radiant slab construction here in Ontario, Canada, is 2" of 1.5 lb/cu ft EPS under the slab, with an expansion joint or other insulation around the perimeter to serve as a thermal break between the slab and any uninsulated concrete foundations etc. 1" of XPS has theoretically roughly the same insulating value as 2" of the higher density EPS and should have more than enough bearing capacity.

The tubing is going to heat the slab substantially uniformly, given that concrete is much more conductive than either air or XPS. Moving the tubing up or down an inch or so in the thickness of the slab isn't going to matter much in terms of heatloss to the soils. A little more concrete cover over the tubes will make temperatures a little more uniform between tube runs- more comfortable in bare feet, but not an issue either way in a shop.

The soils are not an infinite heatsink. A thermal gradient will form over time in the soils. What's the thermal conductivity of the soils? It depends greatly on moisture content and the nature of the soil material. But the benefit of going to 2" of XPS or 3-4" of EPS is doubtful unless energy is very expensive where you are. Just think: how cold are concrete slabs in buildings with forced air heating and ZERO insulation under the slab? They're not running at earth temperature, for sure.

You will need a capillary break under your slab (a layer of crushed stone) and a vapour barrier on top of your XPS unless you are permitted to use the XPS itself as a barrier by taping its seams.

Radiant slabs have a lot of mass and tend to not respond quickly to return after a night-time temperature setback, or to avoid overheating in spring/fall when you have cool nights and warm days. That said, in my own home, we have a combination of radiant floors and radiators which are separately zoned and the system responds to nighttime setbacks without any trouble at all and gives us excellent comfort.

The big thing you need is a properly tuned outdoor reset controller, which sets your boiler loop temperature setpoint based on measurement of outdoor and indoor temperature. That, plus a modulating condensing boiler, can give you excellent thermal efficiency. In my heating system, regrettably I'm the outdoor reset controller...but our efficiency is still spectacular.

 

[hydtools]

My system here in Colorado has the temperature offset control to adjust heat input based on outdoor to indoor temperature difference. I leave the thermostat set at one temp and do not setback at different times of the day. My south-facing windows help with daytime heating of the main living space.

Ted

 

[fastline12]

In this structure, there is basically a shop area and office area. The office area is getting a Climatemaster geo system for cooling but as a heat pump system, will also double as a forced air system to maintain comfort.

What I am ultimately trying to quantify (if I even can?) is basically the heat loss from the slab to the soils. I realize soil comp and such will play in here but I just need a reasonable way to calculate it.


As far as summer, we are planning to run well water to cool the system and slab. Not really building "cooling" but limiting any potential heat gains from the earth either from our radiant heating or the summer heat. Surface temps of the soil will hit 90F so if we can keep the slab cool, yet above dew point, it should take the demand off the cooling system.


The numbers I am using right now is assuming 50F for soil temp under the slab insulation? I know that will go up but I am not sure how to account for the heat leakage through the soil. I am also assuming a btu transfer of 1sf-*F so a slab of 1000sf held 20F over air temp would transfer 20k btu/hr.

We also had planned to install the vapor barrier under the slab insulation to minimize potential water logging. I am also hoping that the perimeter insulation will help reduce any moisture trying to get to soils under the slab thus further decreasing its conductivity. The water table is more than 40ft.

 

[hydtools]

Assuming 80F slab and 50F soil can you not calculate heat flow?

Simplified to flow through the styrofoam: k = 0.02 Btu/hr/ft/F, L = 1 in. k*(deltaT)/L = 0.02 * 30 / (1/12) = 7.2 Btu/hr/sq.ft.

6000 sq.ft * 7.2 = 43,200 Btu/hr

Ted

 

[fastline12]

Did I miss something? I am using ASHRAE tables directly for 1" of XPS at an R-5 or U-.20. U*(dT)*sf = 36,000btu in this case?


As far as the calculation, we are probably at an understanding but I am inclined to think there is more interaction with the soil because as that heat leaks through the insulation, the temp of the soil in contact with the insulation will go up, probably closer to 60+, in which there would have to be some sort of temp gradient and values applied for a certain thickness of soil based on certain assumptions of the soil conductivity. ? IE, there is more resistance to heat flow than just assuming the bottom of the insulation stays at 50F.

 

[racookpe1978]

But you have to start somewhere.

Iteration through several approximations is a common heat transfer essential tactic.

You still have not apparently accepted the need for including both radiation heat transfer and convection heat transfer in parallel from the upper surface, and conduction heat transfer through the slab thickness through the insulation thickness through (some assumed) dirt layer thickness.

Don't try to make every assumption perfect at the start. Start with the assumptions and see what the next step should be.

 

[hydtools]

There are studies that investigate time, heat flow, temperature gradient in the soil, etc. I just selected a simple condition which seems to be worst case. Yes, as the soil gains heat the heat flow will decrease to some steady state. Will that steady state be reached before the heating cycle adjusts to interior conditions? You would need to add interior demand to the ground leak to get close to a total system demand.

Was the linked document of any use to you?

Ted

 

[moltenmetal]

Yes, the soil offers substantial resistance. You can't just ignore the thermal conductivity of the soil. But the insulation provides the controlling resistance in a radiant slab system.

The 2" of EPS or 1" of XPS typically used under a radiant slab in a basement or slab-on-grade is an economic optimization of heatloss versus insulation cost and labour to install it. If your energy cost is higher than typical or the soils are more likely to be damp, then use more. If your energy is cheap, you might get away with less- but not less than 1" of XPS- that's just a practical lower limit.