This is a common question from people who want to retrofit a solar thermal space heating system. The simple answer is "Yes, you can integrate the two systems." The reality of the process is a little more complicated.
If your solar thermal system has no storage, the heat gathered by the collectors goes straight to the ducting. When heat is needed, both systems are switched on, so hot solar fluid circulates to the heat exchanger and the furnace blower distributes the heat around the building.
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.
Combining a solar thermal heating system with an existing hydronic installation requires not only plumbing and solar equipment but some extra control gear – both electronic and hydronic – to divert fluid when needed. There are several options available in most cases.
In some situations, you will be able to wire the solar thermal setup into the existing thermostat control system so that the water supply passes through the solar storage tank's exchanger when there's enough heat to do some good.
Although it is usually preferable to connect your solar thermal space heating installation into a grid-tied system and have the two work together, it's not always possible. Some retrofits simply do not lend themselves to integration, so you will end up with two separate systems. They can still work together, just not to the same extent.
One of the oldest known man-made solar thermal space heating systems was built by the Romans, who constructed radiant floors for their bathhouses over 2,000 years ago. The systems heated the bathing water and the buildings by circulating hot air under the floors, then up through multiple chimneys. Today, radiant floors are not only the most comfortable heating system available, but the most economical – especially when tied into solar thermal heat generation!
Active solar thermal installations come in two main types for space heating: liquid-based and air-based. The third type – high-mass systems – is completely different to active systems and deserves separate treatment.
High-mass installations use a sand bed or pit underneath the building and a network of Pex tubes to gather, store and deliver heat to a radiant floor. They are extremely economical to run and provide very comfortable heating for the whole home. Remember to complete your installation in spring, so that the system starts gathering heat as soon as possible!
The simpler option for solar thermal space heating is to install what's known as a "dump" system. In this setup, there is no storage tank: the collectors heat the domestic supply when the sun is shining and send that heat to some kind of liquid-to-air interface, such as a radiator, a baseboard heating unit or a fan convector.
Although they are extremely efficient and produce very comfortable radiant floor heating, high-mass solar thermal systems can be difficult to live with, especially at first. They require changes in habits and it may take a few years to figure out the optimal approach for your particular situation. This article will provide the background knowledge and some pointers to help you along the way.
