i did a reshingle  and my girlfriend wants me to put one of the hot water panels i took off the roof, onto her house.  could i just take the cold line to the panel and run the out straight to the hot water heater?  or do i need other things in between?

If you hook it up like that, at night the water will actually be made colder, and it could in fact freeze.

The idea is that you want to preheat your hot water, so you use less energy to heat it. You need a storage tank for the solar-warmed water, and you need a controller which senses when the outside collector is warmer than the tank so it can turn on a circulating pump.

Trouble is one panel is not enough to do anything. And if you are in a northern state, a flat panel collector will not be efficient enough anyway. You should have evacuated-tube collectors.

Flat panels are fine in the Northeast but you can't run directly into your water heater per code from what I've been told. One panel is fine but limited btu's and probably not worth the effort. Two would cut it. All you have to do is buy a small external heat exchanger that AET has and a few pumps and a diff. controller.