Controls for an Integrated Hydronic and Solar Thermal Setup

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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.

High-temperature boiler setup

Many low-heat delivery systems use a high-temperature boiler and temper the overheated fluid down to the right level by running it against the colder, returning fluid. This is common practice in systems with a combination of low-heat and high-heat delivery equipment and in manifolded or injection-loop systems.

If your system is set up like this, you will have to install a set of relays and switches to bypass the boiler entirely.

Modulating boiler setup

If your existing system has a modulating boiler which can detect the temperature of incoming water and fire up only when it's too cold, you have an easy option. Install a differential temperature controller to measure how hot the fluid is on the return trip from the heating delivery equipment and compare it to the temperature in the solar storage tank. When the tank is warmer than the fluid, divert the flow through the solar tank's heat exchanger by means of a motorized three-way valve (or a zone valve).

If the solar tank doesn't quite do the job, the boiler will detect any shortfall and fire up (on low power if it's an efficient model) to top off the heat once the fluid arrives there.

Two-stage or dual thermostat setup

An alternative is to install a two-stage thermostat in the building to control delivery (or two separate thermostats, whichever suits your needs). Stage one controls the solar heating system and is set slightly higher than stage two, which controls the grid-based heating. If you're using two thermostats, each controls one system and they are set similarly.

When the temperature in the building drops, stage one (or the first thermostat) kicks in and triggers the solar thermal installation. As long as there is heat available, the second stage remains inactive but, as soon as the solar energy is expended and the temperature drops lower, the grid-tied system kicks in and acts as backup.

You will need to use an aquastat (or setpoint thermostat) on the solar heat storage tank to check whether there's heat available for delivery. If there is, the aquastat should start up a circulating pump to push fluid through the solar heat exchanger. If not, the pump remains inactive and the backup system will kick in once the temperature drops below the stage two temperature setting.

Dump system setup

For solar thermal heating installations with no storage, the generated heat is usually delivered direct to the low-heat heating equipment. Radiant panels work very well with batch systems and, of course, the system only provides heating for as long as the sun shines.