Residential Geothermal Heat Pump - Where does it work?


Let's get one thing straight:  Geothermal power production is very different than residential geothermal.  Geothermal power production is when wells are drilled over an area of high thermal activity in the earths crust, then water is put down the wells to create steam, which turn a turbine, which create electricity.  Residential geothermal usually refers to a geothermal heat pump.  The ground is still used as a source of heat (or cool) for your home, but the temperatures are much, much lower.

I was stumbling around the internet tonight when I realized  Popular Mechanics just put up a great article on residential geothermal.  The article has some nice diagrams and pictures, and is a good introduction, but it's not very in depth.  From the article:

Geothermal works more efficiently because the system’s mild starting point (the article is referring to the nearly constant temperature of the earth below the frost line) creates an efficient shortcut to the target temperature. Imagine a 100-degree Florida day or a 0-degree Michigan night: Spot the system 50 degrees, and it doesn’t work so hard to get the house comfortable.

So to sum it up, if the ground where the geothermal wells are drilled is 50° F year round , then the geothermal heat pump is going to pump water into the ground (which brings the water temperature close to the ground temperature), then use the water in a heat exchanger to pass the heat it has gained in the ground to your home.

In most parts of the U.S. this works in the winter (when the outside air temperature is usually colder than the ground temperature) to heat your home.  But this also works in the summer, when the ground temperature is cooler than the outside air temperature and the water that is pumped into the ground is used to cool your home.  It heats/cools your home year round!  And it is more efficient than using electricity/gas/oil to heat/cool your home because it is only moving heat from one source to another (the ground <--> your home) not having to create the heat.

With all that being said, the burning question I have is how well geothermal heat pumps work in different parts of the country and for different home types.  While there are multiple websites extolling the virtues of geothermal heat pumps, there are few sites that really highlight where they work and more importantly, where they dont.  I wouldnt want to spend $30 - $40 k on geothermal heat pump system then realize it really doesn't satisfy my heating/cooling needs.

Take my situation for example:  I live in Atlanta where it gets real hot in the summer, and can get pretty cold (but rarely freezing) in the winter.  I'm not that worried about the summer cooling (but maybe I should be.  Would a geothermal heat pump have enough cooling power to cool the house on a 95 degree day?), but more worried about the winter heating.  My town-home is 3 stories, but we only really heat/cool the upper two.  If it gets down to 40º F at night, but the heat pump can only get the inside temp to 50° F , am I still going to have to rely on a natural gas furnace to get to a comfortable 68° F?  None of the articles I've found have really addressed this. Also, how well does it handle multiple story dwellings?  I guess these systems must work pretty well though, because my utility's sister utility recommends geothermal heat pumps right next door in Alabama.  The California Energy Commission also has a pretty good review on geothermal heat pumps, so they must work there, but California has desert regions and very cold regions.  I'd love to see some information on how geothermal heat pumps work in both!

Basically, a geothermal system can never heat up your home to a higher temperature than the ground temperature all by itself.  So something is going to still have to make up the difference.  (see comment below.  I made a big error when writing the post originally) I still think this can be a useful technology, but it would nice to be able to know when to invest in a geothermal heat pump, or if a solar PV array or new windows would make the better investment.  Does anyone have a geothermal heat pump that could shed some light on this discussion?

Additional Links:

Check out Scientific American's "Solar at Home" blogger George Musser's experience with a Geothermal heat pump.

Energy Savers : Geothermal Heat Pumps

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At 1400 feet the temperature captured is 180 degrees, with a 10 well system your costs for heat energy become zero for the life of the home. The initial start up costs are a $10 a foot per 1400 feet per 10 wells. The return on investment begins immediately by removing the need for fossil fuels, so no payments for gas or oil.
I had a 5 ton geothermal unit installed in my Peoria, IL home when it was built. Home is 2 story with walk out finished basement (total finished sq.ft. 4700). The unit was installed with supplemental electric heater and desuperheater (which provides excess heat to 2nd water heater tank). My home is all electric with natural gas only for gas fireplaces. When I listed my home for sale I told the listing agent that I was averaging anywhere $150-$200 per month for electricity with bills occasionally up to $250. Agent didn't believe me until he received 12 month copies of my electric bill from the utility. I'm told from neighbors with similar size homes that it's not uncommon to get $400-500 monthly bills. Peoria gets bitterly cold minus single digits F and 90s with high humidity in the summer. In addition, my wife keeps home very cold in summer 72 degrees, and 75 degrees in winter. The unit has no problem with these loads. It performs better in summer as it actually blows cold air. In the winter if you feel the air being blown it just feels warm, hence takes more time to heat home on cold nights. Very infrequently electric supplemental system kicks in (when there is a very rapid large drop in temperature). However system catches up quickly and again don't see much effect on bill. Just amazed by how well it works i'ld estimate it cuts bill 60-70%. You can barely hear the system except fans blowing ventilation thru the home. The only downside on our system has more to do with sizing of vents through my home. Because our home is essentially 3 floors in the summer time the lowest level will be low 60s and the upper floor low 70s, which can be a bit too cold downstairs when it's 95 degrees outside. The systems ability to cool the home is remarkable. I'm sold on geothem. Lastly the system never required any maintenance except for cleaning air filters. I believe in the time I owned the home, system paid for itself in savings (4 years). System cost $17K versus the $10K traditional high efficiency heating and AC unit would have cost (add. cost $7K). Hope thats helpful.
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Mike in S. MN. We are building a large one story with an exposed lower level home beginning this Spring. It is very close to the Mississippi River just south of Ferryville, WI (about an hour south of La Crosse). I am most interested in the names of your contractors and the type of system that you installed. The home will be in the country at the end of a level farm field overlooking a trout stream so a horizontal system will work fine for us. I know that getting a contractor with a lot of experience is most important. Thank you so much for your help.
HUGE error in the article. "Basically, a geothermal system can never heat up your home to a higher temperature than the ground temperature all by itself." NOT true. A heat pump could heat or cool a space to any temperature higher or lower than the source or dump as long as the place you're taking heat from remains above the vaporization temperature of the refrigerant. You're not moving temperature, you're moving heat. The limitations of how much higher or lower the pump can make the space will be determined by the capacity of the pump at the given temperatures and the losses from the space. The temperatures also affect the efficiency of the heatpump. Once the heat source reaches a given temperature the COP becomes 1 and it would be just as well to run resistive heating. (Note: this temperature is lower than the switchover temperature for your home. Your home may require a higher switchover temperature as the capacity of the system is overwhelmed by the demand as losses grow in lower temperatures.) For a simple example, air heatpumps can still heat homes to 70 even if the outside temperature is in the 30's. Why would you think ground source heatpumps are any different?
We are building a large home about an hour South of La Crosse, WI and want to go geothermal. We have the ability to use the horizontal system. I would appreciate you giving me the name of the contractor that you used as I know that using one with much experience is most important. Thanks
I recently installed a geothermal heat pump system in my home (1200 sq ft - does not include basement square footage - total is 1700sq ft) ( I live near Ottawa,Ontario,Canada) and the temperature can go below -20 celcius sometimes, an average temperature for december to march is -10 to -15. I received my hydro bill for the past two months that it has been installed, and it has tripled. I was not expecting this much of an increase in my hydro bill, so this caught me by surprise as i was under the impression running the geothermal system would be cheaper or comparable to running a conventional fossil fuel burning furnace. I'm hoping someone may be able to shed some light on this for me, if my energy consumption seems correct. I went from using 600-800kw/mnth (this was in the summer with two window AC units running to using 1900kw the first month installed (nov-dec) and 2300kw the next month installed (dec-jan). Thanks to anyone who may be able to help clarify this.
ckmapawatt's picture
Tania, I'm not an expert in goethermal systems, but it sounds like yours either wasn't sized correctly, or it is just not a great application for geothermal due to the cold winter temperatures. It sounds like your backup heat is electric and this is coming on in times of extreme cold and running so much that it isn't too cost effective. I would talk to your installer for more insight into this or try and find a home energy expert to help you.
Hi chris, Thanks for your response. We did have our auxillary heat on at first, but we have turned it off on our breaker panel so it doesn't actually kick in. its been like that for the past month and our KW usage hasn't made a difference, we are still using around 2000kw for the past month (februaury). I'm just concerned that if it costs me this much in electricity to heat my house, will it be the same for cooling, at this point this isn't saving me any money. I was also assured (when looking at installing geo) that these systems are advanced enough that if installed properly will work fine in my climate. We even had them put in a 4tonne system when we only needed 3, so more than enough for our house.
It has been stated many times in posts that a system needs to be properly engineered. I am not a HVAC technician, nor do I play one on TV, so hopefully I don't use terms improperly. Because the unit is oversized (4 ton vs 3 ton) this causes what is known as short-cycling. Basically it takes quite a bit more energy to start the compressor than to continue to run it. Your unit is turning on and off more frequently than a proper sized unit, and therefore using more energy. (If I have misrepresented anything, please correct me.)


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