Two small low flow SDHW systems based on mantle tanks were tested side-by-side in a laboratory test facility for SDHW systems at the Technical University of Denmark. The systems are identical with exception of the solar collector loops and the circulation pumps. The most important data for the systems are shown in table 1.

One of the systems is equipped with a low flow SOLAR 15-65 pump and a so-called “lifeline” solar collector loop based on a %” copper pipe for the hot solar collector fluid pumped from the solar collector to the mantle tank and on a 5/16” copper pipe for the cold solar collector fluid pumped from the mantle tank to the solar collector. Both the hot and the cold pipes are insulated with glass fibre and enclosed by a PVC jacket with a diameter of 38 mm, see figure 2. The length of the lifeline in the tested system is 16 m. The other system is equipped with a normal circulation pump for solar heating systems, SOLAR 15-40, and a solar collector loop consisting of 33 m 10/8 mm copper pipes insulated with 9 mm INSUL-TUBE insulation.

The thermal characteristics of the lifeline have been investigated [2]. The heat loss coefficients of the hot and the cold pipes of the lifeline depend on the temperature difference between the hot and cold pipe and the temperature difference between the hot pipe and the ambient air temperature. For the hot pipe a typical heat loss coefficient is 0.1 W/K per m, while the heat loss coefficient of the cold pipe is typically placed in the interval from -0.4 to 0.2 W/K per m pipe. That is: In periods