[stevehull]

Pipe resistance (and thus the size of the pump) is HIGHLY dependent on the diameter (radius to the 4'th power). The length is only a minor contributor (unless you are talking of hundreds of meters long). The gravity head is the other issue and we talked of that in the last post.

In my roof top solar water heater, it is about 30 feet (~ 10 meters) above the storage tank and the total pipe run length was about 200 feet. It is three collectors just about the same size as you talked of. The water flow was set up for about 5 gallons per minute (20 L/min) as a drain back system with a 1/10'th HP pump. Has been working since 1992 - with one notable problem.

The only thing I would mention is that a section of copper pipe needs to be used if you are using a drain back. Here is why.

Assume the system shuts off for some reason in the daytime (power outage). The collector heats up to several hundred degrees (doesn't matter if C or F) in the hot sun. When the power is restored, the water in the drain-back supply is pumped up, goes into the collectors and immediately flashes into super heated stem - and it melts plastic pipe in the return.

Use a thick walled copper for the first dozen meters right off the solar panel on the return to storage if you are using a drain back. The copper has sufficient thermal mass to absorb the flash heat in the above situation.

Steve