Drainback Solar Thermal Systems

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If you think a drainback solar hot water system fits your needs, you must consider a wide variety of rules, limitations and requirements, as outlined below.

Array choice and mounting

The choice of collector array for drainback systems is more complicated than for a pressurized system: some evacuated tube collectors cannot be used at all (check with the manufacturer) and any collectors that use serpentine riser tubes are also ruled out.

The array mountings have many requirements:

  • The array must be installed above the drainback tank. In most cases, this precludes ground-mounted arrays.
  • You must take the pump power into account: unless the pump motor has more than 0.5hp, the distance between the top of the water in the drainback tank and the top of the collectors must be less than 28 feet (8.5 meters).
  • The collectors must drain toward the array inlet (the bottom manifold, where the solar fluid enters) and should be mounted at a slope of 0.25 inches per foot toward the feed inlet.
  • Collectors must never be mounted with the absorbers' riser tubes horizontal: the tubes will sag with time and trap fluid instead of allowing it to drain properly.

 

Pipe choice and mounting

This is also more complex than for a pressurized system. The most important feature – and the one which must be remembered no matter what else you do – is that the system must drain quickly and completely when it is switched off. Additionally:

  • All pipes must be a minimum size of 0.75 inches and made of hard copper.
  • Corrugated steel piping must not be used, for reasons of efficient drainage.
  • All pipes must be mounted at a slope of at least 0.25 inches per foot, towards the drainback tank.
  • All horizontal pipes must be supported every 4 feet (1.2 meters) or less to ensure that the pipes do not sag.
  • Avoid using 90-degree pipe elbows wherever you can: two 45-degree elbows provide quicker drainage.

 

Solar fluid choice and maintenance

If you are installing a drainback solar hot water system in a location where freezes never occur, you can use simple water as a solar fluid. Not only is it a lot easier and less expensive to manage, but it's more efficient than any mixed fluid.

However, if there is even a remote possibility of a freeze, you may want to use a 30% mix of propylene glycol as insurance against burst pipes. You'll lose a bit of efficiency but you'll gain peace of mind.

You can monitor the amount of solar fluid by using the sight glass on the drainback tank when the pump is not running. If you use a water-glycol mix, make sure you remember to check the ratio when you perform normal maintenance. In most cases, this means a check every 10 to 15 years.