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Designing for High Temperature Geothermal Systems

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Traditional geothermal hydronic systems are limited to a maximum load temperature of +/-120 degrees. Recent design and refrigerant changes now allow a few of these systems to attain temperatures in the 145-150 degree range. The capability to attain higher leaving load temperatures opens a number of new possibilities for the application of geothermal hydronic systems for our energy conscious customers. In particular are applications that currently rely on higher temperature hot water boilers for both heating and domestic hot water.

There are a number of factors that make high temperature geothermal systems attractive as a primary heating source for older structures with either cast iron radiators or copper fin-tube baseboard. Every application requires careful consideration to the heat loss of the individual rooms, and the effective radiation that is applied to that room. These factors include:

  1. Intentional oversizing of cast iron radiators in the early years of the past century, (a result of the Spanish Influenza of 1918 and a need for ventilation air). Oversized radiators along with energy improvements to windows and insulation may allow many of these older systems to function with a substantial reduction in fluid temperature. See Dan Holohan’s book Every Darn Radiator at for ratings for cast iron and convector type radiators.
  2. Modulating condensing boilers are capable of higher efficiency, and can modulate to match boiler temperature with the actual radiation load.
  3. ASHRAE BINS weather data supports the fact that 50% of a home’s heat loss represents 95-97% of the total winter heating degree hours. This means that if the hydronic system design maximum fluid temperature falls within the operation range of the high temperature geothermal system, the geothermal equipment may operate in a lower temperature/higher efficiency (3-4 COP) range for 95-97% of the winter hours, and need only run in the high temperature (COP 2.2-2.3) lower efficiency range of its capability for only 3-5% of the winter run hours.

All of these factors allow for the application of high temperature geothermal systems as a source of primary heat for perhaps 95-97% of the winter operation hours, yet allows the high temperature geothermal system to remain in operation, or to use supplemental heat from a modulating high efficiency gas or electric boiler to help meet worst case cold weather conditions for the remaining 3-5% when higher temperatures may be required to maintain comfort.


  1. Controls for the high temperature geothermal systems should be based on the load side return temperature, and be fine-tuned with the system in operation at maximum load conditions. This is necessary to ensure that the system flow rate and high limit temperature control setting do not cause the system to cycle off on the high refrigerant pressure safety limit.
  2. Whenever possible, operate the system using an outdoor temperature reset controller. If functioning via a thermal storage tank to manage smaller zones and using a simple “On-Off” hydronic temperature control, wire the zone control “boiler run” signal in series with the return line temperature sensor so that the system will not remain in operation trying to manage storage tank stand-by thermal losses during periods in which no heat is required. This will save immensely on needless operation time, and prevent the system from constantly cycling to maintain a thermal tank at its maximum temperature.
  3. If managing domestic hot water for a customer, avoid indirect water tanks that use boiler coils inside the tank to make hot water. These are intended to operate at 180+ degrees and do not have the temperature differential to adequately make domestic hot water at 120 without short-cycling the geothermal unit. Instead, consider using a tank like the Turbomax (, which uses the tank volume for the load side water and manages domestic hot water via a series of copper tubes that are suspended inside the tank. I’ve never had a “not enough hot water” complaint from a customer with one of these tanks.

I hope that you find this helpful and remember to always read the manufacturer’s installation manual from cover to cover.

Jeff Persons
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Posted In: ACCA Now, Hydronics

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