As I look back on my past, I'm grateful for the many opportunities I've had with the geology, water chemistry, water well and geothermal heat pump industries. One of my earliest assignments in these ventures was to study the effects of water chemistry as it related to scaling incrustation and corrosion of piping systems. Early on, I learned the importance of the galvanic series (Figure 1). The galvanic series is a guideline to indicate the susceptibility of dissimilar metals to form a battery cell which will corrode a less noble metal and deposit it on a metal which is higher on the galvanic scale. Lessons learned from investigating failures of a multiple number of piping situations taught me to avoid the use of ferrous metals in “open systems.” If working with an open system containing natural water or well water, the materials of choice were always polyethylene, PVC, copper, brass or stainless steel. To use galvanized or black iron fittings, which are very low on the galvanic series, was simply asking for a failure within a few years. The majority of geothermal heat pump manufacturers have imparted this same knowledge into their piping materials suggestions. Yet to my knowledge, no manufacturer has taken time to educate anyone about the consequences that may result if lower-cost, dissimilar metal fittings are used. Nor have they mentioned that when ferrous metals are used, as is the case for cast iron body loop pumps, proper loop fluid chemistry is critical for longevity of the system.
We can apply this to the selection and mixing of antifreeze. Low-cost antifreeze solutions typically lack the needed corrosion inhibitors to prevent galvanic corrosion. Premium-grade antifreeze solutions are often double the cost of uninhibited antifreeze. The extra cost buys broad-spectrum corrosion inhibitors that protect a wide range of metals within the system. Inhibitors alone will not provide total protection. Ultimately, the behavior of an antifreeze solution and its ability to protect or corrode a system depend upon the type of antifreeze, its corrosion inhibitors and the quality/purity of the makeup solution. Low-purity makeup solutions will quickly deplete even the best inhibitors and render otherwise safe antifreeze corrosive.
Distributors working with geothermal equipment have a great deal to gain if they provide a complete package for the system, which includes pipes, fittings, water treatment solution when needed for southern climates and antifreeze for those northern climate installations requiring freeze protection. The value added for this service can cost between $300 to $800 per system. If the distributor doesn't provide this function and leaves it to the dealer, contractors might take numerous shortcuts, resulting in equipment failure, dissatisfied customers and repeat warranty claims, none of which help to increase profits. Instead, both you and the dealer suffer substantial lost time, lost profit and declining consumer confidence.
Installing contractors need to know why certain water-treatment solutions are used and the importance of maintaining a proper solution concentration. They need to understand the consequences and liabilities they are exposing themselves to when not using the recommended system fill, and what actions may be needed to recover a system if the solution becomes corrosive. To these effects, it is very likely that your local water-treatment manufacturer's representative would jump at the opportunity to sponsor a class for you and your dealers. The educational benefits of such a class will provide a win-win-win for you as the distributor, your rep and the dealers who attend.
Figure 2 lists a variety of system solutions I've encountered and their suitability for use with geothermal loops.
I prefer teaching from examples rather than cut-and-dry tables of rules and regulations. You can learn a great deal from the experiences of others and how they resolved their problems. The following is a series of four case histories of systems “gone sour” due to a lack of knowledge on the part of the installing contractor. Hopefully, you will identify with one or more of these cases and discover a solution to avoid similar misfortune in the future for yourselves and your dealers.
Commercial Office Building: In this instance, we have a commercial two-story office building of 25,000 sq. feet. The geothermal loops are all fusion-welded polyethylene, manifolded to a welded black iron building piping system. The piping system serves 22 geothermal units with brass valves and flex-hose connectors at the equipment locations. System antifreeze fill was 80% city water with methanol as the system antifreeze. The original system operated with a minimum return fluid temperature from the loop of 40 degrees. Over the years, the cooling dominant nature of the application caused the minimum return temperature to rise above 60 degrees. The building first encountered problems approximately eight years after completion when a series of leak sites developed at threaded connections between brass valves and black iron interior loop piping. Repairs required using liquid carbon dioxide freezing cuffs to isolate the leak sites, removing the leaking valves, cutting and rethreading the black iron pipe, and restoring the system into operation. After analyzing the failure sites, we determined that the methanol solution was highly corrosive. Methanol is an excellent solvent and a primary reason that NASCAR crews need to replace the fuel cells (gas tanks) in the cars they maintain. The mixture of methanol with city tap water made for a conductive solution and a galvanic cell between the brass valves and black iron pipe. The need for repairs continued until the system was completely flushed and cleaned, and refilled with water containing industrial boiler/tower water inhibitors and sacrificial anodes to help protect the system piping, much as the anode in a hot water tank helps protect the steel shell of a water heater. The cost for these repairs ran into the $20,000+ range but saved complete replacement of the piping system for the short term.
Residential horizontal closed loop for a 5-ton geothermal system: (Figure 3) The geothermal loop is filled with softened well water and 20 percent methanol. The owner experienced three loop pump failures in fewer than eight years. He replaced two failures under product warranty. The third failure occurred out of warranty and cost the owner more than $1,800. By this time, the customer had lost confidence in the installing contractor and contacted our office to remedy the situation when he experienced a fourth failure. Upon inspection, we discovered that methanol in combination with the high-sodium softened water had eaten through the pump bodies. As fluid leaked from the system, the pumps became air-locked, overheated and burned out. To clean the system, we first replaced the flow center and pumps, and flushed the loops with tap water, followed by 120 gallons of deionized water which we circulated for several days, followed by another 120 gallons of deionized water and 24 gallons of inhibited propylene glycol antifreeze to attain a freeze point of 20 degrees. The total cost for flow center replacement and a complete system cleanup flush and fill was nearly $5,000. Had the contractor flushed and filled the system with an inhibited antifreeze solution during installation, the proper antifreeze solution would have added $450 to the total cost. In hindsight, the installing contractor's decision to use softened site water with methanol cost more than $10,000 in additional warranty materials, labor and cleanup costs plus the total loss of consumer confidence and any opportunity for repeat referrals for an otherwise very nice installation.
Residential 10-ton geothermal system: (Figure 4) Vertical closed loop of +/- 20-year age, shared between two 5-ton geothermal units. This system functioned reasonably well for 20 years with little maintenance until a contractor, with minimal knowledge of the system's history, replaced one of two 5-ton systems. The original equipment, it appears, was totally sealed and filled with calcium chloride mixed with city tap water. It appears the solution was likely “in equilibrium” with the loop piping and had not created any major corrosion issues. When the contractor replaced the defective machine, he also installed two self-purging, atmospheric flow centers. He must have been oblivious to the fact that the clear fluid in the system was calcium chloride, which becomes extremely aggressive with the presence of oxygen. Now you have atmospheric flow centers that incorporate an open but capped solution tank. To further compound the problem, the contractor compensated for lost fluid by adding methanol as the makeup solution. The combination of air contact in the atmospheric tank plus the methanol created a toxic corrosive cocktail that destroyed and ate out six loop pumps within a two-year period. Correcting the problem resulted in a lawsuit, considerable lost time on the part of the distributor, installing contractor and homeowner. The cost to replace the corroded flow centers and completely flush the system with deionized water plus add inhibited propylene glycol as the solution fill amounted to nearly $10,000. If we were to factor in the expenses for failed loop pumps, labor and legal fees, the expenses to the installing contractor were likely in the $30,000 range.
Office building 8-ton system: (Figure 5) Vertical closed loop of +/- 20-year age. Likely system fill when installed was automotive antifreeze (ethylene glycol) mixed with drilling or site water. The office manager called one afternoon to announce the system was not cooling and there was a bad smell in the mechanical room. This type of call generally sets off all types of alarms and had me on the way immediately to find out what had happened. Upon arriving, I found the floor was wet around the base-mounted loop pump. A horrible stench filled the air and the system pressure was down, but I did not see a leak. A quick, temporary hose cross-connection to city supply would let me repressurize the old loop to help find the leak. Before reaching 10 psi, the face blew clean off the circulating pump. Upon failure analysis, it appears that contaminated water may have filled the original system. (Bacteria love glycol and will reduce it from antifreeze to something similar to vinegar or silage, however you choose to describe the foul odor it creates.) The resulting acidic solution had slowly dissolved the impeller and face of the pump until the failure occurred. Cleanup as with the previous examples involved replacing the pump and flushing the loop with deionized water. Only this time, the first flush had iodine added to it to help disinfect the loop. Once flushed, the system was refilled with deionized water and a 20% solution of inhibited propylene glycol. Annual monitoring will serve to verify if the antifreeze holds its strength and inhibitor balance. The replacement pump plus cleanup expenses for this system ran in the $7,000 range.
In each of these cases, methanol or a glycol mixed with site water was largely responsible for the failure of the loop pumps or piping system. In my experience, it appears that virtually all geothermal manufacturers as well as the International Ground Source Heat Pump Association (IGSHPA) provide pressure drop tables for designing closed loops using methanol as an antifreeze. In addition, every closed-loop drilling contractor we work with also uses methanol as their vertical loop fill solution. In each case, we find ourselves either providing deionized water to fill the loops on site, or we flush the loops with deionized water and test the electrical conductance of the return water to assure that all mineralized tap or drill rig water has been flushed from the system.
Methanol is in common usage as an antifreeze solution in the drilling industry. It is inexpensive and makes for easy freeze protection of rig water pumps and lines. However, like smoking, old habits are very hard to break. Methanol, or for that matter any antifreeze, either with or without inhibitors, when mixed with a mineralized or contaminated water source will act like a cancer to the geothermal piping and pumping system. There is no easy cure other than to clean it out completely, or take precautions in the beginning to see that it does not occur.
Mixing Antifreeze Solutions
Antifreeze suppliers provide guidelines for the allowable chemical content of city water to which you may add antifreeze. Rarely do I find a water supply in the field that meets these criteria. To maintain the best possible performance of the corrosion inhibitors, the antifreeze solution must be made from clean deionized or distilled water. The presence of bacteria, iron, calcium, magnesium or sodium (softened water) in makeup water will combine with corrosion inhibitors, rendering them ineffective and causing problems in the future.
Antifreeze solution is available as a premix or as concentrate and mixed with deionized water on-site. Deionized water should always be made from a chlorinated, bacteria-free city water supply. To use any other water source as a mix agent is to gamble away future time and profits. All major cities have water equipment and service providers who manage deionized water column rentals and exchange them on a regular basis. A conductivity lamp on the column discharge line will tell when the column has exhausted its resin capacity and is in need of replacement.
As product distributors, you are in the best position to educate your dealers on the methods for proper geothermal system solution fill. The loop solution will be as a boiler/tower fill in southern states using low or nontoxic corrosion inhibitors, or as an inhibited ethanol or propylene glycol in the northern states. After all, these solutions are a key part of the long-term dependability of all geothermal installations. As distributors, you might as well be a part of the merchandising chain rather than outsourcing this possible profit center to others or, worse yet, leaving the loop solution fill to chance and trusting that your dealers will understand water chemistry and take the initiative on their own. Failure to follow appropriate procedures for filling geothermal loops will create negative cash flow and erode consumer confidence in a technology we so desperately need.