Geothermal HVAC Systems: Pros and Cons

Every residential and commercial building needs an optimum balance of heating and cooling, throughout the year. A geothermal heating and cooling system uses the earth as a heat source during winters and a heat sink during summers. The heat sink or source could be the ground itself or a reservoir of water such as a lake or it could be a body of hot underground water in areas with volcanic or tectonic activity.

The temperature a few feet below the ground in many areas of our plant is relatively constant throughout the year. However, since the outside temperature varies a lot, this relatively constant temperature underground can be used for heating or cooling. In the winter, the temperature underground is warmer relative to the frozen or cold surface so geothermal systems can extract heat from the ground and transfers it into the space they serve using less energy than heating the space by other means. In the summer they do the opposite by taking heat from the space and rejecting it in the relatively colder ground. The heating and cooling is done using an underground web of pipes and pumps that circulate warm or cool fluid to and from the ground. The fluid is usually water-based as refrigerant has the contamination risks in the event of a leak.


The intent of this design is to use the relatively stable temperature of the Earth as a “heat bank” to which heat can be deposited, or from which heat can be withdrawn, as required to condition indoor space. This can increase the efficiency and reduce the overall operating cost of a Heating, Ventilation and Air Conditioning (HVAC) system. If installed and operated correctly, geothermal systems could be up to 40 percent more efficient than an air source heat pump or could provide energy cost savings of around 65 percent when compared with an electric furnace. These systems have many names such as geo-exchange energy systems, ground-source heat pumps and earth-energy systems but the underlying concept of heat transfer from a source in the ground is the same.

 Pros of Geothermal HVAC System

  • The operation of a geothermal heat pump system is smooth and relatively silent. For homes, it is generates about the same amount of noise as a conventional gas or oil furnace.
  • Without any wear and tear from environmental factors like snow and rain, the system works very reliability for a longer duration of time.
  • Geothermal heat pumps simply transfer energy to and from the ground; there is no release of harmful gasses like carbon dioxide and carbon monoxide.
  • This system has a long life due to low wear and tear, as the polypropylene pipes usually used for underground piping are highly durable.

Cons of Geothermal HVAC System

  • Installing a geothermal system will cost much more than a conventional, fuel based HVAC system. Excavating a large area for horizontal pipes or drilling holes in the ground for vertical pipes could take several days to several weeks depending on the size of system and the composition of the ground.  In the case of a horizontal pipe layout, additional costs may be incurred if landscaping must be replaced.
  • A geothermal system can maintain a constant indoor temperature year round, but it is not an ideal for buildings that require very high or low temperatures, as is often the case with commercial buildings or data centers.
  • For many existing buildings, converting to a geothermal system may not be an option due to space restrictions. The high upfront cost for converting to a geothermal system for existing buildings makes payback very long and renders the investment unfeasible. In addition, the efficiency might be reduced due to all the pumping energy required to pump water through the underground loop.
  • In climates where the heating and cooling seasons are significantly unequal (i.e. annual heating requirements are far more significant than annual cooling requirements), a geothermal system might be rendered useless after a few years’ use. For example, in a location where the heating season is 8 months long and cooling season 4 months long, a geothermal system will tend to have more heat extracted from it than added to it annually.  If that location does not have a volcanic or tectonic heat source to replenish the heat removed, the ground becomes successively colder year after year until there is no usable heat left in it.  One of Dimax’s clients commissioned a feasibility study from an engineering firm for one of their residential apartment buildings in the Toronto area.  The study outlined the business case for replacing the existing conventional natural gas and electric HVAC systems, which were nearing end of life, with a geothermal system. The upfront cost of such a project was quadruple what it would cost for conventional HVAC equipment replacements, and furthermore, the estimated savings generated by the geothermal system at today’s energy rates would not pay back the cost premium for some 40 years!  A payback this long does not make sense for most property owners and after the initial shock at the numbers our client chose to file the feasibility study until such time that energy rates are five times their current rate.
  • Geothermal heat pumps usually have auxiliary electric heat or compressors to provide supplementary heat when the ground-sourced water cannot meet the building’s heating requirements. Similarly, auxiliary DX cooling coils are sometimes required for buildings in hotter climates. In extreme air temperatures, this supplementary heating or cooling can increase the energy consumption of the geothermal systems beyond what a traditional heating and cooling system might use.
  • The optimum output temperature of a geothermal system is 100 to 110⁰F (37-43⁰C), which can make it challenging for some buildings to keep a balance temperature throughout the space as the requirements differ. For example, buildings in northern climates can be difficult to keep warm with a heat source that’s only capable of producing heat at 40⁰C.
  • Although generally durable, repairing the underground piping could be very costly if damage occurs.
  • Geothermal systems are more complex than conventional heating and cooling systems, so when used in remote areas with limited access to qualified service technicians, they might be difficult and expensive to maintain over time.
  • If an underground water reservoir is used such as a lake or aquifer, the conditions of the reservoir may change over time. In one example we came across, the building was using an underground aquifer as a heat source/sink for their geothermal system. After a couple of years of smooth operation, the underground conditions of the aquifer changed and the water level rose. This caused a lot of problems in the building like over-flooding of the drains.

Conclusion

A Geothermal HVAC system is capable of providing a relatively clean and more efficient way of cooling and heating than a conventional fuel-based HVAC system if certain conditions are met.  In general, we have learned that buildings in climates where the annual heating and cooling requirements are significantly imbalanced will not be suitable for geothermal system installations or upgrades.  In such climates conventional high-efficiency HVAC systems are more likely to be practical and cost-effective in the long run.  Hybrid systems, which are a combination of both types, can provide a balance in some cases but have twice as much equipment that needs to be purchased, installed and maintained.  Where a hybrid system is being considered a detailed and accurate feasibility study is highly recommended.  In any case, it is important to perform a careful study of the environmental conditions to verify feasibility prior to undertaking any geothermal project. Furthermore, a thorough Return on Investment (ROI) study should be undertaken based on the building requirements to confirm whether the high cost of installation and maintenance can be justified by energy savings in a reasonable payback time period.

 

Follow us on Twitter and on LinkedIn