The Case for Closed-Loop Geothermal Heat Exchangers

Introduction

When choosing the type of ground loop to provide heat transfer for your geothermal heat pump system, you are presented with two primary options, closed and open loops. An open loop system uses water volume directly from a local source such as a pond or lake or a well drilled to tap below grade aquifers.  In this case, heat is moved to and from the water source directly. A closed loop is a very different system because it uses the earth as a heat transfer storage battery, moving thermal energy back and forth as the seasons dictate. There are several distinct advantages and disadvantages to both types of geo-exchange, but we make the case here for utilizing closed, vertical loops.

The primary advantage of an open loop is the cost factor. Open loop wells are considerably less costly than multiple closed-loop bore holes because only two are required in most instances; supply and rejection wells. Whereas, a properly sized closed-loop heat exchanger will require approximately 200 feet of high density polyethylene tubing per ton in vertical loops and perhaps 400 feet per ton in horizontal loops. Obviously, the additional cost here is incurred in excavation, drilling and pipe fusion, which are labor intensive tasks that are inherent in closed-loops.

If this is the case, why would one advocate the additional expense of closed-loops over open-loops?  Open-loops have historically proliferated in geothermal heating and cooling installations and closed-loops are actually now becoming the preferred method for several reasons that tip the balance away from initial installed cost. Primarily, open-loop systems have more potential problems than closed-loop systems because they bring outside water into the loop system and the heat pump (unless a plate and frame heat exchanger is utilized). Because of the uncontrolled nature of ground water, this can lead to clogging, mineral deposits, and corrosion in the system due to natural organic particles, minerals and acidity. Open loop systems also require a large supply of clean water in order to be cost effective. This often limits their use to coastal areas, and areas adjacent to lakes, rivers, streams and abundant aquifers. Wells are unpredictable by nature and water volume can dissipate or vanish at any point during a well’s life cycle, especially if proper testing and planning has not been undertaken.

Water Quality, Volume and Maintenance

A closed-loop system is more reliable and requires less maintenance in the long term. With an open-loop you are also taking the risk that the wells drilled on your property will produce as many gallons per minute of clean water necessary to make the system operate at capacity with the proper flow rate. The water should be tested for hardness, acidity and iron content before a heat pump is installed. Even water which has suitable qualities could change with time to poor quality that causes problems with corrosion and scaling.  One of the largest concerns people have with open loop systems is the potential for scale build up on the earth loop coil and desuperheater. Mineral deposits can build up inside the heat pump’s condenser. Even if a heat exchanger and filter are inserted to create an interior closed-loop to protect the heat pump, fouling can occur in the primary open loop along with diminished capacity.  Impurities, particularly iron, can eventually clog a return well. If your water has high iron content you should be sure that the discharge water is not aerated before it’s injected into a return well. Using water from a spring, pond, lake or river as a source for your heat pump system is a poor choice because of excessive particles and organic matter. They can clog a heat pump system and make it inoperable in a short time. Because of water quality, a geothermal heat pump has a much shorter system life on open-loop. Since a closed loop system simply recirculates a constant volume of clean water, system longevity increases.  Additionally, open loop well pumps are considerably larger than the small circulators used on closed-loop flow centers and require considerably more maintenance and cost to replace in the case of mechanical failure. The potential added cost of having to replace a well or its various components could outweigh the cost differential you had chosen it for to begin with.

Environmental Factors

A closed-loop system presents absolutely no environmental impact to the earth or our aquifers and this is recognized by environmental authorities. As such, no special permitting or licensing is required. In some localities, all or parts of an open-loop installation may be subject to local ordinances, codes, covenants or licensing requirements. Here on Long Island, we are lucky enough to be independent of the reservoir system for our drinking water because we have abundant clean water beneath our feet. However, this water is threatened on a daily basis by chemical run-off, pollution and aquifer cross-contamination. Typically, the upper aquifer may be unsuitable for drinking purposes and is considered gray water because of fertilizer, chemical, fuel, pollution and storm water run-off. Adding to this contamination should not be taken lightly. An improperly installed open-loop system can have a considerable environmental impact in the case of cross contamination. This is why there must be separate intake and rejection points. As such, the New York State Department of Environmental Conservation strictly regulates well water. NYS law requires driller and pump installer registration and certification for open-loop or standing column systems. Driller registration and certification is not required for closed-loop geothermal systems with boreholes drilled up to 500 feet deep. A Part 602 permit is required for all open-loop or standing column geothermal systems with boreholes drilled up to 500 feet deep. Many homeowners are unaware of these regulations and are taken advantage of by installers who do not obtain the necessary permits. This leaves the homeowner with potential legal and environmental complications.

Summary

For all of these reasons, we believe that the inherent risks associated with the installation of an open-loop system justify the additional cost of protecting your investment by installing a more reliable closed-loop system. Combine these risks along with the added longevity and system life that a closed-loop system will provide for your equipment and comfort and you will see that the added investment is well worth the potential benefits for you, your wallet and the environment.

As always, to keep up to date with what’s new in HVAC technology, visit our website at airideal.com and follow us on Twitter @airideal and at our Facebook page!

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Geothermal Boiler on the Horizon

Now that the government has given Energy Star accreditation for water-to-water geothermal heat pumps, you can heat your home hydronically and get your 30% renewable energy tax credit.  This is provided you are interested in installing or replacing a radiant floor heating system.  For those who want to swap out their gas or oil-fired boiler with a geothermal unit, you will have to wait a year or two. Unfortunately, ground source heat pump compression technology in its current state is only capable of a maximum of 120 to 140 degrees F supply water discharge temperatures.  This is fine for radiant floor heating, domestic hot water and ice/snow melt systems, but you will need at least 160 to 180 degrees output to swap out your old boiler if you heat with hot water baseboard radiation or hydro-air (duct mounted hot water coils).

We get many calls from people who want to install geothermal heat pumps, but have baseboard radiation and don’t want to install ductwork or in-floor radiant.  There is not much we can do for them. However, I have it on good authority that a high output water-to-water geothermal heat pump is in the R & D funnel at WaterFurnace Industries.  I can promise you that when it is brought to market, we will be the first Guinea pigs!

As always, to keep up to date with what’s new in HVAC technology, visit our website at airideal.com and follow us on Twitter @airideal and at our Facebook page!