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Posts: 76
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: 565
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: 76
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: 149
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 ;-)

Posts: 31
Reply with quote  #6 
What is a "drainpipe" collector design? Will someone post a picture or drawing of what this looks like? Thanks

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Posts: 3,053
Reply with quote  #7 
I believe it's another name for the aluminum downspout collector:


Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay

Posts: 43
Reply with quote  #8 
I am new here, but I have been a project guy all my life. I want to build a hot air collector. I was going to build a drain pipe collector but if the double screen is almost as efficient it would be cheaper, faster and lighter. My question is, can the intake & outtake of air be side by side or do I need to channel the intake air to the end of the collector? My collector would be about 4' x 25', maybe 4' x 14' is more practical considering my age 85 & installing it on a roof using a drain pipe similar to Scott's.   

I am still tossing ideas in my head. Using aluminum 6-inch facia for the outside framing & inserting 1-inch insulation board inside it & maybe 2-inch insulation board on the bottom, no plywood, I could keep this very light but not as strong. The aluminum facial has a top lip of 1 inch that would cover the top of one-inch foam fastened to the inside of the outside facia frame & that lip would hold sheet screws to hold down the glazing. Then using the aluminum fascia on the 2-inch bottom foam to help cradle it, you could double the strength of the outside aluminum support frame. I'm kinda getting excited about this.


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Posts: 2,461
Reply with quote  #9 

Welcome John!

Placing such a large collector on yours on a roof would certainly make weight one of the top considerations. Building the collector box out of foam, with a few reinforcement points would certainly be light. A few things to consider are that on very rare occasion, critters such as birds or squirrels have done some damage to exposed foam. Although I doubt they are eating it, they may be using it for nesting material. But all of the collectors that were damaged were on ground level. 

Another consideration with a box made of insulation is the wind. Assuming the wind is out of the North and the collector faces south, the winter winds can put a lot of pressure on the large back surface. If the back surface isn’t stiff and strong enough, it could flex enough to push into the inside of the collector box and disrupt the airflow. A thicker or denser back insulation should work to prevent this. XPS insulation sheets comes in high density sheets which are double that of the standard sheet, which would reinforce the back of the collector.

To reduce weight while lifting collector up on the roof, you could also consider making the unit in sections that are secured together once placed on the roof stand. For example, by dividing the collector into say, 4x8 sections, the struggle to move the entire unit is greatly reduced. 

As far as the type of hot air collector, using screen as your absorber is likely the cheapest, lightest and most effective builds. The way you configure the screen material inside the collector can vary depending on your preference. Some configurations, such as the ZeroPass, needs a LOT of CFM flowing through them as they collect the most heat. And to capture this heat effectively requires very good airflow. On a unit the size you are considering, 500 CFM or more would need to flow through your system. Otherwise the collector could run too warm inside and be less efficient due to heat loss.

In any case, it sounds like you have a very good grasp of the concept. Don’t hesitate to ask any questions you may have. Depending on where you live, the need for heating can be days away. Winter is Coming!


Greg in Minneapolis


Posts: 43
Reply with quote  #10 
Thank you so much Greg for your consideration of my questions. This took time on your part. I will address each one of your concerns. They make a lot of sense to me. In 1975 I bought a small farm with a woods. I set my house back 100 feet from the edge of an open field & dead center facing south in case I wanted to do some solar projects in the future. So as winter comes on the leaves will drop & allow the sun will come in nearly full force. As the build take place I will keep you & the site informed.
Thank you so much.
John of Indiana.


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