Ground Source Heat Pump Sub-Slab Heat Exchange Loop Performance in a Cold Climate

TO3 2.1.2: Sub-Slab Heat Exchanger

This report presents a cold-climate project that examines an alternative approach to ground source heat pump (GSHP) ground loop design. The innovative ground loop design is an attempt to reduce the installed cost of the ground loop heat exchange portion of the system by containing the entire ground loop within the excavated location beneath the basement slab. The horizontal sub-slab approach presented in this report will reduce installation costs by approximately $2,500/ton (Oberg 2010; Appendix A) compared to those of a conventional vertical bore well. For the 1.5-ton system installed in the cold-climate, 2,772-ft2 , two-story unoccupied test house in Pittsburgh, Pennsylvania (hereinafter referred to as the Pittsburgh Lab Home), a traditional vertical well system is expected to cost $17,800, compared to $14,000 for the horizontal sub-slab coils.
Prior to the installation and operation of the sub-slab heat exchanger in the Pittsburgh Lab Home, energy modeling using TRNSYS software (TRNSYS 2007) (in addition to traditional design efforts) was performed to determine the size and orientation of the system. Several design considerations were fundamental to the sub-slab GSHP design. Being a low-load home, the GSHP capacity is less than that for a typical house of the same size. Standard design of a GSHP allows for a margin of error for system undersizing, but with a low-load home, the margin of error is reduced. Additionally, a low-load home has longer shoulder seasons with reduced peak loads. The impact on the GSHP design is that heat is rejected and withdrawn from the ground in smaller, individual loads but over a longer time period than with a traditional GSHP. This design characteristic directly relates to the climate region where the house is located. A balanced climate region with a relatively equal number of heating and cooling degree days will have the effect of balancing the deep earth temperature and will help prevent overheating of the system.
This report analyzes the performance of the sub-slab heat exchanger for the approximate 1.5-year monitoring period of October 2011 through February 2013.