The 90-340 relay has two sets of dry contacts, which don’t have a connection to a power source. Energized or “wet” contacts DO connect to an electrical source. The contactor coil is an electromagnet that energizes the relay and pulls in the contacts; when the contactor isn’t energized, the path between 1 and 2 or 4 and 5 is normally closed. On the flip side, the path between 1 and 3 or 4 and 6 is normally open when the contactor coil isn’t energized.

For the contacts to work as they’re intended, they need to be wired as a switch, not a load. When you have a load, you need a power supply and a common for work to be done. Switches only exist to make or break a circuit.

When we use an EcoBee or smart thermostat (with humidity control) with a dehumidifier or an AC with dehumidification mode, we would connect ACC+ straight to DEHUM (DH). (ACC+ is an accessory terminal that we can program.) Dehumidification mode on AC systems lowers the fan speed. When we don’t use humidity-controlling thermostats, we would use a jumper between DH and R at the terminal block to energize DH all the time (and NOT dehumidify).

Using a 90-340, you could connect the thermostat (ACC+) to one side of the contactor coil and back to common on the other side of the coil to make the circuit, forcing the system OUT of dehumidification mode. Under these conditions, the contactor coil would be energized and take the system out of dehumidification mode.

When we use an actual dehumidifier, we have to take a 24v signal from the dehumidifier transformer and energize the DH terminal to turn the dehumidifier on. Dehumidification mode for a dehumidifier is the opposite of dehumidification mode on an AC unit. On a dehumidifier, DH must be energized, whereas it must be NOT energized in the case of AC dehumidification mode. We need the relay because we need a set of dry contacts, and it allows us to work with a separate transformer: the dehumidifier transformer. When we wire a dehumidifier to the relay, we want to make sure we just have 24v out and back in.