I have a Chronotherm IV being used wirelessly to control one of two furnaces and one of two AC units which control the upstairs temp. With the warmer weather, I turned this thermostat to cool but it did not turn on the AC unit outside. I changed the batteries and checked power at the power disconnect located outside by the AC unit. The thermostat does not seem to be linked to the AC unit. Is there a way to reset the link?