Head of a pump under pressure

Im working on a solar water heater project at home and need information on how high a pump will pump under normal house (60psi) water pressure. I want to put a pre heat tank ahead of my home hot water heater using a solar collector 20 ft above the tank on the roof. The pre heat tank is an old 20 gallon water heater that will be plumbed in series with 3/4 inch pipe. I want to pump from the tank up to the roof and back, circulate, using a small solar pump and 3/8 inch tubbing. The pump is rated at 9 ft head but would it being under pressure change the head? I hope this explains what I'm trying to do.

That's a thought provoking question.

It would seem to me that just like pool pumps that pump up to solar panels push the weight of the water up to the panel, but then the weight of the water coming back down pulls a vacuum which helps pull the water up. In other words it's circulating almost by itself. Being under 60 psi shouldn't make any difference since it's the same everywhere in the pipes.