Direct systems (called draindown systems) are not suitable for installation in DFW and North Central Texas because the water will freeze.

Anytime someone tells you that you can make your own solar hot water system out of a window frame and some copper tubing, that is true, you can. However, that is a direct system and will almost certainly eventually freeze in North Central Texas, producing quite a disaster for your home.

What is a direct or draindown system?

The domestic hot water is heated directly in the collector. If it is not drained before freezes, the pipes can burst. 

It's best not to try this system in the DFW area. Great for the tropics, though.