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Sizing a Geothermal Heating System

Posted by Taylor Hendricksen on Jan 16, 2012 in Design | 0 comments

There are a couple of different philosophies as to how to size a geothermal heating system.

The primary question is if the system should be sized to handle the highest loads that the structure will ever see or if it should be sized to handle the majority of the load but during peak loads a conventional element be used as a supplement.

For me the answer is one of economics. If the system were an open loop system and I had the available water to size the heat pump to handle the peak load then I would do that. The added cost of meeting the peak load with the heat pump involves nothing more than upsizing heat pump itself and pumping more water to it. If the system were a closed loop and I needed to build all that much larger of an earth loop to handle a peak condition that will only exist a small percentage of the time I may opt to design for 80% of max load and include a conventional back up to handle the peak demand times.

The second consideration for sizing a closed loop system is not the percentage of the load it can be expected to meet, but the minimum earth loop temperature that will be acceptable in terms of system efficiencies. Closed loop systems will be removing heat energy from a specific mass of earth and as they remove heat the mass will become colder. The colder the mass becomes the harder the heat pump will have to work to remove heat from it. Provided the heat transfer fluid has sufficient freeze protection the heat pump can operate down below 20 degrees F but the colder it is the less efficient the operation will be. Each heat pump manufacturer will have data on the efficiency of their product at different load conditions. Use this data to guide your sizing decisions and keep in mind that if you cool your earth loop much below 20 degrees your system will not be able to operate at all. Leave yourself some room for error.

The goal should be to get the maximum economic advantage from your geothermal heating system and this is a balance between operational efficiency and installed cost.