There are several types of on demand hot water systems including: recirculating systems, demand type pump systems, thermo-siphon systems, and point-of-use water heaters. These systems may or may not save water in your home depending, upon your specific situation and the type of system you install.
Few studies have evaluated the water saving potential of these devices. A 2000 study in Westminster, Colorado evaluated on demand hot water systems in six homes and in a group of control houses (Mayer and DeOreo, 2000). This study found evidence of reduced shower usage in the six homes with the on demand systems. However, these six homes used more water for baths than did the control group. The combined shower and bath usage was identical between the two study groups. The study concluded that there was “no statistically verifiable overall savings” associated with the on demand hot water systems, either because of the effectiveness of the systems or because of the small sample size.
A recent unpublished study in San Jose, California evaluated on demand hot water systems in about ten single-family houses (de la Piedra, 2001). This study found very limited water savings in the study homes that were substantially less than the manufacturer had claimed.
If these devices are to included as part of comprehensive water conservation programs they must be shown, through independent evaluation, to save water. At this point water saving potential of on demand hot water systems is uncertain.