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Posts: 2,223
Reply with quote  #21 

I like what you experimenting with the new ZP build. Like you, I don't want another two part collector. That's why I had to curtail my latest ZP. As a single 4x16 foot unit, it didn't fit up the basement stairs from the workshop. Had the fall weather stayed mild a couple of weeks longer, I might have been able to finish it up in the garage. But it was not to be. And like your ZP I had to make the back screen frame a bit thinner as the ¾" frame partially blocked the narrow 1" gap in the connecting wall gap. The stud track framing simply didn't have the depth, especially since I also had a ½" plywood back. Hence, I couldn't widen the connecting gap. I won't be using stud track in the future. In many locations where the stud track was reinforced with wood, including the entire perimeter of the collector, rust formed where the metal and wood met. Not good...

I had screens at both ends of my ZP and I see no problem with them being in the plenums as long as no screens impede the path of air. After all, the air in both of the plenums swirls like crazy, so why not add the extra screen area to better transfer heat to the air. 

I do think you could get away with pulling the air through the collector instead of pulling. The narrow 2' wide channels won't have the problem the 4' wide units do, where the air that is drawn through the channel doesn't spread out the full width. I did play with a simple deflector, attached to the intake manifold, that deflects the draw air to the sides instead of straight down the middle of the screen gap. In any case, your turn is likely going to need a little bit of help if you pull the air so the air doesn't bunch up on the inside corner of the turn. 

It might be worth experimenting with the airflow pushing versus pulling before you do a final sealing of the unit. I don't think it will take too much more CFM input than the 2x16 unit. It will be very interesting to see how the two unit compare, both flow and output temp-wise. 

Greg in MN


Posts: 201
Reply with quote  #22 
Greg,  When I built the 2x16 I reconfigured the ductwork so the fan pushed air into the intake (it was the other way originally on the slanted 2-screen).  I'll probably leave it pushing, at least at first.

I think I may be at the end of the road with stud-track frames too.  They really looked like a good answer, and they are for about 90% of what you need to do . It's that last 10% (fixed, limited depth mostly) that trips you up somewhere.

Since I'm committed already on the new 4x8 build, I'm looking for ways to make at least one of the screen frames thinner.  I had limited success with the generic aluminum screen frames; they were really light and low-profile, but I couldn't get the screen as tight as I wanted, and I wasn't sure I could double the rear screen (I ended up leaving it single).  Also, the framing seem to be a seasonal item, at least around here.

I came up with a couple ideas: (1) Make the rear screen frame out of a piece of 1/2 plywood, with most of it cut out.  Little worried about out-gassing where it's cut, although there may be 'greener' varieties.  I could paint it completely with primer before the black coat; that might seal it enough.  Also, I don't know if it would be stiff enough.  (2) Bring the front screen frame out a little bit past the track frame.  The felt weatherstripping I purchased to re-do the gasket between the 2 sections on the 2x16 is quite firm.  I doubt I can squeeze the 1/4 inch felt much more than 1/16 inch with my bare hands.  It worked pretty well where I used it; it made a good seal.  I covered it with foil tape, since it was outside the collector proper, but that may not be necessary for a glazing gasket.  If I glued strips around the front edge of the frame, where the glazing rests, the screen frame could stick out maybe 3/16 inch past the track and still be flush with the felt gasket when it was slightly compressed.  I figure I need 1/4 inch more frame depth to have a 1 3/8 inch screen gap with 1x2 screen frames (I don't have a plywood back), so I could get the gap out to 1 5/16 - close.

Looking down the road, I'm considering frameless, basically a polyiso box with aluminum flashing skin.  I could still have plywood strips around the sides, between the polyiso and the flashing, as anchor points for screens and/or angle glazing hold-downs.  The plywood could even go inside the polyiso.

It might even work for a 2x16 (or 4x16) - with a little extra time and money, you could make a 2-layer polyiso back, two 2x8 sheets end to end, backed by 3 sheets of different lengths, say 5+6+5.  Have to draw a picture.  Seems you need to do something to avoid a 'hinge in the middle' if everything is built from 8 ft. long material.

Sorry to hear you had rust problems with the track frame and plywood.  have to keep an eye on mine.  How did your design work out with the square tubing and snap-together corners?  Or is that the one that got put on hold for winter?

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Posts: 2,223
Reply with quote  #23 

Yup-The square tubing frame ZP was put on hold. As I putting up last years ZP I noticed it needed some help. Not only was the unit rusting alone the edges, but I needed to open the unit up in order to add a braces to keep the glazing panel hold downs from ripping out from the air pressure. Last year I had to make do with external braces. Unfortunately, as soon as I opened the unit up in the garage, the weather turned cold enough to prevent some of the needed repairs. But the time I finished repairs, the weather has turned cold enough that I could no longer finish building the new ZP unit outside, and it would be too large to fit up the stairs from my workshop. 

I think you may want to reconsider using screen frames, just not the ones you find in the big box stores. They are thin metal and narrow. Here is a site that does nothing but screens. They carry thick and wide frame pieces, in lengths up to 8' long, so there is little waste, which makes the prices as good or better than the cheap frames sold at the big box stores. They carry metal and plastic corners, both inside and outside mount. You can also buy frames either 5/16" or 7/16" thick, either of which is far thinner than you could build a wood frame. And a thicker, wider frame should be plenty strong enough to allow for tight screens. You might also consider making screen frames smaller. So instead of 7' or 8' long screen frames, make two smaller frames. It takes more corners, but you should be able to make the frames tighter than larger screen frames. Krautman made large frames, but added a mutton as well as sheet metal triangle reinforcements at the corners and muttons. Seemed to work pretty good, but with the thin framing pieces he used, the sure felt like flimsy, but there was no sag.


And if you are going to try out my Chicken Wire screen holder, you may not need a support all the way around the perimeter. And if you do need full perimeter support, I'd rip board stock before plywood as there would be no glues to worry about.

One big advantage you have with any lightweight build is your collectors are permanently mounted. Moving and storing the collectors certainly takes its toll as the collectors get banged and tweaked each time I have to move them. And being they are out of the prevailing winter winds, the backs don't need bracing like my units that sit exposed to the winter winds.

Greg in MN

Posts: 201
Reply with quote  #24 
You've convinced me to take another look at aluminum screen frames.  Quality Screen certainly has everything you need, even black aluminum screen (no painting required), although it is only available in 100 ft rolls minimum.  The price might send me back to fiberglass, which is a lot cheaper.

I could make only the back screens with 7/16" aluminum framing and still use 1x2 wood for the front; that would give enough depth for a 1 3/8 inch screen gap.  The reason I am thinking of wood for the front screen is that I didn't plan to attach the center divider wall to the back of the collector (not much there anyway - just polyiso and flashing).  I was going to make the screen insert so it could attach at the sides only.  The center divider would join between a center rib on the front and back screens, forming sort of an I-beam running down the center of the screen assembly.  It would still work if I attach the divider to the front screen only, and have the divider pressing down on the back screens.  In this plan, the back screens would be only half-width, so the two adjoining frames would meet in the center.  I am working on a sketch right now.

If I make both front and back screens out of the aluminum frames, I will need some sort of 'skeleton' to hold the center wall in place.  Thinking about that too; it could work.

The biggest problem I had last time getting the screens smooth and tight was the somewhat 'kinky' sisal rope I used in place of the standard vinyl spline.  The frames themselves didn't bend much, since I used a cross-bar every two feet, and added flat corner brackets at all the corners.  Do you know what Krautman used to hold his screens?  If he used vinyl spline, did he have any problems with heat?  Did he reinforce or cover the spline with anything?  (glue, tape, crimps?)  One more question (sorry) - did he use 2 layers of screen on the back screen?  I wonder if you need to drop one size on the spline to do that.

Posts: 201
Reply with quote  #25 
Getting ready to shift gears on the internal design of the 'turnaround 4x8 ZP:  Most likely plan now is to use separate 7/16 inch aluminum screen frames for the upper, lower, and turnaround areas instead a single wood frame assembly.  This is leading back to 'ultralight' assembly methods (like on the 2x16 ZP), as explained below.

What's different is that I now have to figure out the center divider - whether it will be part of the absorber assembly or permanently attached to the back panel.  Leaning towards the second idea.  Since there is no real structural back, just polyiso and flashing, I thought about making the divider out of 1 inch polyiso and gluing it to the back.  To test the idea, I made some test panels, using (1) Gorilla Glue (polyurethane), (2) Liquid Nails, and (3) Silicone.  I made one test panel with the foil facing removed from the back, and a second with the foil left on.  I also made a single panel with the foil left on and the mating edge of the divider wrapped with foil tape (the tape on top is not part of the joint).  I only tried silicone with this last method.  I put a 2 1/2 inch construction screw into each sample to hold it together while the glue dried.  Here's a photo.  The panel with the foil peeled off is on top:

Glue samples, 012818.jpg 

Next day (yesterday) I checked the samples.  They all felt pretty strong.  I could flex the whole assembly before any of them broke.  I haven't tested to actual failure, but the samples without the foil seemed a bit more solid; on the others I could hear the foil bond just starting to 'crinkle' like it was about to separate from the foam below.  I was surprised the silicone held as tight as it did; I think that would be a usable option.  The Gorilla Glue my be the strongest - it felt like a single piece.  The Liquid Nails was also strong, but it still had a smell, even today (2 days after).  By the way, the Gorilla Glue was old (kind of thick) so it bubbled out quite a bit.  The silicone was easiest to smooth out into a nice bead.  The silicone and Gorilla Glue left no noticeable smell.  In summary, I think I can make a center divider that will be strong enough.

Second problem is what to attach the screens to.  (I was going to screw through the sides of the track frame with the wood screen frame.)  I got some pieces of poplar, 1 3/8 x 1/2 and 1 3/8 x 1/4, real nice stuff, straight, no flaws.  Tentative plan is to make small 2-layer inserts out of 1/2 inch poplar and 1/2 inch polyiso and glue them into the stud track, poplar side out, where the screens will attach, then fill in all the rest with 1 inch polyiso.  The center is a little harder; idea here is to 'inlay' pieces of the 1/4 inch poplar (have to cut out a pocket in the polyiso beforehand) and probably also run a strip of foil tape over the raw top of the divider.

Still working out how to attach the screens, but I'm hoping to attach the back screens to the wood inserts and then drop the front screens onto some protruding screws with a spacer between the 2 layers.  That way I'm not trying to screw into the wood right at the outer end.  All these methods are borrowing heavily from the 2x16 ZP, which was a little knick-knacky to assemble, but such is the consequence of changing plans mid-stream.

Also still looking for a way to make the center divider part of the screen assembly and just attach at the sides of the frame; not as far along on that idea.

While we're stuck with weather too cold to glue or paint, I made the snap disc bracket and drilled all the holes for that bracket and the electrical box that will go behind it on the back wall.  Here's the bracket:

Snap disc bracket 02, 012818.jpg 


Posts: 201
Reply with quote  #26 
Been working on the 4x8 ZP when the weather cooperates.  I cut the 1 inch polyiso strips to fit around the inside of the frame.  The strips have small wood blocks, made of 1 1/2 x 1/2 poplar, 'inlaid' half way into the polyiso for attachment points for the screens.

I had to re-figure everything when I decided to go back to aluminum screen frames, including the center divider.  This was originally going to be part of the screen assembly, but now I need something attached to the back panel.  Since the back is just polyiso with flashing skin, I made the center divider out of another piece of polyiso.  I peeled off the foil from the back and glued the divider in place with paintable silicone.  I added some 3 inch construction screws to hold it in place while the glue dried, and (carefully) weighed it down with a 2x4 and some bricks.  The center divider has more wood blocks, now on both sides, to correspond to the blocks around the edge.  All the polyiso strips have foil tape on the top edge.  Here are photos of the wood blocks glued in.  You can see how the top and bottom blocks are staggered:

Wood insert 01, 030118.jpg 

Wood insert 02, 030118.jpg 

I'm still not quite sure the center divider is strong enough, but it feels pretty solid now that the silicone is cured.  I also added a bead of silicone all the way around the bottom of the divider.  Here are more photos of the assembly:

Top piece, 030218.jpg 

Divider, glued in, 030218.jpg 

Divider with screws, 030318.jpg 

The last 2 photos show the polyiso strips added to the end, after the divider was set.  There is another strip on the far end with wood blocks to help hold the turnaround screen and to provide anchor points for curved strips of something (probably aluminum flashing) to help guide the air around the turnaround.  Thinking of making the curved strips just wide enough to fit between the screens.  In the next photo, you can see that the turnaround area will accept a screen the same width (22 1/4 inches) as the upper and lower screens, hopefully keeping the ZP channel as consistent as possible.  All the wood blocks here are 12 inches from the corner:

Panel, insulated, 030718.jpg 

My 90/110 snap disc just showed up from SupplyHouse.com.  I already made the bracket, so I'll put that together, and then the box will be ready for paint.

Still working on how to attach the aluminum screen frames to the wood blocks:  Thinking of a 'C' shaped metal bracket with the legs 1 3/8 inch apart (to match the planned screen gap).  A hole in the center of the 'C' to screw the bracket to the wood block.  This will hold the back screen against the back panel.  The top leg of the C will have a hole to accept a screw running through the front screen frame and into the bracket.  Have to prototype this.  A machine thread would be best, although a sheet metal screw may hold good enough if it doesn't have to come in and out too many times.  Maybe a clip on nut that slides over the edge of the bracket.

I'm putting together an order with Quality Screen for the 7/16 frame material, internal metal corners, cross-bar and clips.  They have everything (unlike Home Depot).  I still have a bunch of charcoal fiberglass screen, so I'll probably use that.  I'm tempted by the black aluminum screen available at Wholesale Screen and Glass (no painting!), but a 36 in. x 50 ft. roll is $58, so I'm still thinking about it.

Also thinking about what to use for spline - regular window spline, O-ring material, jute twine, solid aluminum wire.  Maybe I'll 'bake test' a piece of normal spline in a solar cooker, see how it reacts.



Posts: 77
Reply with quote  #27 
You do nice neat work Don. I like the tight fit of the Iso. Why don't you just make blocks to hold the screens in...I don't think the blocks would interfere with the air flow and would be way easier to fit tightly against the frames... a spot of glue might help too.  Anyway, I will wait to see how you make out with that phase.  Thanks for sharing!

Posts: 554
Reply with quote  #28 
One thing to consider with a U-turn collector like this is how to keep the glazing tight against the divider.  I have a U-turn regular screen collector (not ZP).  Differences in pressure between the house and outside often caused the twinwall to pull away from the divider a little.  That lets some of the air go over the divider.  I had to add a 1x2 strip of wood on the outside to keep the twinwall pressed against the divider.  It is held in place with a wire running side to side that can be tightened.

Kevin H

Posts: 201
Reply with quote  #29 
Kevin, I hear you about the difficulty of maintaining a good seal on top of the divider.  This is my second U-turn design; the first was this old slanted 2-screen clunker:

4x8 2-screen, 030817.jpg 

It actually works pretty well, even though the turnaround area isn't quite wide enough.  This one had a divider made out of 2x6 lumber screwed to the plywood back.  It weighs a ton, and is kind of slow to start up, which is why I have been focusing more on low-mass internals.  This unit has Sun Tuf corrugated glazing, so I could screw through the glazing into the divider.  It's not really made to take apart; there is silicone on top and bottom of the foam closure strips.

I can't really do any of that this time; in fact, I even debated whether to cut my losses and abandon the new project, thinking I had 'built myself into a corner' over this very issue.

On my 2x16 ZP, I used some 3/4 x 9/16 aluminum C-channel across the glazing to help hold it down.  It worked pretty well for a 2 ft. span, but I think it is too flimsy to span 4 ft.  I'm looking at other ideas - 3/4 or even 1 inch EMT, or maybe 13/16 deep Unistrut.  Even some decent 1x2 lumber may work.  I even thought about a similar idea to yours of running steel cable around to the back and tightening it with a turnbuckle.  I don't really know what it will take yet.

I hadn't considered running a piece lengthwise, directly above the divider, but that may be another solution - maybe run the cable through some EMT (lengthwise) and add more cables cross-wise over the EMT and around to the back.

Haven't decided on the glazing itself yet either - a single 4x8 piece of twin-wall is probably best, although I could use two 2x8 pieces with an H-profile  in the center.  Not sure about running the joint horizontally, but I may try it, since 2x8 is what is available locally (HD).  I considered drilling through the center of the H profile, through the divider, and out the back, then fitting in some 1/4 all-thread with a sealing washer on the front and a wide fender washer or metal plate on the back, pull it tight with nuts.  Seems like there's a lot to go wrong, like drilling the holes crooked for example, or introducing leaks.  One thing I did do, which may help, is make the center divider slightly higher than the sides, so a stiff, straight brace across the front will pull the glazing against the divider.

I will probably keep going on this project (don't know if I'm cheap or just stubborn), try to make it work.  I guess I still haven't come up with the design I'm looking for (simple to build, non-critical, 'anyone' can do it), but I don't think I will be attempting any more U-turn designs.  No more 2-piece designs with end slots either.  I think it's down to 'one piece, one way'.  Minimum useful size seems to be about 32 sq. ft., and I would like it to be light enough to pick up.  Thinking (next round) of another 2x16 1-piece ZP, which makes good use of available materials.  You could up-size by feeding 2 units in parallel.  3x12 is another possibility although there may be more material waste.


Posts: 183
Reply with quote  #30 
This 6" fan from ipower at $57 is a lot cheaper than the Suncourt.  I've been running one now for just over three seasons and it's quiet and powerful. 


Where are you in Colorado?  I'm in Harvey Park neighborhood of Denver. Would enjoy getting together with you and sharing ideas.  I like your build quality.

Denver, CO

Double screen hot air collector
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