Homeowners looking for an ecologically friendly way to heat and cool their homes may consider using a ground heat exchanger in place of traditional central heating or air unit. Installing these systems takes a team of trained professionals and does require excavation of part of the property.
Once the system is in place, a pressure test ensures there are no leaks before putting the system into service.
Ground Heat Exchanger System Overview
As the name implies, is a heat exchanger capturing and dissipating heat to the ground. It uses a series of smooth-walled, plastic or plastic coated pipes buried approximately 5-10 feet underground. At this level the soil temperature is around 50 to 73°F year-round, the deeper the burial, the more stable the ambient ground temperature is.
Pipes with smaller diameters are less efficient at moving air than larger diameter tubes. While there is some debate on whether it is more energy efficient to pull air through a long tunnel, called a solar chimney, or use a fan, it is understood that sharp angles are not desired. Using bends, no greater than 45-degrees allows for smoother, more efficient air flow.
There are three configurations for a ground heat exchanger.
A closed loop uses a blower to move air from inside the home through U-shaped pipes through the earth and the back into the house.
An open system draws air inside from an intake pipe through cooling tubes of long straight tubes, bringing air into the home.
A combination system is a mixture of the two which draws in some fresh air from outside but still runs most of the air through U-shaped lines to circulate.
Ground Heat Exchanger Requirements
When installing a ground heat exchanger, there are specific requirements that all systems must meet. While metal piping is an option, the best choice for this system is polyethylene, or high-density polyethylene pipe is the best choice. Should you insist on metal, it must have a plastic coating for more excellent durability. (Read The Lifespan of Steel, Clay, Plastic & Composite Pipes.)
Workers should choose pipes and fittings which can handle the stress and pressure requirements for this application. Using weaker lines and connectors will only result in leaks and damage to the overall system. Pipes with thick walls do handle more pressure. However, they are slow to transfer heat to and from the ground.
Designers should also use the proper anti-freeze solution in closed loop systems. While the pipes do have a higher stress tolerance, there is a risk of stress cracks due to the possible extreme temperature differences within the line to that of the ground.
However, the antifreeze solution should not contribute to this issue.
Tips for Pressure Testing Ground Heat Exchanger
Once installation of the ground heat exchanger is complete, workers must pressure test the system. The test should occur before any grouting or backfilling of the site occurs. By testing the system before covering, workers can visually assess each line and fitting for leaks and cracks.
To begin a pressure test, flush the system with water. This action removes debris such as dirt or pipe shavings from the pipe. Debris inside the lines will not damage them during use. However, it can damage the pumps attached to the line and limit water flow through the system during the pressure test.
Each line needs to have air purged from it. Air trapped in the system may block water during the pressure test. It can also corrode metallic components causing failures to occur.
Use Water Not Air
Once the system is free of debris and excess air it is time to fill the lines with water to achieve a steady pressure in the line. Recommendations from the International Ground Source Heat Pump Association (IGSHPA) state workers should pressurize the line to whichever is least of either 150% of the pipe design pressure or 300% of the operating pressure.
Pressurization should remain for at least thirty minutes to one hour. Worker should check flow rates and pressure drops during this period and compare them to the values calculated during the design phase.
Due to potential safety hazards, workers should not use air to pressure test the system. Unlike water, air is compressible, much like a large spring. If a pipe should crack during the testing, the pressure from the air inside could cause the broken line to become a projectile which could injure someone or damage the system further.
Make-up Water Requirement Doesn't Indicate Leaks
Due to the nature of PE pipe, there is a chance that it may expand during the pressure test. This expansion forces workers to add more water to the system to maintain pressure. Adding “make-up water” to the system does not indicate a leak.
It is normal to have to add water to the system during the initial pressurization. However, if there is no pressure stabilization, then a leak may be present. Workers should then inspect the connections and lines for water discharge.
All ground heat exchange systems require a pressure test to ensure there are no leaks. Open and closed-loop experiments should use water instead of air during the testing for safety purposes.