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Drain back solar.

joseph annon
joseph annon Member Posts: 51
I have a question about how to provide air relief in a drain back solar system that a client wants. The problem as I see it is that the run is more horizontal than vertical with a couple of steps in the piping. Slope will be pretty good about 1/2"+ per foot for a majority of the run. Length is my concern we are running about 125' horizontally under ground. Most of this run is on a slope and we can get below frost line. Total elevation from tank under house to collector high point is about 22' I am concerned that the pipes will not allow air into the collectors quickly enough to prevent freezing and that I will have to install a relief at the collectors to allow air in to drain the piping but then that eliminates the siphon for the flow of the water meaning I have to keep the larger pump running constantly. 

I am tinking about using a spring check on the high point in the collector bank and set it so that it opens to allow air in. I know that there are vacuum breaker for pool collectors but this system is going to hotter than the breakers can handle. I tried years ago to get  large drain back array working but the airvents seemed to create problems and the collectors constantly stamed through the air vents.



  • Robert O'Brien
    Robert O'Brien Member Posts: 3,483

    to install manual for a Beckett drainback sytem we're in the process of installing. Panels up,piping next!

    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • hot_rod
    hot_rod Member Posts: 19,278

    all the pipe 1/4- 1/2" per foot and it will drain. You do not want any air admittance or vacuum breaker in the piping. You want to keep the air trapped bubble in the system.

    When the pump starts the air bubble in the collector is pushed back into the drainback tank. When to pump stops the water will flow back and the air rise to the highpoint. It's a gravity sport.

    The air will end up in the high point, just like it does in hydronic systems when you don't want it to :)

    I like to run pressurized drainbacks at 30- 60 psi. This assures some NPSH to eliminate pump cavitation, and also raises the boiling temperature of the water to prevent steam hammer when water hits a 350F collector!

    Remember too, you do not need to drain back all the water, just the portion that could freeze, the collectors and the piping until it is below frost level. If the underground piping is deep enough then that line could stay "flooded." But it will drain back if sloped.

    Another option is a glycol mix in the fluid. This gives you extra protection should that underground line settle some day and form a trap that will not drainback.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • joseph annon
    joseph annon Member Posts: 51
    edited August 2010
    Drain back

    Very helpful Hotrod.

    The drain back tank is installed. I believe that it is an atmospheric bulk tank of  200 gallons under the mechanical room. I am waiting for specs from the owner. The piping we will install will be below frost but the  piping coming out of the building is probably only  a foot or so below grade. I was planning on insulating and sleeving all the piping to maintain the structure of the insulation. So will there not being an airbubble in the system affect the drain back? How long will it take to drain down? I don't see how I am going to avoid steam for at least the first moments.

  • hot_rod
    hot_rod Member Posts: 19,278
    the steaming

    isn't a problem, other that all the noise it makes. As long as the collector is rated for drainback.

    Can you calculate how much fluid is in the collectors and piping from the tank out to the array?

    Can you get into the 200 gallon tank?

    Another concept that is gaining steam (no pun intended) is draining back into coils inside the storage vessel.

    I first say this concept at the Euro Solar Expos. They use collectors with small diameter tube, usually serpentine, and small diameter S&R linesets, usually 15mm, around 1/2"

    Then they use solar tanks with large diameter 1-1/2" coils. So all of the fluid drains back into the coils. It is a careful balance act.

    They actually sell drainback kits built this way, you get a pre-determined amount of tubing so the fluid volume all fits into the coils when it drainsback.

    I was able to mock this system up at my shop with 2- 4X8 collectors, 36' of 3/4" solarflex and a Caleffi tank with an upper and lower coil piped in series.

    I also built a model with all clear tubing to see how the coil looks when it is only 1/2 full of water, when the system fills and is harvesting.

    So maybe you could get enough copper coil inside the tank to do a pressurized DB, and add the glycol if needed to protect that shallow portion.

    Here is a shot of the Wagner system. Vailant and a few other big name hydronic companies had similar systems on display.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Phelps_Clarke
    Phelps_Clarke Member Posts: 6
    Did you find a good vacuum breaker?


    Looks like I am in your same situation with a solar hot water system with an atmospheric drainback system I am looking for a good quality vacuum breaker to go on the roof. Did you have any luck?