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58impala

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Reply with quote  #1 

I have a 6 ft by 16 ft solar collector mounted on a screen porch roof.  I chose to use 4 inch steel stove pipe as my collection medium which equals about 100 lbs of steel within the collector.  Heat up is ok but slower I think due the steel mass; however, the collector runs about 30 minutes after the direct sunlight is gone.  I obtain 120 F in winter (January) at 150 cfm.  Question:  Has anyone else used steel pipe in their solar collector?

SolarInterested

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Reply with quote  #2 
Welcome to our forum. I don't remember anyone using steel stove pipe but downspouts, pop cans and flexible ducting have been used.

Mike

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gbwillson

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Reply with quote  #3 
Welcome 58impala-

I know people have used stove pipe, especially if it was free. But as a general rule, having any extra mass inside a collector isn't a good idea. As you noted, it makes the collector slow to heat up and slower to cool down. So in most cases, the extra mass offsets any advantage. There is a distinct disadvantage to using heavier gauge tubes or pipes. With thin metal, the heat from the sun moves rather easily to the back, or shaded side of the tube shape. With thinner metal such as pop cans or aluminum downspouts, the heat from the sun can surround the air passing through. Thin metals allow the collector to heat up instantly once the sun comes out from behind a cloud. The front side of a tube will most certainly be hotter, but back side of thick tubes may warm very little compared to the thin metal tubes. 

All that being said, I certainly wouldn't change out an existing absorber. Based on your numbers above your output temps are a tad high, but that could simply be personally preference. You could capture more BTU's of you were to increase the CFM to bring down the output temp into the 95-100˚F range. But again, your output very good, and is giving you a lot of cheap heat.

If you are wanting to capture or store heat, you can store it as long as the storage is located INSIDE the home. A storage medium such as sand, water, spare bedroom, or even an insulated basement can all be used to store heat that is slowly given back to the rest of the house late into the evening and overnight.

Greg in MN


58impala

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Reply with quote  #4 
Thank Greg...your right i had extra 4" steel tubing from another job.  I added to the bulk and built this. We are in SC and the collector approaches 140 F (output) in the spring...crazy hot in the summer if i turn it on.  I was going to heat hot water via back flow thru a gas hot water heater with no gas piped to the unit.  

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gbwillson

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Reply with quote  #5 
58impala-

After seeing the photo of your collector while it was being built I'm very surprised you get 150CFM out of the exhaust. So is that is one long tube running back and forth? That looks like it would be over 100' of tubing zig-zagging back and forth inside the collector! If you can, try and get the output temperature closer to 100˚F. It will give you a lot more BTU's. At 140˚F, you are at the point where not only are you wasting heat, but you begin to do damage to the collector. Do you have any more photos you could post while the collar was being built, as well as a finished photo on top of your porch?

Greg in MN
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