Drainback Solar System Pumps

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Unlike pressurized installations, drainback solar hot water systems have a fair amount of head to overcome before they start operating properly. Consequently, they need more powerful pumps. The choice of pump is absolutely critical to the system.

The pump on the solar loop needs to be powerful enough to push the solar fluid out of the drainback tank to the top of the collector array. It has to be capable of doing this quickly, so you'll need a high-head model that can overcome the head measured as outlined below:

  • Measure from the bottom of the drainback tank to the highest point of the collector array.
  • Add 4 feet (1.2 meters) to the measurement.
  • Add the friction head for all the piping in the solar loop.

Once the solar fluid has filled the loop and is flowing, the pump won't need to do much work – gravity will lend a hand – but the startup requires more immediate force than a PV-powered DC model can provide. You'll have to go with a 120V AC pump using a differential temperature controller for switching on and off.

However, once the fluid is running, you don't want the pump to rush it around the loop too quickly, before it has a chance to heat in the collectors. There are two solutions for this:

  • Choose a high-head, low-flow pump that has a powerful start and a weaker constant push.
  • Install a variable-speed model with a timer to reduce the flow rate once the initial kick-start has overcome the system's static head.

Whichever model suits your needs, you should install it below the bottom of the drainback tank in a vertical pipe so that it pumps water straight up to the collectors.