The market for lower-cost passive-solar domestic water heaters (PSDWH) has been limited by the problem of freezing and bursting of insulated supply and return lines that connect to the thermal storage on or under the roof. Using insulation to protect the pressurized piping—as is current industry practice with PSDWH—severely restricts the market for PSDWH.

Freeze-protection valves (FPV)—also called "dribble valves"—protect water pipes by inducing a small flow of warm water through the piping when the temperature drops below the FPV setpoint (typically ~35°F for piping protection), thus preventing freeze. The valves are commonly used in many industries to protect piping from freezing in exposed locations as far north as Alaska. It is possible to use FPV to protect the supply/return piping from freezing in passive systems, when the valve is mounted.

NREL conducted an experiment to better understand the flow rate through the valves as a function of ambient temperature and warm water temperature. The data was the basis for predicting the long-term annual water consumption of using FPV for pipe-freeze protection. If 1,000 gallons/year is considered acceptable water consumption, the potential market for passive systems (considering only the pipe-freeze aspect).

NREL's research showed that markets could be extended by more than 2 orders of magnitude with the 1,000-gallon limit and approximately 1 order of magnitude for a very small 100-gal/yr limit (less than 1 day of average household use). Continued research in these technologies will help further penetrate existing markets and expand to new geographic markets.