Back when modern radiant floor systems were first installed, copper or steel pipes were used. They were embedded in concrete under the floor and hot water ran through them to heat the building. It doesn't take a genius to see where the problems came from:
There is an upper limit on how much Pex tubing you can use in a radiant heating installation. If the circuit is too long, the solar fluid will cool off before it reaches the end and you will get uneven heating. For standard-sized Pex tubes (half-inch or 5/8-inch internal diameter), the circuit length should not exceed 300 feet – which is conveniently the size of the coil supplied by many manufacturers.
Before you begin laying Pex tubes into your heating area, you must prepare the ground. The area should be flat and level, with the insulation in place and a vapor barrier under the insulation. If you are going to enclose the Pex tubes in a slab, you can cover the insulation with reinforcing wire or rebar.
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.
