Heat Pumps and Electric Heating - DIY air conditioner as ground source heat pump???

I have a cabin that I heat to about 45F all winter with baseboard electric. It is costing me about 200 month during the worst months. I was thinking on doing this to cut my heating bill by 300%. Take a Goodman 1.5 ton 14 SEER R410a ($1150) outdoor compressor unit (usually for an air conditioner, not the heat pump model) and put it inside the cabin. Take 100' of copper lineset ($400) and instead of going to a indoor evaporator like a normal air conditioner, I run the lines 8 feet underground in a 100' long trench outside. I run it in the normal direction and it should heat the cabin, right?

I have moist sugar sand soil that doesn't move and has no rocks or anything to cut the outdoor copper coil. It seems like this is much much cheaper then a water source geothermal.

Any feedback from the experts would really be appreciated. Thanks!

I'm sure the Pro's will be around to tell you WHY it won't work...but I don't think it will.

You won't be able to gain enough heat from the ground into the lineset. And I think they call it an evaporator for a reason. Where will the evaporation take place? Were you just thinking of connectiong the lines together at the end?

For that kind of money and work...I would think you could just get a mini-split (?) system.

No expert...but I pretty much understand basics...

Lets see what the HVAC guys have to say.

Thanks gunguy45 for your reply. I guess the mini-split are air source heat pumps (and air conditioning). I don't want air conditioning so if they make a heat only it still wouldn't be right. I'm up here in northern wisconsin and really need it to work efficiently when the temp is -30F. An air source heat pump like the mini-splits work best when the temp is above 50. Since I only want to keep the cabin from freezing, an air source heat pump would be running inefficiently most of the time. At 0F an air source heat pump is basically running off the backup heater unit.

You brought up a good point about connecting the lines together at the end of the outside trench run. I have no idea if there is some special reservoir or part the converts the 3/8 liquid line to 3/4 vapor line. A refrigeration tech could certainly connect the two together with the proper reducer but I think there is probably more than that.

As far as the ground transferring enough heat to evaporate the coolant, I really think that would work. I don't know the exact temp of the coolant (in a normal air conditioning system) when it gets to the evaporator unit but it must be around freezing or below freezing because ice on the coils is not uncommon. The air passing by the cold coils is 75F in a normal air condition system so there is a 40F difference and lots of heat exchange. But air is a lousy conductor. Moist earth is much better. The ground will be about 50-52F and the temp of the coolant in the copper tube in the ground will be around 32F (This temperature is my biggest assumption/guess and where I need an expert to reply). Over the 100' of copper tube in direct contact with the ground, I'm fairly confident that it will heat up to about the temperature of the ground.

I appreciate your reply and don't want to contradict you only to clarify some points.

I like the way you think, but, I am sorry to say it just won't work. What about an LP furnace or replace the elec baseboard with elec hydronic baseboard that would be cheaper to run.

The compressor will not be able to pump all that oil and refrigerant! 60 feet is about the max!

So how do the DX geothermal systems work if only 60' of line is possible? I wouldn't give up yet; I'm going with a horizontal water-water system, starting up this week...It's a long haul, but I hope its worth it. Don't blame you if you don't want an LP appliance running while you're gone.. You may have no need for heat the next year.

Greg

geo has and water in the lines not refrigerant.

Thanks for more input and opinions. More clarification:

1. It is (will be) a direct exchange geothermal heat pump with refrigerant in the the buried copper lines. If the evaporator and condenser/compressor unit are at the same level (compressor unit in the basement where the coper exit/enter the ground) the maximum length is 150' equivalent. Subtracting for elbows and such 140' would be the very maximum for a single state compressor. Plus I think this limit is more for the liquid 3/8" line and the expansion valve could be just outside the house and therefore a short liquid line, with a long vapor line. It doesn't take much work to move a gas through a horizontal tube.

2. I think it is excepted that a ground source heat pump is the most efficient, cheapest to run, enviromentally friendly system available for cold climates. The reason everybody doens't have one is they are too expensive up front and people sell and move in a few years before the payback. I'm trying to design a simple system that people (me) can afford. There are 4-5 companies that sell similar to what I'm trying to do, but they are bigger and more complicated than what I'm looking for, and most likely won't sell to a doityourselfer. Nobody is going to sell a 15,000 BTU furnace in northern wisconsin but that is all I need at the energy efficient cabin.

3. Considering that the trenching and drilling is such a big cost for a geothermal or direct exchange (DX) geothermal system, how about this: When the 8' deep excavation is made for the foundation it is usually excavated 3-4' beyond the footing dimension. After the foundation is in, but before back filling, bury 150 feet of 3/4" copper tube at the bottom of the excavation, outside the walls, as far from the foundation as possible (4'). It is a $500 investment that has the potential to save thousands it this technology is ever used. It sure beats digging up the yard as an after thought.

thanks again for the input... I'm gonna keep thinking on this one...

I saw a bumper sticker to day that read "Don't believe everything you think." Which is true in this case as it can't be as simple as I'm thinking or everybody would be doing it. I did find a bunch of information contained in patents related to this design. The patents might also be a reason it isn't catching on since the holder for most of the Direct Exchange/Evaporation geothermal heat pump patents is a lawyer ;)

Anyhow, it was interesting to read the patents chronologically and see how the system design evolved with trial and error and necessity for certain applications. Most exciting, the exact sizing and length requirements for the underground copper refrigerant lines were given. The good news is the lengths are within the limits of the compressor for a 18K BTU system.

Issues still open:
1. Do I want to pay extra for the heat pump version of the compressor and possible run it backwards for cooling?

2. Do I want to use the large off the shelf wrap-around condenser coil/fin that surrounds the outdoor compressor unit? Building a cold air return and duct work to house a compressor and heat exchanger that is 3' x 3' x 3' is not as small as I had hoped. But the goal was to stay cheap and it is hard to start piecing together components and be cheaper then a complete 14-16SEER outdoor compressor module for around 1000-1200 buck. I guess for a cabin or garage just having the unit sit there without any duct work would work fine... maybe ad some ceiling fans... it works for word burners.

You have to be some kind of engineer .........right:wall:

It is possible to get your system to work, IF you can keep the oil coming back to the compressor. The DX system is actually quite complicated in that the amount of refrigerant required between heating and cooling varies greatly, so the system has to store the excess refrigerant in a receiver tank depending on the season. If you only need heating then your system charge would be consistent. One manufactuer builds a system and uses multiple loops of 1/4" copper tubing. The small diameter keeps the velocity of the refrigerant high enough to carry the oil back to the compressor. A 1 1/2 ton unit would need about 9 loops of 100' copper to dissipate the cold refrigerant. If you were to use large diameter tubing you culd end up with oil pooling in the lines. If you have a lake or pond that is at least 8' deep you can simply sink your copper coils and use that as your heat sink.
Good Luck

Thanks speedster1. Your reply was very timely because I was trying to figure out the exact line sizes. I am only considering heating so that should simplify things as you mentioned. Since this is the evaporator part that is in ground we will be pushing the liquid through an orifice where it vaporizes/expands and pulls heat from the surrounding ground. The gas/vapor line has to carry the refrigerant gas and oil back to the compressor. So I want the oil at the compressor but not any liquid refrigerant, just gas/vapor refrigerant. Right?

This is what is confusing as heck to me. I know gas can go up hill but the oil in the vapor line isn't going to go up hill unless the pipe was completely filled which it can't be in the vapor line. I have seen numerous example of DX systems with the liquid and vapor lines going into a deep vertical well and are being used for heating. How does the oil make it straight back up 300' in a 1/2" or 5/8" line??? Also in my split AC system at home, I have the evaporator in the attic (I have hydronic heat so no vent in the floor) a liquid trap was required in the 3/4" vapor line at the evaporator since it is above the compressor/condensor. This prevents refrigerant liquid, that happened to make it through the evaporator for some reason, from migrating to the compressor. How does the oil ever make it over the trap? Still confused.

I don't have a pond or lake so it will have to be in the ground.

thanks for the info...