Protecting a solar collector from freeze damage increases the complexity and cost of most SHW systems. Virtually all SHW systems incorporate copper tubing in the collectors and/or piping systems. Water-filled pipes will burst from a hard freeze, since water expands as it freezes. Burst piping is difficult to predict, as many factors influence the conditions for freezing. Outside temperature and the length of time the temperature remains low are two important factors. Others are the tubing diameter and effectiveness of pipe or collector insulation.
The least freeze-tolerant systems are direct forced-circulation and direct thermosyphon systems. The relatively small (usually 1/2-inch or less) riser tubes in thermosyphon system collectors make these systems susceptible to freeze-burst damage at about 20°F ambient temperature. Any system with an integrated tank outside the heated space (ICS and thermosyphon) will have 3/4-inch potable water lines to and from the tank. Even if these lines are well-insulated, they are still prone to freezing between 0°F and 10°F ambient. Progressive-tube ICS systems are less tolerant and will freeze at higher temperatures (about 10°F to 20°F ambient) due to uninsulated 3/4-inch connection tubes inside the collectors.
A drainback system’s freeze protection relies on the water in the collector loop draining back to the reservoir, leaving the collectors and piping filled with air. The collectors and all piping must be sloped so they will drain when the pump is de-energized. A path for air to rise from the drainback tank to the top of the collector must be included in the system.
For their freeze protection, antifreeze systems use the same concept used in cars: a 50/50 solution of antifreeze and water. SHW systems use nontoxic propylene glycol instead of the highly toxic ethylene glycol used in most automobiles.