Technicians commonly mix up their wiring between fan coil relays and 90-340 relays. Fan coil relays and 90-340 relays look similar on the surface, but power feeds into them differently.

When techs feed a relay, they want to bring power into the 1 or 4 terminal on the 90-340, but the power goes into the normally open (NO) contact on the fan coil relay. Instead, the motor winding typically goes to the common terminal on a fan coil relay (equivalent to 1 or 4 on a 90-340). The blower speed tap connects to 1 or 4 on a 90-340 relay. The normally closed (NC) contact connects to the heat sequencer. Unlike relays, heat sequencers are NOT magnetic loads; they are heaters that have time delays due to thermal expansion that occurs inside each time the circuit is made.

Backfeeding complications may occur when techs mix up the wiring on relays, especially to the heat strips. In short, just make sure you connect the blower to the common on the fan relay, not the power.