Let’s get right down to it. We know agents want to know more about these systems but it can be intimidating looking at a geothermal system but we promise you it’s not going to be difficult. Essentially Geothermal heat pumps are like a central air conditioning unit with two key differences. One is the absence of that loud and ugly outdoor unit; that’s been replaced with a radiator (a heat exchanger) designed for a liquid. The second piece is a reversing valve, just like air-to-air heat pumps: this allows the heat pump to switch from cooling the house and heating the ground, to heating the house and cooling the ground. The thermostats work the same inside the house, and the ductwork is identical to any other forced air HVAC system. The next part is where the word GEO comes into play, we mentioned earlier that this type of system has a radiator designed for a liquid and we need to get that radiator a liquid for it to work.
Geothermal heating and air conditioning can be broken down into two major categories; “Open Loop” mostly installed prior to 1992 with a few modern exceptions; and “Closed Loop” installed post 1992. Each of these can be broken down further into sub categories each with their own pros and cons and all of which save money, create a comfortable home, and reduce our carbon footprint.
Open loops use well water; the water is pumped out of the well at 50 to 55 degrees and through a water heat exchanger (radiator). Heat is extracted from that water, and using a refrigeration circuit, adds that heat to the air through the airside heat exchanger (radiator). The water leaving the system is colder than it was when entering and needs to be disposed of. Some open loop systems return part or all of that chilled water to the same well that it came from; others dump that water onto the surface of the ground or into a separate well on the other side of the property. As that cold water migrates back to the water well’s pump it picks up heat from the ground around it, returning it to 50-55 degrees.
Closed loop systems in Connecticut usually use what we call a vertical loop as opposed to horizontal loop that’s popular out west. This is for a few reasons but mostly because horizontal loops require six-foot deep trenches to be dug around a property and digging in Connecticut isn’t exactly easy with the amount of boulders we have. Drilling vertical loops have better contact with bedrock so we need fewer loops to do the same job, which makes it easier to install on smaller lots and in urban settings.
Closed loop systems work almost the same. They use the same heat pump used in an open loop but instead of a large well pump like we have replaced them with small circulator pumps. Because the system is a “closed loop” it doesn’t need as much horsepower to pump, and less horsepower means less electricity used. Inside that closed loop is a mixture of an environmentally safe antifreeze and water, usually propylene glycol a food additive that also has antifreeze properties. That fluid is pumped through the heat pump where heat is extracted and goes back out to the earth, through the loop in the yard, picking up heat from the ground or bedrock and returns warmer than it was (but not back to 50-55 degrees). Over the length of a heating season the closed loop will gradually get colder. A properly designed loop will range between 55 degrees at the beginning of a heating season to 32 degrees in the coldest nights. That’s still plenty of heat for the heat pump to do its job but it has to work a little harder to get it.
In the end we are left with a list of pros and cons between the two types of systems (Figure A). Pay attention to that “Coefficient of Performance” part, more commonly listed as COP for short. This is a number to help you gauge the efficiency of a piece of equipment. Electric heat is 100% efficient and that equals a COP of 1. That means that 100% of the electricity you bought was turned into heat that made it to the inside of the house. A system with a COP of 4 is 400% efficient. This sounds outrageous doesn’t it? Look at it this way, pretend that water in a geo system is free electricity but you have to buy electricity to get it. You buy 1-kilowatt hour from the power grid and use it to run your heat pump. That heat pump extracts 3-kilowatt hours’ worth of free heat from the earth and combines it with the kilowatt-hour you purchased, to deliver 4 kilowatts worth of heat to the home. It’s buy one get three free! One question you might get from a potential homebuyer is “Yeah it’s 400% efficient electric heat but electric heat is expensive! How does it compare to oil and other types of heat?” (See Figure B)
|Open Loop||Closed Loop|
|55 degree water has more heat||Costs more to pump it||Costs less to pump it||Water has less heat in it toward the end of the heating season|
|Coefficient of performance of 3||Maintenance on well could be needed||Coefficient of performance of 4 or better|
|Costs less than oil, propane, and electric heat||Water treatment may be needed||Only requires air filter to be replaced twice a year for service|
|Costs almost half to air condition the same space||Requires a well with lots of water||Costs less than oil, propane, and electric heat, and natural gas|
|Reduces carbon footprint||Costs half the price to air-condition the same space.|
|Reduces carbon footprint a lot|
Fig B (estimated annual operating costs for 1 year heating and air-conditioning on a typical 2000 sq. ft. home based on May 2017 fuel prices and electric rates)
|Compare Operating Costs for Same Loads||BTU / Unit||Fuel Unit Cost||Price / Units||Efficiency||Annual Heating Cost Est.||Annual Heating and Cooling Cost Est.|
|Geothermal Open Loop||3410||0.1600||$/kWh||3.00||$902||$991|
|Geothermal Closed Loop||3410||0.1600||$/kWh||4.00||$676||$743|