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Conflicting requirements in small drainback system

JoeS Member Posts: 1
I am working on the design for a solar thermal drain-back system for my office.  The system will be for DHW heating as well as a display for training purposes.  The system will have just 2 collectors, one flat plate and one evacuated tube collector, and they will be piped in parallel on the roof in order to show differences in efficiency under the same t(i)-t(a) conditions. (We will monitor the delta T across each collector separately before the hot water mixes and comes down from the roof.)

So, the question I have is about pipe sizing.  I have read that in order to be confident that all water will drain when the circulator shuts off, pipes above the drain-back tank should be no less than 3/4 inch dia.

I've also read that in order to establish a siphon in the return piping from the collectors, water speed should be greater than 2 ft/sec.  And in order to have 2 ft/sec in 3/4" pipe, the flow rate must be at least 3 gpm.  This is more than 3 times the flow rate of almost all collectors I have looked at.

Is there a way that I can use a drain-back system that will drain-back properly and still have the manufacturer recommended flow rate?


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    It depends....

    The recommendation for using 3/4" minimum is from people who have employees who don't know the difference between 1/4" per foot, and flat. The pitch requirements for 1/4" per foot, in and of themselves is greater than it need be.

    Provided that all piping slopes consistently back to the reservoir, without traps and potential vacuums to hold water in suspension, the water WILL drain back into the heat of the house.. It's plumbing. Water seeks its own level, and it runs down hill :-)

    Establishing a siphon on the down comer can also be done by putting just a few feet of smaller diameter pipe. down near the reservoir, within the heat of the house. Once it establishes a vacuum (siphon), it will draw any air standing in the column down to the reservoir, and maintain good flow.

    Even in situations where you do have a need for 3/4" downcomer, you can put two pumps in series, and put one of them on a time delay timer, that drops it out once siphon has been established. Where there's a will. there's a way ;-)

    The rule of thumb has typically been, what ever your supply pipe is, based on flow and velocity, the downcomer should be one pipe size less to assist in establishing a siphon.

    What is CRITICAL, is to insure that when the array goes into drain back mode, that the return pipe from the array is not immersed below the water line of the tank. It can be as simple as a 1/4" hole drilled in the side of the pipe, 2" above the flood level of the tank, but it HAS to be there. If not, it will allow water to be held in the collectors. and they WILL freeze and break.

    If in doubt, throw a light solution of glycol (~30%) into the drain back module/loop and sleep better at night. (this would obviously be expensive for a large volume DB system...)

    In any case, consistent, trap free, well vented piping WILL drain back to the module/reservoir regardless of size (within reason)..


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  • hot_rod
    hot_rod Member Posts: 22,262
    size to flow rate

    sufficient to assure the water can carry the air back down with it. There is some good sizing info, formulas, etc in the idronics 6.

    We built a 3 collector (75 sq. ft.) system at the shop. It is piped with clear tube, filled with colored water. It's plain to see how the system drains down with this demo. It's piped with 1/2" ID tube, supply and return.

    Oversizing the return can cause the water and air to part ways and not clear properly.

    Pressurization is another important part of drainback systems to consider. That topic is discussed in Idronics 6 also.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • sunny
    sunny Member Posts: 5

    i've read the same thing: 3/4" pipe for the supply & return. i hadn't heard about downsizing the return, have you done that mark? i've been wondering the same thing about flow rates: .88 for the panels i'm considering (3). i guess i was hoping the variable speed control would work it out...
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    MANY times...

    The normal standard for circulation is .02 GPM/Sq. Ft.

    So for a 32 square foot panel, = .64 gallons, or 1.92 GPM for your 3 panel array.


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