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

One response to “Be Careful When Jumping Out a Blower”

  1. 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.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Related Tech Tips

4 Silly Mistakes of The New HVAC Tech
We've all been new at one time or another. So, there is no need to get all judgy about some of the mistakes new techs make just because they are inexperienced. However… These are some very preventable mistakes that occur due to simple oversights and carelessness that need to happen 0% of the time. 1. […]
Read more
What is a HSO (non-bleed) TXV/TEV?
There is a lot of misunderstanding about the HSO (hard shut-off) or “non-bleed” TXV (thermostat expansion valve) and what makes it shut off, why it exists, and how it “magically” opens. Once you understand the forces inside the valve, it is quite simple, obvious, and sadly devoid of any magic. The Three Forces  The TXV […]
Read more
Remove "Weep" Plugs on Motors
  In this 60-second tech tip video by Brad Hicks with HVAC in SC, he shows us how and why to remove the weep port plugs on a condensing fan motor. I know from experience that motors can fail prematurely when this practice isn't followed. Remember that motor orientation dictates which weep port plugs are […]
Read more
loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from