Very few solar thermal space heating installations provide 100% of a home's heating requirements year-round, so a grid-tied backup is almost always required. It is obviously a lot more efficient – particularly financially – to use the same heat delivery equipment for both systems wherever possible.

There are three mediums that grid-tied space heating systems use to deliver energy to your home: water, air and electricity. The only one of those which won't integrate with a solar thermal setup is electricity. Solar thermal works by gathering heat from a collector array and that heat can't be passed to electricity for distribution like it can be passed to air or water.

However, the real compatibility question comes from the operating temperatures of the two systems. Solar thermal space heating installations commonly run at temperatures between 120°F and 150°F (49°C to 66°C). They may peak a little higher, especially when not in use, but that's a good baseline temperature range.

Looking at heat delivery equipment, you'll see that they fall into two main categories: low-heat and high-heat. A low-heat source delivers lower overall temperatures to a large surface area; a high-heat source delivers much higher temperatures in a more concentrated fashion. For example, a radiant floor is a low-heat source which delivers between 90°F and 130°F (32°C to 54°C) over an entire floor surface; hydronic baseboard radiators are high-heat sources which deliver 150°F to 190°F (66°C to 88°C) and concentrate their output into an area the size of the baseboard around the walls of the room.

Now consider those two pieces of information together: a solar thermal system delivers appropriate energy for a low-heat setup but is unlikely to be able to handle high-heat equipment unless you make significant adjustments (which will reduce its efficiency and heat production volume). So what does this mean in practical terms?

• If the existing grid-tied heating system uses high-heat delivery equipment (e.g. baseboard radiators), it will be incompatible with a solar thermal installation.
• If the existing grid-tied heating system uses low-heat delivery equipment (e.g. radiant floors), it will be compatible with a solar thermal installation.
• If you have no existing grid-tied heating system, you should choose a low-heat option to ensure compatibility and lower overall costs (for both systems).

To further clarify the situation, here's a concise list of some of the systems that are compatible or incompatible with a solar thermal heating setup:

Compatible: domestic hot water, radiant space (floors, ceilings, walls), forced-air, pre-heat air (or make-up air), make-up water on commercial boilers, industrial processes, crop drying.

Incompatible: electric heating, high-temperature equipment (radiators, etc.), steam heating.