Climatemaster. The Tranquility 22 Digital series geothermal system has one of the smallest footprints in the industry, making it ideal for retrofit projects or installation in tight spaces, the company says. Efficient enough to meet EnergyStar Tier 3 requirements, the system is available in several capacities, all of which are eligible for an uncapped 30% federal tax credit on residential geothermal systems. The unit features a two-stage compressor, variable-speed fan, communicating digital controls, and plug-and-play functionality. 877.436.6263.