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Posts: 3
Reply with quote  #1 
I have been wanting to build a solar hot air collector for my 24'x36' shop in midcoast Maine for years now and finally have some time now being mostly retired.  Shop has 6" walls with FG insulation.  I was a builder so i am familiar with tools and materials.  I am limited to about 48" high due to the south wall having lots of windows which already help in the winter but am hoping to get more heat on sunny days.  I have settled on a double screen collector (unless someone can convince me otherwise) and the outlet(s) will have to be at spaces between the windows.  The tempered glass that I have is 46"x76".  If I use all 4 pieces of glass, my outlets would have to be a few feet in from each end.  I am assuming that for a 100 sf collector, I will want 2-150 cfm fans.  The inlets would need to be 12" above the bottom of the collector in order to avoid having to cut through the rim joist and crawl around in the dirt in the crawl space.  I'm 69 years old so would rather avoid this if possible.  I realize that inlets would ideally be right at or near the bottom.   Some  specific questions I have are 1. whether there is an  ideal location for inlets and outlets on a long width and short height collector?, and 2. is a 2x6 box deep enough or should I use 2x8?  I would also love to have any comments and or advice or links on how to proceed.  I have looked at a lot of information on the internet but have not found enough to guide me through this.   In addition, I have another piece of glass which is pretty close in size that I could use to extend the collector another 77", though I am thinking that 25' long should be enough.   Thanks for taking the time to read this.  Larry


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Posts: 3,058
Reply with quote  #2 
Your requirements bring to mind TWO collectors, which will be easier to build and install than a single collector 25' long.  The center-feed style also reduces backpressure and improves airflow, and meets your requirements of the outlets being in a few feet from the ends.  As to what type of screen collector, that's for others to recommend. collector.png 

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

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Reply with quote  #3 

I'd say welcome Larry, but it looks like you have been lurking in the shadows for some time. 

You have a great grasp of what you are looking to build. A 100sf collector will give you a lot of cheap heat for many years to come. But you mentioned that if you want to use all 4 panes of glass, you have to have the inlets a few feet from each end. Is there a structural or equipment issue that would block building access at the ends of a 25' collector? If so, how far apart are these available access points? A photo or detailed drawing would help.

If this is to be a single long collector, with the intake at one end and the exhaust at the other, 2-150 cfm fans will not be enough. The actual CFM will only be a fraction of the free air rating of a given fan. A collector of this size and shape will collect far too much heat for such small fans to handle. But this is a good problem to have. Fans are cheap, and can often be found for free. If you build this as a ZeroPass configuration, which is a two screen but with the air flow passing between the two screen layers instead of through, you will need somewhere in the neighborhood of 400+CFM output airflow. So the fan or fans would need to be rated higher than 400 cfm total. The general guide of 3 cfm of airflow is a bit outdated, especially for a collector such as yours. Higher airflow is needed to keep the output air temps lower and more efficient. If the output temps are too high, you are simply wasting heat.

Many, if not most in one end and out the other collectors won't have much of an issue with inlet placement as there are usually workarounds, if needed. Most have them midline, so on your 4' high collector your inlets could be 2' from the top or bottom your collector. And the depth on a wall mounted unit could easily be either 6" or 8". A deeper unit gives you more room, but takes slightly longer to warm up. But it also keeps the warmed air farther from the cold glazing. 

There are several very nice collectors that are similar in size and shape to what you are looking at building. Here are some examples for you to check out:






Greg in Minneapolis

Posts: 3
Reply with quote  #4 
I am attaching a pic of the shop.  My reasoning was that the outlet(s) would have to be between windows, but the ZP would make more sense since I could put the in and out in the center.  I have a bench along the entire wall but all of the drawers are in the middle section with open shelving towards the ends.    I have been browsing through the links you sent and will start to try to formulate a plan based on ZP.  One question that comes to mind right now is about whether to strip off the cedar shingles.  The sheathing is 1x8 pine boards, so I probably would have to put separate back on the collector anyway.  I'm sure I will have more questions as I go along.  Thanks.



Posts: 149
Reply with quote  #5 
I understand your dilemma about the siding. It is beautiful and would be a real commitment to strip it off for a collector, but I found it a good idea for my water heater collector, but that was just cheap vinyl siding.  It gave me a good flat surface to build out from though.  it also made it easier to flash and seal the edges.  

As far as the 2x6 or 8 frame options, I am building a zero-pass collector with a 2x6 frame and have found that it is plenty of space for my design.  I built everything inside the frame (dadoed the back to inset a plywood back and glazing for the front) , 1 inch of ISO, 3/4 inch screen frames with a 1 1/2 inch gap and still have and extra 3/4 inch space between the screen frame and glazing.  If i were to have used 2x8, I think the extra depth might cause more shading problems early in the day and in the afternoon when I lose my good sun. 

This looks like a great opportunity.  I think you might be surprised how much heat you get from it.  I agree with Greg that you might want to consider more air movement, either with larger ducts and fans or with multiple ducts and fans.  Two 12 foot collectors might be easier to design and build, but I gather from your analysis that you are up to the construction challenges of this project.  Good luck!


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Posts: 2,461
Reply with quote  #6 
A single, large, long ZP collector will also give you considerably more BTU's than the total of two smaller collectors, even if the two smaller collectors have the same square footage as the larger ZP. The single unit allows more time in the collector for the air to heat up, which in turn means you need a lot of airflow to keep the output temps under control for maximizing BTU output and efficiency.

Only reason to split might be if you are working by yourself, and need help moving or mounting or it doesn't fit in the workshop. In which can you might consider splitting the unit for most of the build and connecting the two sections for final assembly.

Greg in MN
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