Hi Paul,
Well, here is something to think about -- it may or may not work for your situation.
Build a fairly large storage tank as follows:
Sides, top and bottom made with plywood or MDO plywood, and stiffened with 2X4's turned edgewise, and which lap over each other at the corners for a strong shear joint. Tank is lined with 1 or 2 layers of 2 inch polyiso insulation, which is in turn lined with an EPDM liner. The top of tank should be removeable to install heat exchangers etc. The EPDM is made for pond and tank linings, and is good up to 180F, and has a long life. Its about $1 per sqft, and can be found at places that sell pond supplies. The storage tank could be filled with plain water, which is just used to store heat -- it could have something it to prevent growth, but should not be anything poisonous to people. I think that this kind of tank can be built up to several hundred gallons, and should be fairly inexpensive.
As a 2nd thought, maybe the insulation should be fit over the outside of the plywood, in which case you could use the extruded polystyrene (the pink stuff). You might even be able to work out a way to just use fiberglass batts on the side with rigid underneath.
Three circulation loops come into the storage tank:
Loop 1: goes to the solar collectors and uses antifreeze. The heat from this loop is transferred to the tank by a heat exchanger that is a coil of copper pipe immersed in the storage tank.
Loop 2: goes to your hot water heater and circulates water directly from the hot water tank when the storage tank temperature is hotter than the water at the bottom of your hot water tank. Again, the heat exchanger is just a coil of copper pipe immersed in the storage tank.
Through a part of the year, the hot water tank elements could be turned off. I believe that the double wall heat exchanger requirement is met by this arrangement, in that there are two copper pipe walls between your potable water and the antifreeze in the solar loop (which should still be non-toxic). You would be able to detect a leak in either because the storage tank overflow would start flowing.
Loop 3: goes to your floor heating circuit and circulates water or antifreeze depending on the design of the floor heating system. It might even be possible to just circulate the water directly from the storage tank to the floor system without a heat exchanger if the floor system components can stand some oxygen in the water.
So, this amounts to 2 or 3 pumps, 2 or 3 coils of copper for heat exchangers, and some plywood, insulation, EPDM for the tank, and probably two controllers. Maybe not too expensive or complex?
The pumps could probably the Taco or Grundfos pumps that are commonly used in radiant floor heat systems (you may have one now for your floor system) -- these are fairly cheap, and can be found on ebay sometimes.
In the winter, I would think about whether you want to let the tank get up to maximum temperature before using water in the floor loop. The cooler the water circulating through the collectors, the more efficient they are and the more heat they will collect. If you read the part of this page about collector efficiency, it might clarify this:
http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm
If this is getting to be more than you wanted to get into, then I would just go ahead and do your original hot water plan. Maybe with less than all six collectors. Its better to be actually getting hot water from solar than be thinking about a system that might do both hot water and floor heating :-)
Gary
PS -- size of the storage tank -- this is based on making the tank large enough to store the collected heat for one full sunny day:
Not sure where you live, but 140 sqft of panels operating at 50% efficiency on a sunny winter day at 40 deg latitude would produce about:
Heat out = (140ft^2)(1600 BTU/ft^2)(0.5 efic) = 112,000 BTU
The idea here is that each sqft of collector gets about 1600 BTU/day of sun on a sunny winter day at 40 deg latitude.
if you say the tank should store this 112K BTU of heat when the tank temperature goes from 90F up to 150F, then the number of gallons would be:
Tank Size = (112,000 BTU)/((150F-90F)(1 BTU/lb-F))
= 1870 lb or 230 gallons or 30.7 cubic ft
If the tank was a cube, it would be 38 inches on a side (not counting thickness of plywood and insulation)
You might want to make it a somewhat larger than this, but probably not smaller.
