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Posts: 75
Reply with quote  #1 
I'm really partial to drainpipe collectors, I guess because they're so elegantly simple and work really well even though not to the efficiency of the screen units.  Anyway, here's my question:

I paint my drainpipes with high temp, black paint when installing them in their box. Would I gain any advantage by lining the box with high reflective insulator sheets? Would reflective inner surfaces bounce more solar energy back to the pipes rather than being absorbed by a dark lining?  I realize some radiation would be reflected back out through the glazing so what's the trade off if any?  

Appreciate the members views on this and especially info from anyone who's tried reflective insulation with DP collectors.

Thanks, GOM 


Posts: 564
Reply with quote  #2 
Here are my opinions for a tube-type collector like the downspout:
- Use the maximum number of tubes that will fit in the collector width.  Some people think that gaps are a benefit, but since the tubes are what is heating the air I want as many as I can fit.
- Use the thinner aluminum downspouts if you have a choice between two grades.
- With no (or very little) spacing between the tubes, only paint the top black.  The bottom, if painted black, will radiate more heat out toward the insulated back than a lighter color.  You want the tubes to heat the air, not radiate heat toward the insulation.
- With no (or very little) spacing between the tubes, leave the shiny foil on the polyiso below the tubes unpainted.  No sun is hitting it, so no need to paint it black.  With downspouts I would probably use the white ones or try to get the paint off the back.
- For the small area on the sides above the tubes, it probably doesn't matter much if they are black or left shiny (mine are black, mainly because I spray painted the tubes in the finished box).
- For a rectangular downspout (like 2x3 inch) have the 3 inch side toward the sun (minimize the amount on area not hit by the sun).
- Put the tubes in parallel with a plenum at each end (no snaking back and forth through a single tube).

Kevin H

Posts: 75
Reply with quote  #3 
KevenH,  Thanks very, very much for your informative response!  GOM 

Posts: 1
Reply with quote  #4 
What about pop riveting the downspout ends together to form a semi-solid block in order to minimize the length of the openings between the collector area and the plenum areas?

  • Where to pop rivet them (how deep into the downspout ends)? In the plenum free area (no concerns about leakage around the pop rivets)? In the collector-plenum wall area (reinforce the pop rivet area)? Somewhere else. Where? Why?
  • What material would be the best sealing material to put on the downspout-to-downspout faces? High-temp silicone?
  • What kind of gasket material would be good to add in order to air seal the pop rivet-to-inner downspout faces?
  • In the collector area, could the downspout block butt right up against the back polyiso insulation in order to reinforce the back wall? Can the downspout block butt right up against the side polyiso?
  • What is the optimal distance between the front face of the downspouts and the glazing?
  • Which type of single-layer polycarbonate material is better for solar insolation, flat or wavy?

Posts: 137
Reply with quote  #5 
As far as your question here, if seems to me that painting the inside of the box will help to increase the temperature in the whole collector, thereby making the downspouts easier to heat from the direct sun that hits them.  Any reflection of light from the shiny lining of the box will also reflect heat, so if the lining of the box is more visible from outside the glazing, it is also reflecting heat back out of the collector. I have not done any testing to come to this conclusion other than noting the temp of my empty box increasing after I painted it black.  Common sense is not very scientific ;-)
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