Be Careful When Jumping Out a Blower
A tech diagnoses a failed blower relay or board so they leave the blower jumped out by putting a terminal multiplier on the common terminal of the relay/board and connecting the fan speed tap right to power.
There can be an issue with that.
Some electric heat fan coils have a heat/blower interlock where the heat relay/sequencer back feeds and brings on the blower across normally closed (NC) contacts. The purpose of this is to ensure the blower comes on with a heat call without the need for a G call.
In some circumstances when you put in a terminal multiplier and constant power the blower, the G call sends that constant power back to the Heat strips and brings them on.
Not good… high power bills, melted wires, fire, death and stale doughnuts.
So, if you are leaving a blower jumpered out to run constantly I advise doing it separate from the board completely.
Coincidentally the photo at the top is a setup that will not back feed because it uses two isolated circuits on the Heat sequencer for fan and heat…. so the photo I chose wasn’t the best. Cut me some slack, the blower assembly was sitting right behind my office.
Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade