result in lower temperatures in the heating mode when the earth coil is acting as an evaporator pressure drop results in warmer temperatures in the inlet portion of the coil and colder temperatures toward the outlet the varying evaporating temperature results in a decrease in the log mean temperature difference between the coil and the earth which adversely impacts performance somewhat of an offset to the degradation of lmtd is the fact that increased refrigerant velocity increases the refrigerant side heat transfer coefficient and air source evaporators are normally circuited with up to a 7 psi drop r 22 and still obtain performance about equal to coils circuited for a two to four psi pressure drop if greater pressure drops are encountered the coil performance is decreased and additional circuits are required to meet the heating and cooling requirements an insufficient pressure drop indicates that the tube diameters are too large and the charge of refrigerant will be large if the tube diameters are too small the pressure drop will be too great and coil performance will suffer a careful calculation will determine the optimum pressure drop without degrading performance the factors involved include cost of tubing cost of refrigerant and cost of installation if longer heat exchangers are required this generally results in 10 11 psi pressure drop as being optimum in the future with greater costs for copper and refrigerant and labor higher or lower pressure drops may be more cost effective the demonstration project has shown that three vertical 100 foot heat exchangers consisting of 80 feet of 3 8 inch od tube 100 feet of 1 2 inch od tube and 80 feet of 5 8 inch od tube work well with a 4 ton heat pump as do.