2 Responses to “Dr. Ben’s Solar Hot Water System Last 25 Years!”


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  1. Ryan Williamson

    Dr. Ben,
    In your top 3 mistakes videos you suggest a slope for collectors of 1″ in 20ft. That seems quite small to me. I was planning on putting in a slope of 1″ for every 4-6ft on panels that are mounted flat to a 5/12 pitch roof. I understand that too much header slope can fill the upper supply side dead end stub and cause it to freeze. How much slope will cause that to happen and is there an advantage in the faster drain back time with something like 2-5″ of slope over 20′ Less slope will look better on the roof but I don’t want to risk ever having a freeze up event.


    • Dr. Ben


      You raise a good question. The fact is, you can put all the slope you want on a drain back collector system if you block the down stub. One way to do this is to sweat a female adapter on the down stub. Then make up a sausage sized silicon rubber rod. Insert the rod into the pipe up to the first riser tube. Then put a plug in the end.

      We used to take a silicone rubber caulking tube and squirt a finger size rope into water. That will prevent it from sticking to itself and is a good method for making silicone “rope”. Then we cut the rope into lengths to form a bundle. We would cram a bundle into the header pipe and screw in a plug. Problem solved. Or so we thought. Sometime later we saw one of the rope sections floating in the tank. The obvious solution is to glue the rope pieces together so they can’t separate.

      So, this method will work, but you need a plug that is too big to break loose. It reality, you don’t need to block the whole pipe, you just need something to absorb the expansion of the water when it freezes. The problem is keeping it in place. Another technique is to place a metal disk into the manifold up to the riser, then fill the remaining tube to the end with silicone caulking and insert a plug. The caulking must be stable to 350-400 degrees to stay put.

      Now, back to your original question. You can calculate the slope necessary to have water stand in the pipe halfway up the diameter. I don’t remember the number I originally calculated, because it depends on the distance from the end of the manifold into the last riser in the absorber plate. The number would be different for different collectors.

      You can do the calculation for your collectors if you want. The math is based on the short leg of a triangle. Assume the collector slopes down to the left. Draw a horizontal base line. Then draw a line sloping down from the right end of the line. That is the collector slope. Make the length of this line be the distance from the outside end of the manifold pipe to the first riser inside the collector box. The third side of the triangle connects the base line to the slope line. If the third side is greater that 1/2 the diameter of the pipe, then the water in that stub will stand above the center line and could burst the pipe on freezing. Note that the triangle plane must be perpendicular to the ground, not parallel to the roof slope.

      We really don’t need a lot of slope for a collector to drain. Most collectors have 1in diameter manifolds and if they were perfectly flat, the water would drain below the center line and cause little problem. However, piping and mounting techniques are not perfect, so there a bows and wiggles in the piping of a row of collectors. If the bow is big enough to cause a trap where water stands halfway up the diameter, then we have potential disaster.

      Tilting the collector 1″ in 20 feet is a discipline that makes us pay attention to what we are doing. It is not enough slope to cause a water trap in stub end(s) and it is not so much that it looks ugly or out of place running up the shingles on a house. We are calculating runs of 140 feet and that little bit of slope can cause a rise of 7 inches, which is not trivial.

      So you can have it both ways, but whichever way you choose, you need to do it right.

      Dr. Ben

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