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The pump of your solar water heater keeps running

Does the pump also run at night or when it is cloudy? What is normal, what you can safely check yourself and when to call an installer.

The pump of a solar water heater should run when the collector on the roof is warmer than the water at the bottom of the storage tank. If it also runs when that isn't the case, say at night or under heavy cloud, it uses power and can even cool the tank down.

That isn't always a fault. First check whether it is one of these ordinary reasons.

When it is normal

  • A summer night after a hot day. Many controllers have a function that cools the tank back down at night through the collector when the water has got too hot. The pump then runs for a while. That is intended and stops by itself.
  • Frost protection. Some controllers run the pump briefly now and then when it freezes. The manual says whether yours does.
  • Just after starting. In a drain-back system the pump first runs at full power for a while to fill the collector.

What you can check yourself

  1. The controller's mode. If it is set to manual (often "Hand" or a hand symbol on the display), the pump always runs. Set it back to automatic.
  2. The temperatures on the display. Look at the collector temperature at night. If it reads much higher than the outside temperature, the sensor is probably measuring wrong, or isn't in its place.
  3. The temperature at the bottom of the tank. If it shows a very low or impossible value, the controller thinks the tank is cold and keeps pumping.
  4. An error on the display. Write down the code and look it up in your controller's manual.

Don't open pipes or loosen anything: the fluid can be hot, and in a pressurised system it is under pressure.

When to call an installer

  • A sensor shows an impossible value, or the value no longer changes.
  • The pump keeps running while the controller is set to automatic and the temperatures look right.
  • You see fluid leaking, or the pressure drops (see also The pressure of your solar water heater drops).

Replacing a broken sensor is usually a small job. Don't leave it for long: a pump that runs all night carries away heat you collected during the day.

How to see it sooner

You only notice a pump that keeps running if you look at the display at the right moment. With the BoilerBuddy Alpo you see the temperatures and the pump in a chart per day, and you get an alert when the pump runs while there is no heat to gain.

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