Most residential buildings that have an existing grid-tied heating system use a hydronic rather than a forced-air system. This is generally better for integration with solar thermal installations, though that's a big generalization: there are so many different layouts, implementations and requirements that it is impossible to state whether integration is guaranteed.

At its simplest, this type of integration simply adds an interface to the existing piping so that the solar thermal equipment can pass gathered energy along and save you money on grid-tied use. Bear in mind that solar thermal installations are low-heat generators, so if your existing hydronic systems is based around high-heat delivery mechanisms (such as baseboard radiators), the resultant mix will be inefficient and impractical.

If you're installing a new build, everything is a lot simpler. Your best bet will be a primary loop-secondary loop setup, which simplifies design while maintaining the flexibility to integrate multiple heat sources and loads in one hit.

If you're integrating solar thermal with an existing system, the best advice you will ever get is this: find the person who installed the original system and talk to them . If that's not possible, find a professional who is up for the challenge of the project. You can, of course, take the whole thing on yourself and you stand to learn an enormous amount about plumbing and heating in the process. But don't expect it to be easy, especially if your existing system is old or complex – or both!

One simple step to make integration easier is to install a solar thermal system that uses a large heat storage tank with an internal heat exchanger. This keeps the entire solar loop separate from your other heating and potable water plumbing so that there is absolutely no risk of mixing fluids. It also means that maintenance on the two systems has a clear separation point and you don't need to flush everything just to fix one or the other.