6 Responses to “Different Kinds of Drainback Solar Water Heating Systems”


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  1. Dr. Ben,

    Thanks for a good summary of the most popular systems.

    You said, “single tank solar systems don’t work with gas water heaters, because the flame is at the bottom of the tank where the solar storage is supposed to be.”

    Bradford White would probably disagree, because they now offer a single tank solar storage/heater fired by natural gas.

    For information on how it works, go to http://greenbuildingindenver.blogspot.com/2011/08/important-new-solar-tank-from-bradford.html

    The jury is out as to whether it REALLY works.

  2. wcanterbury

    Thank You very much for the basic designs! It confirms some of my thoughts for a two tank system to preheat my domestic water. I would also like to add some radiant baseboard heaters to heat the lower walkout level of my house so the main level will require less forced air heating.

    Maybe I could add a heat pump later on for cooling during the Summer!?

    Can you recommend any books for a begginer who has a good grasp of most of the hardware in a drainback syatem? Since I am a top level water & wastewater treatment operator who has also worked a number of years for Graingers, most everything is familliar, but there are still a lot of questions! I would rather not reinvent the wheel and avoid all mistakes.

    Thank You for your work!!!

    Wallace Canterbury

  3. Dear Dr.Ben,

    Usually I design Type 2 system without heat exchanger inside the reservoir tank. I think that if we use a “solar” storage as a preheater (without another heat source), it is a good scheme. The exit of the solar storage can go inside the storage of the boiler. So the solar efficency in not influenced by the boiler.
    Furthermore, the design with an external reservoir tank is a more flexible scheme. It’s possible to change the height of the reservoir tank to limit the static head that the pump have to overcome during the start-up of the system.
    Do you agree?
    Best regards.

    • Dr. Ben


      First, type 2 and 3 drainback systems are by definition single tank systems, which are not as efficient as two tank solar systems. There are times when two tanks are too big for the space in an apartment, but there are ways to mount a 40gal conventional water heater over an 80 gallon drainback tank. I understand European water heaters are smaller than US ones.

      If one of these system is going to be used anyway, then installing the drainback reservoir tank high to lower the pump head is a good idea. However, there are specific limitations to both type 2 and type 3 systems that must be considered.

      A type 2 drainback system (exchanger in/on the tank) requires a specially made water heater with the exchanger built in. They usually have the same lifetime as a regular water heater (about 7 years), but cost a lot more. Some argue for externally wrapped exchangers around the outside of the tank, and some favor internal exchangers inside the tank. Either can work well if designed right.

      In a type 3 drainback system (exchanger in the reservoir), there is limited space for the exchanger, so it may not be as big as it should be. Also, if the volume in the collectors and piping is large, the water drop in the reservoir may uncover part of the exchanger. This means you should be careful that all the numbers add up. The exchanger must be the right size and it should not be uncovered as the collectors fill. Type 3 drainback system require two pumps – collector and exchanger.

      In all cases, a drainback system is more efficient than a glycol system and has a longer lifetime.

  4. Hi Dr. Ben,
    am a fan of your site from Germany and find this excellent as they explain everything.
    Here in Europe, only a few people are intelligent to understand the drain-back system.
    We now have a facility is built as a drain back to a storage of 130,000 liters of polypropylene and 200 vacuum tube collectors.
    Video on my site: http://www.agritec-solar.eu

    best regards

  5. michealclark55

    Thank you to explain briefly the most popular water heating system.This system is very useful with the help of this water heating system we save too much energy.For more information visit

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