What sort of piping do I need for drain back solar? The collectors (two flat or one evacuated tube)
will be about 30 feet from a 200-gallon heat storage tank, with a rise of about 8 feet. In the longest section, traversing the attic, the pipes will be buried under 8 inches of cellulose. To manuever around trusses, I'd need solder connections or flexible tubing. Do I need continuous fall? Insulation over and above the cellulose? Any advantage to putting a small tank within 8 feet of the collector (4 foot rise)? I'm doing drain back because I only have room for one storage tank and that one doesn't have room for another heat exchanger.   

I won't be installing until I move in so I don't want a contractor yet. But it will never be easier than now to place the pipes.

Drainback systems are only needed if freezing conditions are present and no glycol is added into solar system. Your solar array seems quite small compared to your storage. Two pipes are needed,properly sized sloping from the panels back to the drainback tank. Also a conduit should be installed for the solar control wiring needed.

Hard copper pipe. Don't use thin-wall. Size for flow considerations; 1/2" or 5/8" probably. Support pipe with hangers - don't just lay it in there. Allow space for insulation and use high-temp insulation.

There are also flexible stainless line sets that are pre-insulated.

Do you really want to go drain-back? It only takes one malfunction.

Maybe you could do something like a Taco X-pump block.

3/4" is the needed pipe sizing.Type L copper is best .SS preinsulated are simple and easy like Solarflex by OmegaFlex. Do not forget the control wire .

I've wondered if one couldn't use pressurized air for a drainback system instead of worrying about slope everywhere. A few seconds of air works great on lawn irrigation systems.

How about PEX?
http://thermalproductsinc.com/wp-content/uploads/2010/07/SRCC-PEX-Policy-Drainback-Systems.pdf

Don't buy more pipe than you need unless you live in an area in which you think you will want to add more panels to your system. You don't want to re-plumb.

Look at your collector specifications for flow rates, but about 3/4 gpm is a start for both flat plate and evacuated tube collectors. That gives you about 1.5 gpm if you go with the two panels. Solarflex, like the acwizard suggested, has a 1/2" product that generates a head loss of 0.08' per foot of pipe length. 30' of run will give you 5' of head out and back. Those numbers are for corrugated pipe (the Solarflex product) and glycol, each of which adds about 25% to head. Using smooth wall copper pipe and/or water as fluid will obviously reduce that, but accounting for fittings will add a little bit back.

Figure $2.50/ft for copper pipe, so $5 both ways and a couple/three bucks for insulation and wire gets you to $8/ft of run plus labor for the copper. The all-in-one products run about $15+ per foot of run (less labor). More for sizes larger than 1/2"

How about PEX?

You might risk it on the return line. How much is it going to save you?

PEX is not a good idea, Pex is only rated to 180F and solar can get stagnation temperatures will above that.You would only have serious headaches later.

Many thanks. Wish I'd been smart enough to buy copper in '09.

Pex is only rated to 180F and solar can get stagnation temperatures will above that.

Looks like SRCC is well aware of the limitations which is why they require low pressures and 3' of copper if temperatures will go above 210F.