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Posts: 2,311
Reply with quote  #81 

I couldn't see the screen covers. A couple of years ago I had spiders make a summer vacation home in several of the ribs of the twinwall poly. I tried everything to clear out the web residue, to no avail. At least it now matches the water stains from the previous year when I stored my ZP upside-down where the rib openings were not sealed and water filled several ribs. Ya live and learn...

"Has anyone 'scientifically' determined what the minimum absorber length is for useful heat gain?  At what air flow?  I'm speculating about a one-piece, one-way 4x10 collector, probably ZP, but maybe 3 inch corrugated aluminum duct.  If I went ZP, I would try aluminum screen frames with taped spline (was going to do that this time, but I went cheap).  In a 4x10, the screen absorber would be about 8 ft. long.  Is that long enough?  10 feet is about as long as will fit inside my work area.  Still want to test the current model though."

I've never thought about a minimum effective absorber size, but think any collector with an airflow path less than 4' is pretty much ineffectual for practical use. Testing so small, on the other hand, could help save build costs while working out a new idea. My thought is that the longer the path the air travels while being affected by the absorber, the better. Having several turns will increase the heat gain, but dramatically slow the flow of air, thus reducing efficiency. The proper, or optimal airflow for a given collector will depend on the size and design of a given collector. That is why I like to have more airflow available if needed. Your new 4x8 ZP should require a bit less air than the upcoming 4x10 due to the 25% increase in collector size, even though they share the same design. Take your new 4x8 ZP and compare it to last years 2x16 ZP and if the absorber gap was the same, I would bet the results would be similar as long as the airflow was the same. Is there any way you could test the two ZP's against each other? A test between the two might give you a clue as to the configuration of your upcoming 4x10 collector. I'd make sure to have enough depth so that inside spacing wasn't an issue. If I could only build as large as 4x10, I'd lean towards your current design with the return so as to take advantage of the entry/exit being at the same end.  But my collectors have to extend from the East or West sides of the house.

Greg in MN


Posts: 261
Reply with quote  #82 

I haven't committed to the 4x10 size yet, it's just one idea.  Also considering 4x12 (need to rearrange the back room a little bit), 3x12, even a one piece 2x16.

I'm juggling several considerations:

1) I wasn't planning to repeat the U-turn design for a screen collector.  The non-structural (1" polyiso) center divider caused a lot of grief, and it was tricky to get everything in plane at the glazing and to seal everything.  I made the divider slightly taller than the frame to ensure that it wouldn't be too low.  It ended up slightly high in some areas and flush in others, which worked out OK.  It would be better to be able to fasten glazing hardware to the forward edge of the divider, but a structural divider would probably need a structural back sheet as well.  Which brings up -

2) Weight.  Need to keep it light enough to pick up and move around during most of the construction, since I don't have anywhere to leave it laid out flat.  That's why I'm always on the lookout for 'ultralight' methods and materials.  I think the aluminum screen frames are the way to go in that regard.

3) Performance vs. buildability.  I think this is what's pushing me toward a one-way design, at least for a screen absorber.  The one way 2x16 was easier to build, except for the joint in the center, which was about as tricky to figure out as the U-turn.  The end slots probably reduced performance some.  One piece/one way seems to be the answer, although I suspect a 4x10 one way might not be best shape for performance.  4x12 would be better, although I had trouble at times even with the 4x8 carrying it around while still seeing where I was going.  That's got me thinking about a 3x12, which, at 36 ft. square, represents a 12.5 % increase in size over both the 4x8 and 2x16.  3 ft. wide would be easier to get a grip on than 4 ft.  I can locally purchase a 4x12 sheet of twin-wall, so there would be some waste with a 3x12.  Not so much for the polyiso, since the scraps are usable for the frame edges.

4) Still looking for ways to build in two sections, but join them into one at some point late in the build.  Solar Dan showed the way on track frame design with his experimental 2x16, although I would use stud for the splice member rather than track ripped in half.  I also decided it is best to do all your splicing in a straight run rather than at the corners.  I think you get the straightest, flattest, track frame by having simple folded corners made from a single piece.  Got some ideas to explore.  A 2x16 makes good use of available materials.  The one thing I don't have a spot for right now is a 4x16 or 3x16 collector.

5) Back to the U-turn concept, I really like Kevin's U-turn tube design - you don't even need a center divider except at the in/out plenums, and you aren't applying pressure on the glazing.  Another bonus is the paint-free air flow path, which he identified as a priority.  Trouble with a tube build is you need a deeper frame, which ties back to the weight issue.  Still thinking about it though, either as a one-way or U-turn.

6) I'm considering the merits of a polyiso back sheet with no solid facing.  Aluminum flashing is pretty light, and I've used it a couple times with good results (3 laps of 14" roll would work well for a 3 ft tall collector), but I'm wondering about gluing some screen to the back side, then painting it.  Might be just strong enough, and the screen would reduce damage from accidental bumps.

Sorry, I didn't mean to ramble, but I'm looking for a lightweight, economical, easy to build, high-performance design (aren't we all?).  I usually end up swiping ideas from you guys and adapting to my circumstances.  I still need to test the 4x8 U-turn more before passing final judgement.  I should be able to swap ducts between the 4x8 and 2x16 when the time comes.  Planning also to build a polyiso square-tube duct, 8" x 8", to replace most of the long input duct on the 2x16.  Probably do that first.  The square duct will stay in place, and I will just have to fit short sections of flex at each end.

Greg - looking forward to seeing your proposed square-tube frame design.  Life beyond stud-track, as it were.


Posts: 261
Reply with quote  #83 
I put some 1/8 in. felt strips between the glazing and cross-bars.  They are just held by friction.  I had to end the strips about 6 inches from the top and bottom, because I couldn't quite force the strips under the EMT all the way to the edge.  I cut off an additional 6 inches (total) for the strip over the plenum; it was a little tighter than the other two.

The strips are slightly loose near the center when the fan is off, but are tight all along when the fan is running (just about all the time with ambient temps in the mid 90's).  Here's what it looks like.  Kind of kills the 'stealth' effect for the cross bars, but it keeps everything down tight, so that's OK:

Felt strip, 070718.jpg 

Now that I can step back, I am reconsidering the possibility of building another U-turn collector to take advantage of the short duct runs.  I analyzed the various problems I had last time, and decided most of them were of my own making, and could be corrected with a little better planning:  I would figure in attachment for the cross-braces earlier in the build, beef up the center divider, use spray paint inside the first time, use aluminum screen frames (was going to do that originally last time) - generally use what I learned by trial and error and improve next time.  Or maybe build a one-way after all, haven't decided anything yet.

Been running the fan for a few days now, I don't smell anything really, so I am hopeful this unit will be OK.


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Posts: 2,311
Reply with quote  #84 
Do you still have the 2x16 ZP? I'd love to see a comparison between the two. The two units still have roughly the same distance traveled through the collector. If it ends up with both having roughly the same output, great! At least it would show that two completely different shaped collectors share a similar result. If the results are different, you could explore the differences and what configuration may have the advantage over the other. Someone might have limited space for one configuration over the other.

Greg in MN


Posts: 261
Reply with quote  #85 
Greg - Yes, the 2x16 is still in place.  I reconnected the snap disc wiring when I installed the 4x8, so I can select which collector controls the fan by throwing a switch inside:

2 collectors, 070818.jpg 

The only bottleneck is the air ducts - I can only connect one collector at a time with the present setup.  Also, I removed the long intake duct (both ducts actually) from the 2x16 because the black plastic that was wrapped around the outside had started to shred (picture shows it when new):

Ducts, covered, 022617.jpg 

This arrangement is not very maintenance-friendly; I had to take the strap hangers apart and remove them to get the duct out (the duct itself looked like new, thanks to the plastic), and then had to fish the duct out carefully so as not to rip the jacket.  This is the duct I want to replace most of with an 8-inch square polyiso tube with duct fittings on the ends.

Part of what I was thinking with the 4x8 was to mimic as closely as possible the internal flow path of the 2x16 and compare the two, like you are suggesting.  I sort of cheated on the 4x8 by enlarging the screen gap to 1 3/8 inch (it's about 1 1/8 on the 2x16), and the screen frames are wood (aluminum screen frame on the 2x16), but I still want to comparison-test them.  I would like to upgrade the 2x16 intake duct first if I can (oops, another variable), but if not I will re-install the 2x16 ducts with new plastic.  Either way, I should be able to swap collectors quickly.

A worthwhile long-term upgrade would be to upgrade the building duct system to run both collectors together, either 2 fans and 2 pairs of ducts, or else enlarge the common portion to 8 inch.  Problem there is the studs and truss tails don't line up along most of the wall.  I just had room in one place for the two 6 inch ducts to fit between the top of the brick cladding and bottom of the double top plate.  The trusses sit right on the plate (no raised heel trusses for this old house).  There are headers over both adjacent windows.  I would probably have to convert to square sheet-metal duct to go through the wall.  It's nasty up there, I just haven't felt motivated yet to rip into it again.


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Posts: 2,311
Reply with quote  #86 
Maybe you could connect the 2nd fan and test the two units with only partial ducting. Even if the exhaust ducts are not connected to the building, you should still be able to make a fair comparison. The most difficult part is matching the output airflow between the two collectors, especially if you use a rheostat to adjust fan CFM. Electronic speed controls are far easier to adjust. I've about lost my mind a few times trying to make the two test units within 1/10th of a MPH. It's VERY touchy!!! But matching the output CFM and comparing the output temps is the only fair way to compare efficiency. 

Greg in MN

Posts: 261
Reply with quote  #87 
I don't actually have a second fan; nor do I have a fan speed control device, at least not a power-management device.  The speed controller recommended and sold by Suncourt for my fan has received very negative reviews, with many claiming that it ruined the fan motor.  I'm not too eager to purchase one of those, although they are not expensive and may be worth it as an experiment.  A second fan may be in the cards however; I can see how that would make testing a lot easier, and save the time spent (and resulting uncertainty) in disconnecting and reconnecting ducts.  There are surely other speed controllers that would work too.

What I do have is this cheap-o manual damper, which could be installed in a straight section of duct:

Damper, 071018.jpg 

If I had a second fan with one of these in each circuit, I wonder if I could match air speed accurately enough?  I have not installed or tested this damper.


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Posts: 2,311
Reply with quote  #88 
I think the damper might be easier than a rheostat in that it would be less touchy. I set my anemometer to MPH since it measured only one digit after the decimal point. MPH allowed me to "sync" the two units CFM if the single digit after the decimal stayed steady for both test units. In my case, I had two identical fans, and once test unit #1 had settled to a constant speed reading, I adjusted test unit #2 to match CFM output. I let both units run for roughly 30 seconds and did a final speed check before letting the two units run for several minutes. At that point I measured the temp output. Windy days were the worst by FAR!!! A gust of wind hitting the glazing would dramatically change the fan speed. 

A rheostat fan speed control can be hard on a fan especially if the fan speed is set very low. Both the fan and the rheostat heat up due to the resistance. Electronic speed controls cost a little more but handle low speeds better. But try the damper before you buy and see if you can get the CFM to settle in at a desired speed.

Greg in MN

Posts: 261
Reply with quote  #89 
OK, I will plan to try the mechanical damper to adjust air flow while testing.

Sorry for the slow response; I've been working on some (non-solar) 'home improvement' projects lately.

Posts: 261
Reply with quote  #90 
While it's still hot summer, I've been making some small upgrades to the mounting frames and outside duct routing for both the 4x8 and 2x16 collectors.  These will hopefully ease testing, particularly switching between the two.

On the 4x8 frame, I removed the diagonal braces, because they were always crowding the intake duct.  Replaced the single long brace with two shorter ones, which opened up the middle.  Before and after:

1) Old frame brace, East, 080618.jpg 

2) New brace, East, 080618.jpg 

The interference was at the other (West) end, but this end was easier to photograph.  (Note: that ugly duct in back is temporary, I've been 'blowing out' the 4x8.)

On the 2x16, I removed the strap intake duct hangers and support rail (see July 8 post).  Instead I installed some cross bars for the intake duct to rest on.  The new cross pieces are the 1/2 EMT that sit about level with the bottom of the duct port:

5) New 2x16 duct shelf, 080718.jpg 

I never liked the long piece of flex duct, so I replaced it with a 15 ft. long section of 6 inch rigid duct with fiberglass insulation sleeve.  Here is the label for the insulation (R6):

6) Duct sleeve label, 081018.jpg 

It only comes in 5 ft. lengths (likewise the duct), so there are 3 sections of each to make the whole assembly.  Here are a few shots of the construction sequence:

One 5 ft. section:

8) Sleeve on duct 02, 081018.jpg 

Joining the duct sections:

9) Duct sections screwed, 081018.jpg 

10) Duct sections taped, 081018.jpg 

Insulation sleeves taped:

11) Duct with insulation, complete, 081018.jpg 

The whole 15 ft. piece is pretty solid; I can pick it up in the middle and the ends don't sag at all.

Finally, I wrapped the insulated duct with black plastic, held on with some plastic hanger-strap 'belts'.  I didn't want to tape the plastic to the insulation, since I know from experience that the black plastic degrades a lot faster than the protected mylar duct underneath.  The belts are secured with small tie-wraps.  In the future, it will be easy to cut the tie wraps and pull the plastic off for replacement.

15 foot stogie:

14) Duct wrapped with plastic, 081118.jpg 

Here it is set in place behind the collector:

16) Duct on frame 01, 081118.jpg 

So now this piece becomes semi-permanent 'infrastructure', which just needs short flex sections at each end (still n4eed to lash it down loosely to the cross bars with some nylon cord first).  The flex at the collector input can probably stay connected, with just the section at the other end needing to come out when swapping over to the 4x8.  The output duct is pretty short already so I didn't change anything there.

As a side note, I had previously considered making the intake duct out of a long square tube out of polyiso, but when I priced out all the necessary materials it was quite a bit more expensive (and more work) than replacing with rigid.  The black plastic will still need periodic replacement but it will be a lot easier now.

I'm considering one further mod:  I got a couple 6 inch wye adapters and caps:

13) Wye and cap, 081018.jpg 

Wondering if I can install these on the in/out ports of the building plenum, with one branch assigned to each collector.  This will provide quicker, easier access to the flex ducts going to each collector.  I can just cap the branch not in use during comparison testing.  I may do some tests to see if or how much this disrupts air flow.

I also wonder about running both collectors together.  I'm pretty sure the airflow from my existing fan and common ducts will be less than optimum, but having a whole collector sitting there idle doesn't seem too optimum either.  Have to experiment with this.  There may be no performance advantage, and the collectors may run too hot.  Assuming there are no problems, I could live with reduced efficiency if the total heat produced is more.  I know - what I really need to do is replace/upgrade the fan and duct system or add a second one; just don't know when I will get to that.

First, I'm going to re-test air flow on the 2x16.  Curious what replacing 15 ft of flex with rigid will show.

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