Graham FisherBy Graham Fisher

Updated November 2026

A DIY drain back solar water heater uses a collector loop that empties by gravity whenever the pump stops, leaving the glycol-free water in a reservoir and the piping dry. Building one means sizing a collector, placing a storage tank above the collector’s lowest point, running a small pump with a differential controller, and making sure every pipe drains at a steady slope. This guide walks through the build in seven steps, with a materials checklist and the mistakes that most often cause drain-down failures.

How a Drain Back System Works

When the collector is warm enough to pump, a controller starts a small circulating pump and moves water from a reservoir up through the collector. When the collector cools or the pump shuts off, the water drains back down into the reservoir by gravity. Because the collector and most of the piping sit empty during idle periods, the system resists freezing and does not need antifreeze. The tradeoff is that every pipe must slope continuously toward the reservoir, and the reservoir must sit below the collector’s outlet.

Shopping for one? See our guide: 7 Best DIY Solar Water Heater Options Compared for Every Need (2026).

Materials and Layout Checklist

  1. One or two flat-plate or DIY copper-pipe collectors, sized to your daily hot water need
  2. An insulated storage tank or reservoir mounted below the collector’s lowest return point
  3. A small submersible or inline circulating pump rated for hot water
  4. A differential temperature controller with two sensors (collector and tank bottom)
  5. Copper or PEX piping, insulated supply and return lines, and pipe clamps
  6. Check valves, a pressure relief valve, and a tempering valve on the household outlet
  7. Drain-down and air vent fittings at the high points of the loop

Steps to Build the System

Step 1: Set the Collector and Tank Height

Mount the collector at the angle your latitude suggests, usually between latitude plus 10 and latitude plus 15 degrees for year-round use. Place the storage tank so its inlet sits at least 12 inches lower than the collector’s bottom header. That height difference is what drives reliable drain-back.

Step 2: Plumb the Loop with Continuous Slope

Run the supply line from the tank bottom up to the collector inlet, and the return from the collector outlet back to the tank top. Keep horizontal runs sloped at about one-quarter inch per foot toward the tank. Avoid low loops or dips where water can pool, because trapped water will freeze and split pipe.

Step 3: Install the Pump and Check Valves

Place the pump on the supply side, close to the tank, so it lifts water rather than pushing it through the collector. Add a check valve on the return line to stop reverse flow at night. Install the pump so it can drain without air locking; a vertical pump mounted slightly above the tank is usually easiest.

Step 4: Wire the Differential Controller

Attach one sensor to the collector plate or outlet pipe and one to the tank bottom. Set the controller to start the pump when the collector runs about 10 to 15 degrees Fahrenheit hotter than the tank, and to stop when the difference falls to about 3 to 5 degrees. These numbers are starting points; adjust them after you observe the system on a sunny day.

Step 5: Add Air Vents and Drain Valves

Air pockets at the collector’s high point prevent proper drain-back and can stall flow. Install automatic air vents at the top of the loop and a manual drain valve at the lowest point of the supply line. Test the drain-down by running the pump, switching it off, and confirming the collector empties within a minute or two.

Step 6: Insulate and Protect the Piping

Wrap all tank-side and household-side piping with closed-cell foam insulation. Keep exposed collector headers covered where possible. Insulation reduces heat loss at night and protects piping in cold areas, but it does not replace the drain-back itself for freeze protection.

Step 7: Add the Tempering Valve and Safety Devices

Fit a thermostatic tempering valve on the hot outlet to blend water down to a safe household temperature, commonly set near 120 degrees Fahrenheit. Add a pressure-temperature relief valve on the tank and confirm the discharge line runs to a safe drain. Check local plumbing code before connecting to the house supply.

Common Mistakes

  • Setting the tank too high, so the collector never fully drains
  • Letting horizontal pipe run flat or dip, which traps water and invites freeze damage
  • Skipping air vents, causing air locks that block flow
  • Setting the controller temperature difference too low, which causes the pump to cycle constantly
  • Connecting directly to household plumbing without a tempering valve
  • Closed-cell pipe insulation: keeps supply and return lines from losing heat overnight. View on Amazon
  • Differential temperature controller with two sensors: starts and stops the pump based on collector and tank readings. View on Amazon
  • Thermostatic tempering valve: blends hot water down to a safe household temperature. View on Amazon

FAQ

Does a drain back solar water heater need antifreeze?

No. Because the collector empties whenever the pump stops, the water is not sitting in the panels during freezing weather. Antifreeze is used in closed-loop systems, not drain-back designs.

How high should the storage tank be placed?

The tank inlet should sit at least 12 inches below the collector’s lowest header so gravity can clear the loop. Taller height differences improve drain-down reliability.

Why does my collector not drain completely?

The most common causes are flat or dipped piping, a missing air vent, or a tank set too high. Check the slope of each horizontal run and confirm the vent is open. For electrical or pressure questions, consult a licensed professional.

Conclusion

A well-built drain back system depends on height, slope and controls working together. Build the loop to drain cleanly, set your controller conservatively, and add the safety devices before connecting to household plumbing. Review local codes and consult a licensed plumber or electrician for any connection to your home supply.

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Graham Fisher

About Graham Fisher

Graham Fisher is part of our editorial team and reviews and edits our solar water heaters guides, checking product details against manufacturer specifications and owner feedback.