I have and 8x8x8 foot highy insulated room in my barn that I want to maintain a near constant temperature. Here is my plan. I'm sure it's not new. I searched and found Ground-Coupled Heat Exchanger, most often with earth to air exchange, I plan on using buried pex with water mixture. Here is my plan, and I would appreciate advice, but I'm not interested in purchasing a traditional ground source heat pump.

I plan on burying 300 feet of 1/2 inch PEX about 6 feet deep. In my area, pacific northwest, I will it some ground water even in August, and by winter fully submerged. I will connect the pex into a radiator with a pump and a fan, inside the room. I will need some sort of thermostat and switch to turn the pump and fan on and off based on set back temperatures. I have considered removing the radiator and fan in favor of radiant baseboard exchangers. I could place at the perimeter on the ceiling (and floor). This would eliminate the extra heat generated by the fan and make it quiet.

I tried to calculate the btu loss/gain and one online source was 500/hr, which I thought was a little high.

I'm looking to use this room to hold my home made wine, and home grown fruit/vegetables. Currently the wine is in the barn and subject to the full swing of the temperature through out the year. Too much fruit/veg for the house refrigerator. For the produce/frit, I'm just looking for a place to extend it's shelf life. Basically a root cellar.

Thanks for your time.

interesting,
I would consider a couple different ideas, but not to far off,
pex is cheep, 2 300 foot loops of 1/2, or perhaps a 500 5/8 loop
6 feet is overkill, earth temp in the NW is about 52 at 36 inches, more pipe less burden on the earth heat exchange.
a little myson wall mounted air handler with pump would work well or as you sugest a couple large flat panel radiators, a Johnson Controls A419 can switch on the cool side and relay to the pump electrical.
Dan

You might want to build it first and super insulate it and then see how much energy it uses. I know good wine is sensitive, but is the fan noise really a concern :-).

what temperature are you trying to maintain?

Thanks for the advice, so far...

I'm looking to keep the room at 55 degrees F, +/- 5 degrees... I'm near Seattle and the temperature is relatively mild except for the few week mid winter/summer when it's extreme. If the system could be held to 55 most of the year and then +/- 10 degrees during the extreme ends of the temperature cycle, I would be satisfied.

I really need help determining the BTU gain/loss compared to the external temperature and the factors relating to size of the radiator, volume of the pump (GPM), volume of the fan (CFM) and the length/volume of the PEX.

For example: If my radiator is 2 feet square, and the PEX is 600 feet of 1/2 inch buried 4 feet deep, what would be the volume needed to move through the system to overcome the BTU transfer rate for both GPM and CFM? I assume there is a cross over point of optimization.

The room will have a single door, no windows and be r21 insulated with vapor burrier and near air tight door. I may insulate the concrete floor, but my thought is to leave it bare, no insulation below.

Does anyone sell an integrated fan/radiator/pump with thermostat assembly? Is there a mini-split system that can directly/easily use my pex supply lines without air to air handler?

I did some actual research on this subject when we designed and built our house. I did it for a wine cellar, but the thoughts here are the same.

For one, if you have a room that is coupled to the ground (i.e. not insulated on the ground) but insulated in the walls and the ceiling - as is my cellar - the best temperature that you'll be able to achieve is the temperature of the ground. This sounds silly, but what you might find in your situation is that the temperature of the ground - that is, the floor on the bottom of your insulated room - doesn't vary much from day to day or month to month. I'd try measuring it - the floor temperature - to see where you are.

Then, as you go lower into the ground, the "yearly average temperature" stays the same. That is, the average temperature at the floor (over the course of a year) is going to be the same as the average temperature 10 feet down. The difference is in the swings - daily or yearly. The better insulated you are, the lower the daily swings. The lower you are in the ground, the lower the yearly swing will be.   My guess, though, is at a depth of 4 feet, you won't be able to much better than you can by simply not insualted the floor and being 100% passive.  You might get a degree or two, but I doubt that that will be of much import when compared to the larger issues.

If your room is near the center of the barn, and the barn isn't heated, you might find that the swing even in the room is small enough to satisfy your requirements. FWIW, the average ground temperature in Carmel, CA is 57 degrees, and I calculated my ground swing at about 4 or 5 degrees from winter through summer. But, since my basement is warm (that's where my mechanical plant is) and my ceiling is also warm (it's a heated floor above) I suspect that my temperature is going to maintain at a pretty constant 61 or 62 degrees. MY BTU number was less than 150 btu's/ hour - in through the walls and out through the floor.

You'd probably find a real swing, form plus to minus, given that your room is in a barn. If you'd like, I can send you my spreadsheet which calculates the yearly temperature swing and the "maintained" temperature at a given point in time.

Jeff

FWIW, I'm with Jeff. I don't think you really need an earth loop here. Just use the earth itself.

very different if you want to maintain higher temps, but if you're happy with ground temp, insulate yourself a good block of ground and build a good envelope over it.

What NRT.Rob said- your groundwater temps near Seattle are pretty close to (and slightly lower than) your goal for a 55F median:

http://mb-soft.com/solar/soilmap.gif

A reasonable amount of earth-coupling by insulating a section of surrounding earth down well below the frost line combined with a reasonably tight insulated entry and top-insulating your chunk of earth against your barn's temperature swings would pretty much achieve the temperature range you seek completely passively.

BTW: A vapor barrier might not be the best approach here. In an unheated barn the dew point of the insulated space is likely to be higher than the barn temp for extended periods in winter, and in the summer the outdoor dew-points don't rise and stay above the 55F interior for extended periods the way the do well-east of the Rockies. If you're insulating with batts, a double-studwall of 2x3s with unfaced R8 seperated in the center by 1" XPS sheathing (semi-permeable) would deliver a true ~R20 clear-wall performance with a minimal risk of condensation. (If you want more R, go with 2x4 on the exterior studwall.) Alternatively, a bare studwall sheathed with 3" or 3.5" rigid iso or 5" of EPS takes all of the dew-point guesswork out of it, with minimal thermal bridging.

Remember, the ground under the slab will start heating up in the summer and cooling down in the winter unless you bury the entire room down deep - it's not an infinite source/sink at 53F. But even if it were, I doubt you will get by with just the slab - as you are planning, you need something more active. An air ground loop might be easier. Be sure to insulate the slab around the perimeter.

With a +/-10 F tolerance around 55F methinks we don't have much to worry about- average outdoor AIR temps in Seattle in July-August are ~ 65F: http://www.climate-zone.com/climate/united-states/washington/seattle-co/ The average heating/cooling loads working on that buried thermal mass are quite modest- it doesn't need infinite heat-sinking or a large surface area of thermal contact with soil to support those loads.

If mere earth coupling proves inadequate, using outdoor air & differential thermostats to appropriately heat/cool the space when the it is more than a few degrees off the 55F setpoint would be worthwhile. Even in mid-summer many (even most) nights outdoor air temps drop below 60F. To support a winter heating load an incandescent light bulb would use about as much energy as the hydronic pump and still support the peak loads assuming that the unconditioned barn is tight enough naturally stay 5-10F above nightly lows in mid-winter, which it should. 100watts==341BTU/hr- supporting a 300-500BTU/hr PEAK load with a 600' PEX earth coupled heat exchanger, hydronic loop, and a 65W pump is ridiculous. For the money you'd spend on that you can insulate the 8' cube to R50 on 5 sides and sink R20 a few feet down around the edges for earth-coupling.