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

Comments

Adam
Adam
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.

Related Tech Tips

The Case for Buying a Combustion Analyzer
Last year, I bought my first combustion analyzer. I had just started my own company and felt the additional weight of responsibility to sit down and determine the best way to verify that the furnaces I was servicing were working safely. In the past, the most extensive testing I had done was a visual inspection […]
Read more
Why A/C Systems Freeze
Let's start with the basics. Water freezes at 32° Fahrenheit (0° Celsius) at sea level and at atmospheric pressure. When any surface is below that temperature, and the air around it contains moisture, ice/frost will form. In some situations, ice is to be expected, such as in refrigeration evaporators and exposed portions of the refrigeration […]
Read more
The "5 Pillars" of Residential A/C Refrigerant Circuit Diagnosis
Suction pressure, head pressure, subcooling, superheat, delta T. Taking all five of these calculations into account on every service call is critical. Even if you must do further diagnostic tests to pinpoint the problem, these five factors are the groundwork before more effective diagnostics can be done. I would also add static pressure as an […]
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