There are times when a pressure-glycol system is needed
A steamback solar thermal system. It sounds kind of scary — like what you don’t want to see coming out of your solar equipment.
This kind of solar system design is often referred to as a pressure-glycol system. OK, a correctly designed pressure-glycol system. The equipment is the same, except a steamback system has to be designed for separation of the steam to stay in the panels and not migrate down the pipes.
There are a few scenarios where you might choose this type of design. For instance, when you can’t install the solar collectors above the tank.
Another is when you need to deal with a light amount of overheat protection. Constant stagnation is never good for panels or glycol. The stea-back allows for a light amount of stagnation without the risk of degrading the glycol. But again, it’s important to design this system allowing for the steam to separate from the water.
Another situation fitting for a steamback system is when you’re using European collectors, such as Viessmann. European collectors do not lend themselves to a drain-back system.
This online discussion offers some great additional information about steamback solar.