Geothermal Heat Pump Installation - Tips and Techniques
To ensure a successful geothermal heat pump installation it is vital for all consumers to become well-informed.
Now let's discuss a few basic installation tips and techniques.
Very Important: The success and failure of a geothermal system lies in the quality of its installation.
Ground Coupled - Horizontal and Vertical
How Far Apart Can I Install Each Pipe?
Engineers and scientists have devoted many research hours to answer the above question.
The results of their research:
A closed ground loop can produce a temperature change in the soil up to 16 ft away. The actual distance affected in your application depends upon the difference in temperature between the fluid and the ground, as well as the thermal conductivity of your particular soil.
Does this mean that each pipe should be 16 ft away from any other pipe in the ground loop?
Not necessarily.
We must remember that piping, at a depth of 4-6 ft, can range in temperature +/- 10 F.
So, no geothermal heat pump installation will ever have ideal circumstances. In general, as long as the design calls for it, it is best to keep piping at least 3 ft apart.
This applies to both a horizontal and a vertical bore geothermal heat pump installation.
General Installation Tips and Techniques
Below are some general tips and techniques that apply to both horizontal and vertical ground coupled heat pumps:
- Never place any part of the ground loop less than 6 ft from a basement wall or crawlspace. Never bury the piping less than 2 ft from any buried water line.
- Typically takes a two-man crew about 1-2 days to put in an average residential sized ground loop.
- Due to environmental concerns, it is usually best to use propylene glycol as the antifreeze.
- Any piping above the frost line should be insulated with closed cell insulation like Armacell's Armaflex. The insulation should be protected by jacketing or a UV rated paint.
What if the Piping in the Ground Starts to Leak?
This is where it gets tricky.
When doing the initial geothermal heat pump installation, it is smart to do multiple parallel loops with a little extra capacity.
If a pipe starts to leak, it is generally cheaper to cap or valve it off, than try to fix it.
Ground Coupled - Horizontal Only Tips
How Should the Pipe Lay in the Trench?
The simplest design is a one-pipe ground loop where one pipe is laid in a single trench.
Advantage? Requires the least amount of pipe.
Disadvantage? Requires the greatest amount of ground area and trenching.
Other common horizontal trench designs involve two, four, or up to six pipes in a single trench.
The most common style of installation involves the four-pipe configuration with each one foot on top of the other (not a square corners configuration). This type of geothermal heat pump installation uses two pipes in parallel and requires only small piping.
Some contractors order the pipe in a coiled arrangement so that it can expanded into a spiral formation and laid in the trench.
The spiraled piping arrangement usually requires more piping than the other concepts because there is more heat transfer interference from one pipe to another. However due to the amount of piping, this method often requires the least amount of ground area and trenching.
However, beware!
Heat transfer interference encountered with the spiral configuration can become a problem during prolonged periods of hot or cold weather.
This type of geothermal heat pump installation is generally reserved for smaller residential or mild climate applications.Below is a cross-section representation of the various pipe layouts(Note: the spiral method is not shown).
Ground Coupled - Vertical Only Tips
The vertical bore geothermal heat pump installation is considered the most stable and consistent of the two ground coupled systems.
The reason behind this?
Since vertical bore holes go deeper than 6 ft, the ground temperature is not prone to temperature swings.
The ground temperature stays quite constant.
How deep are the bores?
Typically a vertical bore can range between 50 - 600 ft. This range of depth is typically dependent on the size of the system. It is generally worthwhile to dig the extra depth for large-scale projects. The relative cost increase required to dig deeper bore holes must be compared to entire project capital cost.
In a larger geothermal heat pump installation, it is very important to conduct thermal analysis of the ground at the depth that each bore will be drilled at.
How is the thermal test conducted?
A geothermal test apparatus is used to determine how many bore holes are required to meet the load of the large residential, commercial, or industrial building.
In order to conduct the test, a single bore hole is drilled at a pre-determined depth. This pre-determined depth is typically limited by the capabilities and cost of the contractor performing the work.
Once the bore is drilled, a two-pipe loop (with a U-bend connecting each pipe at the bottom and the two ends exposed at the top) is inserted into the hole.
An annular back-fill is then used to bury the vertical bore, leaving the two pipe ends sticking out of the ground. All piping that is buried above the frost line should be insulated with a closed cell insulation.
The piping is then connected to the test apparatus, which has been pre-manufactured specifically for ground thermal testing. The piping is then filled with a water or antifreeze solution.
The test is conducted continuously for 36 - 48 hours.
The test apparatus determines if the rate of heat exchange is within an acceptable range. It also checks to ensure that the heat rejection/absorption rate of one pipe doesn't interfere dramatically with that of an adjacent pipe.
Groundwater Geothermal Heat Pump Installation
Small Commercial or Residential Systems
Using groundwater for your geothermal heat pump installation can be a very economical solution.
If you are in a rural area, a very likely means of utilizing geothermal energy at low cost is a groundwater pump and dump scenario.
To employ this form of geothermal energy for your heating and cooling need, a well must be dug.
A well-digging contractor is easy to find and has the experience required to understand if the aquifer is large enough to handle a geothermal heating and cooling load.
If the local drain commissioner or code does not allow for rejection of the well water into a ditch or stream, then a second injection well must be dug.
Large Commercial and Industrial Systems
Extensive analysis must be performed if a ground water heat pump is to be used in large commercial or industrial systems.
Are the aquifers large enough to handle large commercial and industrial heat loads?
Is the well water quality such that extensive maintenance will need to be performed?
Iron and lime that could exist in the well water, for example, can hinder the effectiveness of this geothermal heat pump installation.
Lime: The existence of lime in the well water can produce pipe scaling when the water is exposed to high temperature refrigerants. Typically, by introducing an intermediate heat exchanger into the geothermal heat pump installation, this problem is avoided.
Iron:When iron is prevalent in the local well water, it will often coat the heat exchanger resulting in degraded performance. The heat exchanger would need to be cleaned periodically.
What is the most energy efficient means of using a ground water system?
Electrical usage expectations should be considered for groundwater heat pump systems. Depending on the depth of the well and the quantity of well water required, the expected energy efficiency of the heat pump system could be dramatically reduced.
The amount of well water flow must be optimized for energy efficiency. The greater the well water flow, the more efficient the heat pump will be. However, greater flow results in greater pumping power, and therefore less energy efficiency from the system as a whole.
The effect of more or less well water flow must be optimized to consider both of these factors
Typically, the optimization point is somewhere between 1.5 – 2.5 gpm of well water per ton of cooling.
Surface Water Geothermal Heat Pump Installation
In surface water heat pump systems a pond, lake or river is utilized to absorb or reject heat. There are two types of surface water systems:
1. Open loop systems: The pond, lake, or river water is physically extracted using a pump. After heat is absorbed or rejected by the heat pump, the water is returned back into the pond, lake, or river.
- This type of system cannot be utilized for heating when the water temperature drops below 40 F.
2. Closed loop systems: A heat exchanger is lowered to the bottom of the pond, lake, or river by means of weighting (such as cement blocks). A separate glycol fluid is circulated from the heat pump to the heat exchanger.
- The piping material used for closed loop systems is usually polyethylene or copper. Use of copper piping has durability concerns, however it usually requires only 25-33% of the piping length required for a polyethylene geothermal heat pump installation.
- Any closed loop that has the risk of being exposed to a water temperature of 50 F or less has the very real potential of being at or below freezing near the heat pump. A form of antifreeze should be introduced into the the closed loop system to avoid freezing problems.