Be Careful When Jumping Out a Blower


We’ve seen it before.

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

One comment

  1. Adam says:

    Trane/American Standard uses the “interlocks”…

    Now that is one thing i like about Goodman/Amana is that they just run a simple 120v leg from the contactor on the strips to the NC side of the fan board. This way 24V White/Emergency Heat can be engaged without any interlocking at all. Much simpler and better way to do things.

    I always just jump red to green, if no fan i know my board is bad.

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