This study was undertaken to compare the economic viability in Johannesburg, R.S.A., of vertical ground-source and air-source systems. Capital costs and optimum borehole length of the ground system were determined. Monthly heating and cooling capacities, and COPs were evaluated to determine running costs. Payback periods, net present values and internal rates of return were computed. The vertical ground-source system was marginally cheaper than the air-source counterpart.