BEWARE When Replacing Fancy Thermostats
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In this case, we’re replacing a high-end thermostat with a simple truck stock thermostat without advanced features like dehumidification. Most of the terminals are the same (R, Y, G, O, etc.), but fancy thermostats may also have terminals for accessories, such as ACC+, that allow for connection between the thermostat and the dehumidification terminal at the air handler.
You can’t just push the ACC+ wire back into the wall with a wire nut. DH is connected directly to R at the air handler with a jumper, so it would make sense to remove the jumper and make sure it only connects to one pin. However, if there isn’t a 24v call at DH, then the system will run at a slower blower speed; that jumper between the pins is needed if we want the blower to run at its usual speed.
In other words, the blower would always run at reduced speed if we didn’t have a means of connecting the dehumidification terminal to an accessory terminal at the thermostat. The evaporator coil would run under conditions that are closer to freezing, which isn’t ideal. So, that jumper needs to be connected between the pins at DH and R.