CADDY PF01015 Installation And Operating Instructions Manual page 41

To ensure a safe installation, the two furnaces must not, at any time, run simultaneously. To do so, the thermostat
controlling the existing furnace must be connected to your Caddy link board. This way, when a heating signal is sent to
the existing furnace, the Caddy receives the same signal. It will tell the Caddy to either not start or, to go into a shut
down cycle, if the furnace was already heating when the demand for heat was sent.
The wiring for an Add-on installation is shown below. The existing furnace's thermostat must be connected to the Rh
and Aux terminals of the link board on the Caddy furnace. Those two wires must come from the R and W terminals of
the existing furnace's thermostat so the link board receives the heat signal.
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