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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also. That is 400 a month year round. sorry for the mistake
i will help anyone that wants help. sizing a system is easy. take lots of time and put alot of thought in what your doing and you will win.
I live in rockford il. Its on the il wis border. I just installed my first geo unit. Im an irrigation contractor so nothing scares me. I installed a 4 ton unit from ingrams heating and air. Its a mcquay. this maker has been aroumd almost the longest. I made my own horrizontal ground loop. slinky . 70 feet long . 4 of them 2400 hundred feet total. 7 feet down , becouse i am in sand i went deeper. its 12 degrees out right now and it is 72 in my house. this is my first heating and air conditioner ever . I have 8300 in everything. my gas and elec are on the same bill. And i am paying 400 year round in a 1900 sq foot ranch. I have a built in pool. so it has a pump and heater. 25 zone irrigation sys. I run everything. I just put up 2 windmills. small ones from missouri wind and solar. 1500 watts each. and i have 2 killawats of solar panels. next year i will add 3 more k of solar. will see what happens . 400 a month is alot of money . in 5 years its 20 thousand. in 20 years. think about it. total for everthing. under 20. rock energy in wis can blow me
ckmapawatt's picture
Mark, Sounds like you have quite the clean energy system. If you'd like to take picture of the Geothermal, Wind Turbines, and Solar I'd be happy to put them up on the blog.
Tania, What type of het pump do you have, water source -- open vs closed, or direct exchange or other? typically over sizing is not a good idea, right sizing is best; and at times undersize with anticipation of some aux usage may actually be right sized-- as the cost of the ground loops exceeds the ROI of a larger system. Also if it is oversized in canada for heating; I would presume it will be significantly oversized for ac; a common problem of oversized ac is failure to dehumidify-- the air gets cooled quickly but remains damp and uncomfortable. Maybe you have a system with a two stage compressor or a variable fan sped which might help counteract the excess capacity
I live in Southern MN where wintertime it gets to -35F (yes, minus 35) and summer approaches 100F. My geothermal works well in both extremes. You just have to get it sized properly.
Chris - Jeffrey's post from Wikipedia is a good primer on how heat pumps work. A standard AC system works essentially the same way - except that instead of trying to move heat to a ~60-degree heat sink (the ground), it's trying to move it to a 90+-degree heat sink (the air). That's why geothermal is so much more efficient. We installed an Earthlinked geothermal system with our remodel (see the blog post here:, and have been very happy with it. Our house is two stories, approximately 2300 SF, and has been very comfortable & energy efficient even with the crazy hot temperatures in Atlanta this summer. KC
Mike in So. MN--Can you provide more details (contractor, equipment manufacturer, ratings, costs, etc.)? I live in central WI and want to pursue this! Thanks. Mike on Lake Michigan
ckmapawatt's picture
Thanks for the comment KC. It's nice to know you're in Atlanta as well! I saw a post on your blog (which I guess you quite writing on two years ago!) about a tankless water heater. How do you like it? I <a href="" rel="nofollow">wrote a post on a Tankless Heater</a> but I'm not sure if they're worth it. Read the comments on that post. I estimate that I use less than $5 a month heating my water. I can't picture a demand water heater improving it that much.
I've had a Waterfurnace ground source/sink (geothermal) heat pump in operation in central Ohio since 1994. The house is 4700+ sq. ft. with about 4000 sq. ft. of partially heated and cooled basement area, and is very well insulated. It uses a horizontal loop configuration, and consists of two heat pump units, each of 53,000 BTU/hr capacity, and is a forced air system. There is also supplemental resistance heating. The system performs very well. The coldest weather I have experienced is -15 F, which it handled with no problems. Cooling capacity is more than adequate. We rarely hit 90 degrees at our location, but have seen 100 with no problems. As the previous poster indicated, summertime hot water is almost free. Power consumption in the worst month (usually January) for the total house (everything is electric) is 5000- 6000 kwh. Summertime consumption usually runs 1700-2700 kwh/month. In our area this means a power bill of $200/mo or less in the summer, and a bit under $500 in the worst winter month. We keep the temperature at about 71 in the winter and 74 in the summer. Maintenance has been minimal, though I believe I now have a refrigerant leak in one unit. I would get this kind of system again in an instant. The system is very quiet, and I don't have any negatives to report. The only possible drawback is the usual heat pump issue that the warm air coming out of the vents in the wintertime is not extremely warm as you will have from a gas furnace. I have a wood stove in the kitchen and a bottled gas fireplace in the living room, but rarely use them, and then mostly for effect. The heat pump units require a lot of power to start up. I have 400 amp electric service, and they still dim the lights when they come on. I looked a getting a backup generator large enough to run the whole house during a power outage, but the heat pump compressor draw made it prohibitively expensive, so I settled for a smaller generator that does not run the heat pumps. Sizing and installation are critical in these systems, so make sure you use an experienced installer who does a lot of these kinds of systems.


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