Controls for an Integrated Forced-Air and Solar Thermal Setup

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Control in a batch installation

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.

For this setup, you will need an on/off switch (winter/summer) for the solar thermal and furnace blower systems. You might also want to install a heat diversion so that the energy generated in summer goes to some use.

Control in a storage installation

If you have a heat storage tank on your solar loop, you'll need more equipment:

  • A two-stage or low-voltage thermostat
  • A setpoint thermostat or an aquastat
  • A circulating pump (usually AC) and a relay
  • A single-pole double-throw switch if there is central air-conditioning installed

The first item, the thermostat, controls the solar circuit based on the ambient temperature. It should be set several degrees above the setting of the thermostat controlling the conventional heating system (both thermostats are exactly the same model in most cases), to ensure that the solar setup kicks in first and does the most work.

The second thermostat (or aquastat) monitors the storage tank temperature and makes sure that the solar installation only runs when there is enough heat to do some good. It should be set at the lowest temperature at which you want the solar circuit to run. As a rule of thumb, you can set this at between 12 and 15 degrees above the lowest ambient temperature you want in the building – so if you want a minimum of 70°F in the house, set it at around 82°F to 85°F. This will make sure that the conventional heating system takes over when the solar setup has expended most of its gathered heat.

The circulating pump is needed to push the water from your heat storage tank to the heating coil in the ducting (and back again) and the switch is needed to allow both the conventional and solar setups to operate the blower on the furnace.

You will need to wire the furnace fan into both systems. To do this, look for the terminal strip just inside the furnace cover: there will be a row of lettered screws, including R, C and G. Those are the ones that interest us for this installation.

The R terminal powers the thermostat circuit: the wire from there should go to the thermostat, then the setpoint thermostat (or aquastat), then branch. One branch goes to the pump relay, the other goes back to the G terminal.

The G terminal controls the blower: this terminal is also used by the air-conditioning system. To integrate both systems, you wire the solar setup to one "in" pole of the switch and the air-con to the other; a single wire goes from the "out" pole to the G terminal. At the start of winter, you switch to solar/conventional heating; at the start of summer, you switch to air conditioning and turn off the solar setup in