Water is circulated inside the loop and carries heat between the ground and the heat pump. Normally, the loops are filled with water and a corrosion inhibitor to slow the deterioration of metal components. The hole requires no casing and is normally backfilled with sand or gravel and earth from the drilling. Several U-shaped sections can be connected to provide enough heat exchange capacity. One such hole is good for about one ton of cooling capacity.
Horizontal pipe loops demand more land than vertical loops.
The COP of the heat pump is the ratio of Btus of heat produced by the unit to the number of Btus of electric energy used as input to operate the unit. The HSPF is the average number of Btus of heat produced per watt of electrical input. With both of these measures, the higher the number, the more efficient the unit. The HSPF represents an overall seasonal average and therefore is more meaningful than a COP value, which is specified for only one outdoor temperature condition. The advantage with the water-source type is due to the more constant temperature of the water entering the heat exchanger, which enables the heat pump to be designed for maximum efficiency.