Do you think this will work?

[Sea_J]

I have installed radiant floor heat in my garage. I have 1/2" pex tubing with a 40gal water heater. It worked very good last winter, but my question is:

I looked at the national geological survey web site which says that the ground temp in my area stays at a constant 55 degrees year around at a depth of 13 feet. Do you think that if i dug a pit say 20'x20'x20'. and put several thousand feet of pex in it then filled it back in. Would i be able to cirtculate that water and heat and cool my garage? This keeps me up at night.

 

[Mike007]

I have installed radiant floor heat in my garage. I have 1/2" pex tubing with a 40gal water heater. It worked very good last winter, but my question is:

I looked at the national geological survey web site which says that the ground temp in my area stays at a constant 55 degrees year around at a depth of 13 feet. Do you think that if i dug a pit say 20'x20'x20'. and put several thousand feet of pex in it then filled it back in. Would i be able to cirtculate that water and heat and cool my garage? This keeps me up at night.

Theoretically it could work but would not be realistically feasible. It would most likely require a **** load of pex and a huge amount of digging and land. You are basically describing a geothermal heating/cooling system without the use of a compressor. Obviously you could never heat above 55* with such a system and more realisticly with 55* water would not heat much at all.

 

[wb2vsj]

If you are thinking of using your slab as a cooler - look at a glass of ice water on a hot day. Condensation. You floor may become covered in it unless you could remove all the humidity from the air, which an AC unit does for you.

A (slightly ?)more practical approach would be to run that cold water thru a radiator and let the water condense on the coils. 55 deg isn't going to be quite enough to do any real cooling though. Your going to spend a lot more on the effort than your going to get out of it.

As Mike007 said, your basically doing what a geothermal system does. (Friend of mine just had one put in - ~$30K. He got about 40% of that back in rebates)

Neat idea though

 

[pseudorealityx]

Short answer is 'no'.

You'll overload the ground and it will warm up, and then you get no heat transfer. There's a reason why real geothermal systems have LOTS of DEEP holes separated out into a field.

 

[Sea_J]

thanks for the info. Figured if no one else was doing it then there had to be a reason

 

[theoldwizard1]

Short answer is 'no'.

You'll overload the ground and it will warm up, and then you get no heat transfer. There's a reason why real geothermal systems have LOTS of DEEP holes separated out into a field.

Seeing as this is all theory any way, I will disagree.

With a 20'x20'x20' (that is 300 cu yds of dirt !), then filled with water, the worse thing you would do is raise/lower the temp of the surrounding earth 5 or 10 degree.

The "thermal transfer medium" (the water) must be contained so it does not seep into the ground and it needs to be insulated from the ambient (ground level) temp, so the water should only be about 10' deep. That is 4,000 cu ft of water !

This is a complex problem and would definitely take some engineering.

The reason geothermal systems use well is that they are easy to dig. The reason they use multiple wells is to get more contact (surface) area.

 

[Highbeam]

In my area the geothermal installs use wells but more often they use long trenches, sort of a horizontal well. The trenches are dug cheaply by a guy with a hoe vs. hiring out a well drilling rig. Same concept though.

My area also has a ground temp of about 50 degrees. The surface will vary based on ambient temp but since I put a big ol' insulated garage on the surface I expect that the slab temp will be close to 50 degrees. In other words, I don't expect to have to add much heat to my building to keep it above freezing when the floor is 50.

Another trick we use in this area for geothermal is to find or dig a pond and coil the tubing on the bottom. The water in the pond is a big thermal sink. Still a closed system.

 

[pseudorealityx]

Seeing as this is all theory any way, I will disagree.

With a 20'x20'x20' (that is 300 cu yds of dirt !), then filled with water, the worse thing you would do is raise/lower the temp of the surrounding earth 5 or 10 degree.

The "thermal transfer medium" (the water) must be contained so it does not seep into the ground and it needs to be insulated from the ambient (ground level) temp, so the water should only be about 10' deep. That is 4,000 cu ft of water !

This is a complex problem and would definitely take some engineering.

The reason geothermal systems use well is that they are easy to dig. The reason they use multiple wells is to get more contact (surface) area.

The biggest problem is that you're really limiting the surface area is you're digging a single big hole and 'filling' it with water tubing. Most of the interior tubing isn't going to do a damn thing, and since you've got basically a cube, you're really limiting yourself. Given, you'll have a lot of water, so you've got a lot of thermal mass, but you're not giving yourself a lot of place to put that thermal energy.

Like you said, wells are best because you have a single tube that is completely free to transfer heat in any direction all the way down. Typically geothermal systems will require between 8 - 15 ft between wells to avoid heat soaking the ground. Of course, they are not 'easy' to dig since most end up over 200' ft deep, unless you've got the real estate to go horizontal.

Remember that these systems work off relatively small changes in temperature, so if you do increase the ground temp by 5 or 10 degrees, it makes a big difference in terms of cooling capacity.

Disclaimer: the above is coming from the perspective of someone whose been involved with some larger geothermal systems for commercial applications, where a small lose in cooling capacity is a BIG deal. For a garage, obviously temperature is not nearly as critical.

 

[theoldwizard1]

In my area the geothermal installs use wells but more often they use long trenches, sort of a horizontal well. The trenches are dug cheaply by a guy with a hoe vs. hiring out a well drilling rig. Same concept though.

Yep ! Just requires a lot of area !

 

[theoldwizard1]

The biggest problem is that you're really limiting the surface area is you're digging a single big hole and 'filling' it with water tubing. Most of the interior tubing isn't going to do a damn thing, and since you've got basically a cube, you're really limiting yourself. Given, you'll have a lot of water, so you've got a lot of thermal mass, but you're not giving yourself a lot of place to put that thermal energy.

Not completely true.

The coolant inside the tubing will transfer heat to/from the water. The water will transfer heat to/from the walls of the pit assuming they are not insulated.

Remember, the OP just want to circulate a fluid (likely water) from the slab to the the heat sink/source.

• Will it keep the floor at a comfortable temperature. HELL NO ! I suspect the floor will stay somewhere between 40-60F, depending on the season.
• Will the system regulate well. HELL NO !
• Is this a practical solution ? OF COURSE NOT!

Disclaimer: the above is coming from the perspective of someone whose been involved with some larger geothermal systems for commercial applications, where a small lose in cooling capacity is a BIG deal. For a garage, obviously temperature is not nearly as critical.

I suspect the above 3 point are critical design issues for you !

 

[pseudorealityx]

Not completely true.

The coolant inside the tubing will transfer heat to/from the water. The water will transfer heat to/from the walls of the pit assuming they are not insulated.

Remember, the OP just want to circulate a fluid (likely water) from the slab to the the heat sink/source.

• Will it keep the floor at a comfortable temperature. HELL NO ! I suspect the floor will stay somewhere between 40-60F, depending on the season.
• Will the system regulate well. HELL NO !
• Is this a practical solution ? OF COURSE NOT!

I think there's a disconnect going on...

"The coolant inside the tubing will transfer heat to/from the water. "

The coolant is the only water in the scenario. The coolant takes heat to/from the slab, and runs into the pit, which is filled with dirt/rock, not water.

I don't think he's building a huge swimming pool.

Also, he already has radiant heat via a water heater. He's thinking about this as a cooling solution.

 

[maxspeed96ct]

Do a serach on geo-thermal.

But since you posted, give us some details on your heat setup

I plan on running my radiant off a water heater aswell .

you running a gas oil or elect ?

What do you keep the temp set at and etc?

 

[Sea_J]

I have a 24x40 garage. I put slabshield down as my insulator and moister barrier. I used rebar in the floor and zip tied 1/2" tubing at every one foot. I am currently using an 11000watt electric water heater. I put R19 insulation in the walls and R38 (all batting) in the ceiling. Last winter was my first winter with the system operational and i was very pleased with its performance. I usually had it set at about 55 degrees.

I had no previous expierience at installing floor heat. SO PLEASE TAKE ALL INFO WITH A GRAIN OF SALT. Also if you zip tie tubing to rebar make sure to get all of the end clippings from the zip ties or they will float up to the surface of thre concrete.

I am also thinking about switching to a propane boiler and getting rid of water heater. I am curious to know what type of heat source you guys run with your floor heat.