Be Careful When Jumping Out a Blower

We've seen it before.

A tech diagnoses a failed blower relay or board. 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 backfeeds and brings on the blower across normally closed (NC) contacts. The purpose of that is to ensure the blower comes on with a heat call without the need for a G call.

In some cases when you put in a terminal multiplier and apply constant power to the blower, the G call sends that constant power back to the heat strips and brings them on.

That's not good. You end up with high power bills, melted wires, fire, death, and stale doughnuts.

So, if you are leaving a blower jumpered out to run constantly, I advise keeping it separate from the board completely.

Coincidentally, the photo at the top is a setup that will not backfeed. That's 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.

Here is a video on what I'm talking about:

— Bryan


6/20/17 at 08:01 PM

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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