Audrey HayesBy Audrey Hayes

Updated November 2026

A solar hot water system not heating up usually points to one of a few causes: no sunlight reaching the collectors, a circulation pump or controller that has stopped working, air or scale in the loop, or a backup heater that isn’t engaging. This guide walks you through checking each one in order, so you can find the fault quickly and fix what you can yourself.

Step 1: Confirm the Collectors Are Actually Getting Sun

Start with the simplest check. Collectors need direct sun to heat the fluid, and even partial shade from a tree, a new satellite dish or a buildup of leaves can stop the loop from warming. Go outside on a clear day around midday and look at the collector face. Dirt, bird droppings and snow all block absorption, and a glass cover with condensation inside can also cut output.

Shopping for one? See our guide: Direct Solar Hot Water System Picks: 3 Best Options for Pool Heating (2026).

Next, compare the collector temperature to the tank temperature. Many owners use the controller display or a clip-on thermometer on the outlet pipe. If the collector surface feels warm but the outlet pipe stays cold, the problem is probably in circulation rather than in the panels.

Step 2: Check the Pump and Controller

A differential controller compares collector and tank temperatures and switches the pump on when the collectors are hotter. If the pump never runs, you have an electrical or control problem. Work through this list:

  1. Confirm the controller has power and the display is on.
  2. Check the fuse or breaker feeding the pump and controller.
  3. Listen at the pump on a sunny afternoon. A humming pump that doesn’t move fluid may be airlocked or seized.
  4. Verify the sensor leads are seated in their ports and not cut or corroded.

Pumps that run briefly and stop often can indicate a faulty sensor sending bad readings. Replacing a sensor is usually inexpensive and straightforward, but confirm the pump’s voltage and the controller’s rating before swapping parts.

Step 3: Bleed Air and Check for Flow

Air in the collector loop is one of the most common reasons a system stops heating. Air pockets block flow even when the pump runs. Most closed-loop systems have a bleed valve near the highest point of the collectors or the pump. With the pump running and the system cool, open the valve briefly until fluid comes out without bubbles, then close it.

After bleeding, check the pressure gauge on the glycol loop. Manufacturers typically specify a target range, and a reading well below it suggests a leak. A drop in pressure means fluid is escaping somewhere, and you’ll need to find and repair the leak before refilling with the correct antifreeze mix.

Step 4: Look at the Backup Heater and Tank

If the collectors and pump are working, the water may be heating but not reaching the taps at the temperature you expect. Check whether the backup element or gas booster is set correctly. A thermostat set too low, a tripped high-limit switch, or a failed element can leave the tank lukewarm on cloudy days. For systems with a gas backup, confirm the burner ignites and the flue is clear. If you’re unsure how the backup is wired into the system, see Solar Hot Water System With Gas Backup: How to Set It Up Right for the typical layout.

Also check the tank itself. Sediment at the bottom insulates the water from the heat source, and scale on the heat exchanger slows transfer. A hard-water area may need periodic flushing.

Common Mistakes

  • Assuming the panels are broken when the problem is only a blocked or failed pump.
  • Running the pump dry, which can damage seals and bearings.
  • Refilling a glycol loop with plain water, which can freeze and crack components.
  • Ignoring a falling pressure gauge reading and topping up repeatedly without finding the leak.
  • Skipping the controller setting check after a power outage, which can reset temperature thresholds.
  • A clip-on digital thermometer lets you read pipe temperatures without opening the loop. View on Amazon
  • A propylene glycol antifreeze concentrate made for solar thermal loops is the correct fluid for topping up after a leak repair. View on Amazon
  • A multimeter helps you check sensor resistance and confirm voltage at the pump. View on Amazon

FAQ

Why does my solar water heater work in summer but not in winter?

Lower sun angles, shorter days and cloud cover all reduce collector output in winter. Many systems also rely more on backup heating during these months. Check that the backup is set correctly before assuming a fault.

Can I fix a solar hot water system not heating up without a professional?

Basic checks such as cleaning collectors, bleeding air and reading the controller are manageable for most homeowners. Gas, high-voltage electrical work and pressurized repairs should be handled by a licensed professional.

How do I know if the sensor is bad?

A sensor that reads far from the actual pipe temperature, or a pump that cycles on and off rapidly, often points to a sensor fault. Compare its reading with a clip-on thermometer on the same pipe to confirm.

Conclusion

Work through the system from the sun to the tap: collectors, pump and controller, air and fluid, then backup and tank. Most no-heat problems trace back to one of these steps. If you find a leak, a gas fault or an electrical issue, consult a qualified professional before continuing.

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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.