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Freeze Protection

4 min read

Water expands by about 9% as it freezes and splits pipes and collectors, so a solar water heater in a climate that falls below 0°C needs one of three protections: a drain-back layout in which the collector empties by gravity when circulation stops, manual draining before a freezing night, or an antifreeze loop that passes its heat to the household water through a double-wall heat exchanger. Clear nights can freeze a collector even when the air temperature is a few degrees above 0°C.

Part of Thermosiphon Systems

Freeze protection is the set of layouts and fluids that keep a solar loop from splitting when its water turns to ice.

Ice Takes More Room Than Water

Water at 0°C has a density of about 999.8 kg/m³ and ice about 916.7 kg/m³ (handbook values, recalled). A given mass of ice fills 999.8 ÷ 916.7 = 1.09 times the volume of the liquid: an increase of about 9%. A pipe full of water has nowhere to put that volume, so the pipe splits or a joint blows out, usually along its length, and the leak appears when the ice thaws.

A collector also radiates heat to the clear night sky and can fall below the surrounding air temperature, so freezing can occur on a clear, still night with air a few degrees above 0°C. Do not wait for a frost forecast.

Three Protections

  • Drain-back (parent page). Tilt the collector and pipes so that when the circulation stops, the water drains by gravity back into the tank. Every pipe must fall continuously toward the tank; a single sag holds water that freezes and splits the pipe. Check the fall with a level or a pour of water.
  • Drain-down (parent page). Drain the collector manually before a freezing night. It costs nothing and fails the night someone forgets, so reserve it for seasons you do not use the heater.
  • Antifreeze loop (parent page). Fill the collector loop with a water-and-antifreeze mix and transfer the heat to the clean household water through a coil or exchanger in the tank.

The Antifreeze Loop Has Its Own Hazards

  • Toxicity. Car coolant is mostly ethylene glycol: lethal if swallowed, sweet-tasting, attractive to children and animals (Fluids: Oil, Coolant, Brake Fluid). A single leak in the tank coil would put it in drinking water, so use a double-wall exchanger, or separate the loops completely so that a leak shows outside the tank.
  • Heat damage. A stagnating collector sits at 120-220°C (The Greenhouse Effect in Collectors). Glycol held at high temperature breaks down and can turn acidic, attacking metal from inside; renew it on a schedule and test with litmus or a pH strip if available.
  • Less heat per kilogram. An antifreeze mix carries less heat than water, so the loop needs a little more flow or a slightly larger exchanger.
  • Salvaged coolant carries residue. Filter it or use fresh mix.

What You Need

  • A level and a tilted collector for drain-back
  • Drain valves at the lowest points
  • A double-wall exchanger or a separate loop for antifreeze
  • Insulation on exposed pipes
  • Labels on anything containing glycol

Method: Choose And Check

Step 1: Decide whether freezing nights occur at all in the season of use. If not, no protection is needed.

Step 2: If they do, prefer drain-back on a thermosiphon layout (the collector and pipe falls toward the tank), or drain the system at the end of the season.

Step 3: Prove the fall: pour water into the top and watch that all of it runs out of the lowest point.

Step 4: If using antifreeze, keep it out of the potable circuit by construction, mark it clearly, and renew it.

Common Mistakes

  • Relying on insulation alone. It delays freezing; it does not prevent it over a long cold night.
  • A pipe that sags. One low spot is enough.
  • Exposed tank and vent pipe. An ice plug in a vent blocks the safety path.

Safety

Glycol poison, frozen vents and burst pipes

Ethylene glycol is poisonous and tastes sweet; label it, store it away from children and animals, and keep it out of drinking water. A vent or relief pipe that freezes shut removes the pressure relief you rely on in stagnation; keep it short, sloped and clear of ice. A pipe that has split will spray hot water when the system warms, so shut down and inspect it after a hard frost before running it.

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