The experimental work reported here is concerned with the single-phase closed thermosyphon under conditions which are appropriate to a northern application: large length-diameter ratios and heated lengths much greater than cooled lengths. The purpose is twofold: to generate empirical heat transfer data which represent the conservative lower limit of performance (in the absence of boiling or bubbling); and to reveal the thermal and momentum exchange mechanisms which operate at the junction of the heated and cooled sections. The apparatus consisted of a 102-mm-dia, vertical steel pipe of overall length varying from 2 m to 6 m. The lower section of the pipe was heated by means of electric-resistive tape, and the upper section cooled by means of a water jacket. Heat transfer data, plotted in the usual form of Nusselt number versus the quotient of the Rayleigh number and the length-diameter ratio, show the effect of geometry. This data was obtained for 10 ≤ L H /d ≤ 50, and 1 ≤ L H /L c ≤ 20. An empirical correlation is developed.