With the goal of extending the geographical market for passive solar water heaters northward, a room-air natural convection loop (NCL) was investigated for providing freeze protection for supply/return pipes carrying pressurized domestic water through unconditioned space. An NCL model was developed based upon balancing buoyancy and friction in the loop. Calibration factors (Cfric,Cht-trsf) were introduced to scale calculations and account for un-modeled details and correlation bias. An NCL prototype was constructed with 2", 4", and 8" diameters. Least-square regression on data from the 4" diameter duct yielded (Cfric,Cht-trsf) = (2.05, 1.8). The calibrated model was used to provide site-specific design guidance, and to estimate energy losses with and without a thermostatic control to stop flow whenever Tamb > 0 °C.