Putting heat pump underground

[Mamrak76]

I have researched the principles of a geothermal heat pump. I have a standard heat pump (air source) with an outdoor unit. Would it be possible to put the outdoor unit in a chamber underground where the temp is always 50 degrees and extend the copper lines and add extra refrigerant. I think it would always operate at peak efficiency regardless of the outdoor temperature? I know there would be issues with digging a deep enough hole and having access, not to mention replacement. Ambient temperature is why they don’t use air heat pumps in the north. This would eliminate this issue. But any suggestions as to why this wouldn’t work?

 

[Bert_]

The air in the hole will not stay 50* once the unit starts running. You'll end up with hot (or cold) stagnant air. The heat can't transfer to the ground fast enough.

 

[Jackfre]

This was done back in the 70’s in order to qualify for the solar tax credits. They would generally spec a bare Olin collector plate and hang it under the porch, as the refrig only needed about...54* as I recall. For info on Geothermal you might check the Geo-Heat Center, at Oregon Institute of Technology. They are out of Klamath Falls, OR.

 

[theoldwizard1]

The air in the hole will not stay 50* once the unit starts running. You'll end up with hot (or cold) stagnant air. The heat can't transfer to the ground fast enough.

Because air is a poor conductor of heat.

If you really want to try this, you will have to be about 10-15' below grade and then you have to worry about ground water as well as service access.

 

[86turbodsl]

Sounds like a DX unit. Direct exchange. Those were popular for a while, instead of having plastic / copper heat exchanger, the refrigerant would circulate in copper lines directly buried. I think they had problems with leaks / rocks on the copper lines and i've not heard of them being installed recently.

 

[firebirdparts]

The unit doesn't need to be underground; Rather, the heat transfer needs to happen at underground temperatures. You could run copper lines underground, and heat would have to be conducted through the dirt to your copper lines. If you had enough copper in the ground, that would work.

HOWEVER. The beauty of water is that you can move it around. The dirt stays still, but you can just drill a well and bring the water to wherever you want. Water is a fabulous heat transfer fluid, so doing heat transfer with water just takes a tiny bit of space.

 

[theoldwizard1]

Sounds like a DX unit. Direct exchange.

The unit doesn't need to be underground; Rather, the heat transfer needs to happen at underground temperatures. You could run copper lines underground, and heat would have to be conducted through the dirt to your copper lines. If you had enough copper in the ground, that would work.

HOWEVER. The beauty of water is that you can move it around. The dirt stays still, but you can just drill a well and bring the water to wherever you want. Water is a fabulous heat transfer fluid, so doing heat transfer with water just takes a tiny bit of space.

These are more commonly call "geothermal heat pumps". Most people have had excellent result with this systems, but some have not. Sizing the well/ground loop is the key. During the heating season, the water coming from the well(s)/ground loop needs to be close to 50F. Much lower and you will not be able to extract enough heat.

A good installer knows this, but the science of ground loops does not seem to be exact enough to know how much water pipe to bury. With wells, if the water is too cold, you can drill an additional well.

 

[Jackfre]

Build a swimming pool and put a good glazed solar system on it equal to 50% of the surface area. Use a good insulated pool cover. Back when I was doing this we use the “Don Drysdale” cover. That should cover how long ago it was. Pump the water to the heat pump. In the cooling season you are rejecting heat to the pool, so you do not cover it and the pool looses the heat at night to deep space radiation. Ground loop, wells, whatever, 0r....pool. I’d rather have a pool. These systems were very good, but you are spending a pile of dough and there are no longer any geo tax credits. These systems can get really complicated and you need a real hand to do it correctly, so everything is happy together.

 

[86turbodsl]

These are more commonly call "geothermal heat pumps". Most people have had excellent result with this systems, but some have not. Sizing the well/ground loop is the key. During the heating season, the water coming from the well(s)/ground loop needs to be close to 50F. Much lower and you will not be able to extract enough heat.

A good installer knows this, but the science of ground loops does not seem to be exact enough to know how much water pipe to bury. With wells, if the water is too cold, you can drill an additional well.

Both are geothermal heat pumps. Dx were available for a while. They had one less heat exchanger and copper in the ground. Most are like mine, with 2 exchangers, and the freon circuit in the middle. The ground loop is plastic.

Sent from my SM-N910T using Tapatalk

 

[walrus]

I know **** Hill, a Prof at Umaine did a ground source heat pump with coils of black plastic, he froze the ground early in the heating season as his coils weren't nearly large enough. Wells work well but expensive to install.
Depending on your climate newer mini splits(air source) will be far cheaper and will give you heat even in low ambient temps. I know folks here in Maine that run them even in the temps we have now, below 0. One thing though, efficiency goes down and amount of heat output goes down with lower outdoor temps. So you have to plan for that. Backup heat in Maine is a must for a split system. Warmer places????

 

[Mamrak76]

Thanks for all the replies. I just know there’s a huge cost difference between an air source and geothermal. It would be cool to put copper line inside water filled metal pipe and had that buried deep. The whole idea is to get geothermal effenecy for an affordable cost.