Hi,
"Box built from 2X6's with plywood backing, width and hieght approx 3'X 5'.
Layers from back to face
1st 1" thermax on back and sides R6.5
second R11 fiberglass unfaced
third 1/2 thermax R1.5 (i think)"
Thats way way more insulation than you need on the back of the collector. The single layer of one inch thermax is enough. The insulation prevents heat loss out the back of the collector, but remember that most of the heat lost from the collector will be out the front, which is only going to be about R2 depending on what kind of glazing you are using. If the collector backs up to a heated part of the building, then back insulation is even less critical, since the loss goes to a heated area.
Thermaz or any form of the Polyisocynate insulation is good -- don't use the polystyrene (the pink, blue of recycled coffee cup foams) as they won't stand up to the heat.
Consider whether you really need the plywood back or not -- my collector has no back --it just goes right against the siding and works very well.
fourth layer of 1/2" copper pipe (spaced 3" to 4" apart) with inlets&outlest on bottom and top respectivley.
"fifth(collector) Metal or asphalt?? recessed to withing 1" of glazing."
The asphalt shingles would absorb OK, but I'm not sure they would hold up well to the high temps inside the collector. How would you thermally bond the copper to the shingles?
You need some way to thermally bond the copper to the absorber -- otherwise, the absorber will run very hot, and the heat losses out the glazing will be large, and the efficiency low.
"sixth double pane clear glazing in wooden frames."
Glass is good, you might also consider Polycarbonate plastic -- it is good up to 270F, and should work fine. Home Depot sells corrugated Polycarbonate glazing for 1 dollar per squarefoot. Make sure its Polycarbonate and not acrylic (Plexiglass) or PVC, as they won't take the temps.
"At first I had concidered metal roofing(galvanised, corrigated) only but this would be difficult to get a good thermal bond to copper pipe other than random touch. Asphalt shingles would tend to mold to the copper and surface transfer would be more constant. Also I figure that shingle material will help insulate against cooling at night better that metal. "
The collector will cool off at night rapidly -- it radiates well to the cold sky temperature -- there is really nothing you can do to prevent the cool down. Its best just to keep the thermal mass of the absorber as low as possbile, so that when the sun shines on it, it will heat up quickly.
"Someone on the board I believe used rubber roofing (thanks for the idea). I don't have any of that so I concidered asphalt shingle material, however I wonder if temps inside the panel will cause these to melt. For the time being these panels will be used in the winter for constant radiant heat to a floor measuring 14'X12'.Possibly expanded incase of to much heat (in my wildest dreams). Panels will be on a closed system with antifreeze heating exchanger to radiant sysetem, for possible expansion(domestic hot water). A extra gas hot water heater will be used in the radiant system for a heat reserve tank and if need be suppliment durring cold nights. Valves will be places in heat exchanger line to keep panels from cooling water in water tank at night if needed."
Could you build the panels larger? Panel area is the biggest factor in how much useful heat you will get out. It is not that much more work to build a 4 ft by 10 ft than a 3 ft by 5 ft, and you will get three times as much heat from it.
"Well there it is, let me have it. I await your opinions."
There is a lot of material on building collectors on my site:
www.BuildItSolar.com
Take a look at "Projects" -> "Space Heating"
Also take a look in the "Experimental" tab.
Just as a thought, have you considered building air collectors rather than water?
The air collectors could work in parallel with your radiant floor. Air collectors are simplier, cheaper, and don't make a mess when they leak :-)
Gary