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Posts: 303
Reply with quote  #61 

A couple suggestions for your design:

1) The in/out plenums could be a bit narrower.  You're giving up a lot of absorber area with 3 ft. wide plenums.

2) The in/out ports could be lower, more toward center-height.  The way it's shown, you might get reduced air flow across the bottom, particularly with a downspout absorber.  These are notorious for unequal air flow eevn on a good day.  I've seen shaped guideways built into the plenum area, or curved sections for the outer tubes to bring the ends closer to the center.  Kevin made some custom-tuned individual dampers for higher-flowing tubes in his collectors (tuned with an anemometer) to equalize flow, which worked well.  His project thread is full of good ideas.

Are you planning to install a thermal switch (aka snap-disc) in the output plenum?  Most collectors use these to control the fan, or else a sensor which feeds temp data to some sort of smart controller that turns the fan on and off.  You should leave room in there for something, and provision for getting the data out.  On my very first collector, I remembered this only after I had the glazing on.  I had to install the switch the hard way, blind, through the back of the output port.  Hard lesson that.


Posts: 43
Reply with quote  #62 

The plenums are wider than I really want but there are compromises I have to make so the collector can utilize my two upstairs windows for air in and air out. That is the reason the 6 " air duck ports are not centered & also the reason the plenums are so wide. I know I have some problems in the plenum areas. I will take your suggestions to find a way to route the air better in the plenums. The aluminum siding gets really hot & so there will be some good absorption in the plenums. As I build things ideas come to me as I see the work in progress. Yes, I will be adding a snap-disc & will place it in the collector as I build. I may put in a 24-volt system. In the plenum areas, I could add types of collector fins to help the absorption of heat in the plenums but then you have to look out for shadows falling on the aluminum that diminishes the heated area of the siding. As I look at collectors I feel that shadows are not considered enough. When I look at an aluminum can collectors it becomes apparent that only the topmost direct surface is getting the most sun while the curve of the can falls deeper into shadows. So to me that makes the can collector not very efficient. 

Thanks. John in IN

Posts: 43
Reply with quote  #63 
I set here brainstorming. Please critic this idea. This is mass-producing aluminum screening for a hot air collector that has no heavy complicated framework that is no sag & stacked side by side & end to end for the size you need. Very lightweight is what I am looking for. 
John in IN
so simple.jpg


Posts: 303
Reply with quote  #64 
I did some more testing on the 4x11 collector.  Ambient conditions were: sunny, 60 degrees F, light NE breeze.

I re-connected the outside output duct to send the output into the back room.  It goes to my 3 ft. rigid 'test duct' with the cross-vanes in the end.  This is the same configuration I had during the earlier air flow tests.

I tested temperature increase at 1215 pm MDT.  This is 45 minutes before solar noon, but 15 minutes after the time when my collector is square to the sun.  (The wall faces slightly southeast.)  I taped cardboard covers to the glazing to shade the in/out ports.  Here is a photo showing one of the covers and the shadow at the edge of the collector.  I tried to get 'square' with the edge to take the picture:

Port cover + shadow, 101219.jpg 

I measured in/out temperatures the Craftsman thermocouple probe, stuck into the middle of the ports through small holes in the duct stubs.  I had to slide the flex ducts back about an inch to expose the holes.  I covered them afterward with small pieces of foil tape:

Temp probe access hole, 101219.jpg 

The test results:  Input temperature: 65 F.  Output temperature: 109 F.  Temp increase = 44 degrees F.  This was a pleasant surprise; I was expecting around 40 degrees tops.

Since I have been airing out the collector, the angle pieces around the glazing have settled into alignment and the whole thing just looks a little tighter than when I first installed it.  I repeated the airflow test with same setup as before.  Airflow was slightly better: 207.3 CFM (12.0 MPH).

Putting it all together, with a 0.9054 correction factor for my 5500 ft. elevation, I calculated 8258.3 BTU/hour.

          [207.3 CFM  x  0.9054  x  44 degrees  =  8258.3 BTU/hour]

I guess I'm OK with this.  It represents a 36.2% increase over the 4x8 unit for a 37.5% increase in size.  It seems reasonable at least.  Most of the improvement (over the 4x8) was due to the greater temp increase (29.4% improvement), while airflow only increased by 5.2%.  This surprised me; I thought the biggest gain would be in airflow, since the 4x11 has a one-way internal architecture.

I haven't tried adjusting airflow down with a damper or anything, but I think 44 degree temp rise is in the ballpark, so I may leave it alone and just enjoy the heat.  Also - I haven't connected the heater to the front room yet.  That may affect output temperature, since the input air should be a bit warmer.  Waiting for it to get a bit colder, probably another few weeks.  We went down to low teens on Thursday night, but this coming week it's going back up to 70's.  Love that Colorado weather!

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