Water Sourced Heat Pumps Corrosion? Check Your Ground!

Geothermal Heat Pumps Last 25 Years or More!

GEOTHERMAL HEAT PUMPS ARE AMONG THE MOST DEPENDABLE and long-lasting types of equipment available. Proper installation is a key factor in continued trouble free operation.

Proper grounding is important with regard to any piece of equipment that uses water as a heat transfer medium.  When leaks or corrosion occur, the first thing that is normally blamed is the water quality. You might be surprised to find out that the real culprit may be electrolysis.

Plate Heat Exchangers 

It is commonly thought that a plate and frame exchanger or a brazed plate exchanger is needed to protect the water to refrigerant coaxial exchangers in geothermal heat pumps and similar equipment. This can be proven to be a fallacy much of the time.

What happens most of the time when a factory installed coaxial refrigerant water exchanger fails is the result of electrolysis. It has become abundantly clear that this is been mistaken for chemical erosion and corrosion of factory mounted exchangers over the last few decades.

There are several good reasons for the installation of a plate and frame heat exchanger. However, installation of such an exchanger simply to protect condenser piping and the factory mounted water-to-refrigerant coaxial exchanger without regard to the information contained herein may be a mistake.

A Plate Frame Heat Exchanger (PFHX) provides segregation between two different water and/or 

fluids used to transfer energy.  It can be a definite advantage or an unnecessary cost and complexity; it depends upon the application.

A Small "Ding" May Cause Erosion

Electrolysis versus chemical/abrasive wear and corrosion:

A combination of erosion and electrolysis can lead to a rapid deterioration of pipe or heat exchanger integrity. Erosion of any pipe can be caused by high velocities through the pipe and is typically an occurrence where a minor obstruction as a solder-drip, a “ding” in the pipe or even the transition from a male to a female at a solder joint. The intrusion of a solder drip has been shown to increase a 6 feet per second water velocity to a turbulent eddy of over 200 feet per second! That high a velocity will create a “velocity erosion” site. The velocity erosion can go undetected for years OR it can be exacerbated by another serious condition; electrolysis.

Dielectric Unions Won't Stop Electrolysis Without Good Grounding

Conditions favorable to Electrolysis  

Electrolysis can occur in any water or conductive fluid can cause a charge build-up at sharp point (like a lightning-rod). If the charge concentration then passes from the metal edge into the water stream, it is possible that sharp edge can become a “sacrificial anode”.  Keep in mind that copper-oxide is an electrical rectifier so a DC component can essentially plate-away that sharp edge. As the edge gets plated away, it creates new sharp a critical anode surface, creating a self-degrading condition.

Grounding Can Save Unnecessary Service Issues

Of note, A 115 volt device such as a small pump, control and the like derive its 115 volts from one side of a 230 volt line and a return to ground. If the return to ground is a lower resistance through water than the intended metal ground path, the stage is set for electrolytic corrosion.

Check the electrician that installed your dish washer, clothes dryer or clothes washer, they installed a fourth wire electrical ground to insure the internal 115 volt controls for these 230 volt devices do not have a lower resistance path to ground.

A 115 volt device should ever be used in any application, geothermal or otherwise where there is the possibility of the neutral current being returned through the water to earth potential.

Provide a competent and solid grounding path for all installations; especially those that use water as a heat transfer medium.

Best Practices for Geo

For more information on this and other subjects regarding geothermal heat pumps, consult the publications listed below:

This information was abridged from Chapter 6 of Modern Geothermal HVAC Engineering and Control Applications, McGraw-Hill Education, Jay Egg, Greg Cunniff, Carl Orio, July 16, ISBN 20130071792686 / 9780071792684

Jay Egg is a geothermal consultant, writer, and the owner of EggGeothermal. He has co-authored two textbooks on geothermal HVAC systems published by McGraw-Hill Professional. He can be reached at jayegg.geo@gmail.com