Audrey HayesBy Audrey Hayes

Updated November 2026

A solar hot water system leaking water or glycol is almost always fixable once you locate the source. This guide walks you through shutting the system down safely, tracing the leak to its origin, and repairing the most common failure points. Most homeowners can handle fittings and valves themselves, while roof-mounted panels and pressurized tanks call for a licensed installer.

Step 1: Identify What Is Leaking

Before you touch anything, confirm what the fluid is. Closed-loop systems use a glycol-water mix, which feels slick, smells faintly sweet, and may leave a crusty residue. Direct systems leak plain potable water, which evaporates without a trace. Stains on the ceiling below the tank or a damp patch under the roof penetration point to different sources.

Shopping for one? See our guide: 6 Best Indirect Solar Hot Water System Options for Homes (2026).

Step 2: Shut Down and Depressurize the System

Never work on a hot, pressurized loop. Panels can reach temperatures above 180°F on a sunny day, and glycol at that temperature causes serious burns.

  1. Turn off the circulation pump or let the controller cool the array overnight.
  2. Close the isolation valves on the supply and return lines.
  3. Open a faucet at the highest point to relieve pressure.
  4. Use a drain valve to catch fluid in a labeled bucket.

Step 3: Trace the Leak to Its Source

Dry the area with a clean rag, then check each component in order from the roof down to the tank.

  • Panel connections: Look at the flexible connector hoses and the manifold unions. Crusty residue around a joint usually marks a slow weep.
  • Roof penetrations: Inspect the pipe pass-through flashing. Water running down the pipe and out at the roof jack is a common cause of ceiling stains.
  • Pressure relief valve: Discharge from the relief valve is normal only during overpressure events. Persistent dripping means the valve is worn or the expansion tank is undersized.
  • Tank fittings: Check the drain valve, anode port, and heating element gasket if your tank has one.
  • Expansion vessel: A bulging or wet vessel usually means a failed diaphragm.

Step 4: Repair Common Leak Points

Threaded fittings and unions

Loosen the union, clean both faces, and reseal with PTFE tape or a compatible pipe dope rated for your fluid. Tighten by hand plus a quarter turn with a wrench. Overtightening cracks plastic fittings.

Pressure relief valves

Flush the valve by lifting the lever briefly. If it keeps weeping, replace the valve with one rated for your system’s pressure and temperature, matching the manufacturer’s specification.

Roof flashing and seals

Replace cracked boot seals and re-seat flashing so water sheds away from the penetration. Use a roofing-grade sealant only as a temporary measure.

Step 5: Refill, Pressurize, and Test

Once repairs are complete, refill the loop with the correct fluid mix, bleed air from the high points, and check the expansion vessel pre-charge. Restore pressure to the level your manufacturer specifies, then run the pump for 30 minutes and recheck every joint for moisture.

Symptom Likely cause First action
Crusty residue at a joint Slow weep at union Reseal and retighten
Drip from relief valve Worn valve or low expansion capacity Flush, then replace if it recurs
Ceiling stain near pipe Failed roof boot or flashing Replace seal and reflash
Pressure drops over days Expansion vessel or fitting leak Check vessel, then test joints

Common Mistakes to Avoid

  • Opening fittings on a hot loop, which can cause scalding.
  • Mixing PTFE tape with pipe dope on the same joint, which can prevent a proper seal.
  • Ignoring a small weep, which often worsens under thermal cycling.
  • Using a generic sealant on glycol lines that requires a compatible compound.
  • Skipping the pressure test after repair.
  • PTFE thread seal tape helps create a reliable seal on threaded metal and plastic fittings. View on Amazon
  • Glycol-rated pipe dope suits closed-loop joints where the sealant must tolerate heat and antifreeze. View on Amazon
  • Digital pressure gauge lets you check the loop and expansion vessel accurately before and after repairs. View on Amazon

FAQ

Can I keep running a solar hot water system that is leaking?

Running a leaking system is not advisable, because air can enter the loop and fluid loss reduces heat transfer. Shut it down until the leak is located and repaired.

Is glycol leaking from my system dangerous?

Propylene glycol is low in toxicity but still irritates skin and eyes, so wear gloves and clean spills promptly. Keep pets and children away from the fluid.

When should I call a professional?

Call a licensed plumber or solar installer if the leak is at the roof, inside the tank, or involves pressurized components you are not comfortable handling. Consult a professional before any electrical or gas work near the system.

Conclusion

A solar hot water system leaking water or glycol is usually caused by a worn fitting, relief valve, or roof seal. Shut the system down, trace the source methodically, and repair the failing part before refilling and testing. Regular inspections of joints and seals will catch small weeps before they become costly problems.

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Audrey Hayes

About Audrey Hayes

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