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I'm replacing a non-functioning draft inducer on the Goodman furnace in my house (GMP075-3). The previous inducer was mounted with a sheet metal back plate between it and the wall of the furnace. This back plate happens to cover the 3.5" draft hole on the wall of the furnace with a smaller 1 5/8" hole of its own (see pictures) which I happen to find odd. Wouldn't that restrict air flow?

Since the new inducer assembly didn't come with a back plate, I'm wondering if the back plate is actually necessary. The inducer being replaced was new as of 2009 so I'm suspicious that perhaps this smaller hole somehow restricts airflow thus causing strain on the inducer motor which led to a failure after just 3 years. On the flip side, perhaps the smaller hole is necessary because it plays some role in protecting the fan blades from being overexposed to high temperatures via the larger 3.5" draft hole.

Any help from an expert would be much appreciated. I assume the plate is necessary but want to be sure I'm not going to need a new inducer again in 3 years.

The Inducer Assembly

Inducer Assembly!

The back plate and/or mounting bracket with 1 5/8" hole.

Back plate and/or mounting bracket!

The inducer assembly with back plate in position (covering most of the open fan space)

Inducer assembly with back plate covering most of the open fan space!

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I'm not familiar with the whys and where to fors of furnace construction, yet I'm fairly sure the hole acts as a throttle to induce just the right amount of draft. If you omit it, there will be too much draft, which would inhibit heat transfer to the circulating air. In other words, it will cause money to disappear up the flue. While the blower should have lasted longer, this type of blower is very tolerant of restricted air supply, and the reduced inlet would have little impact on it's expected lifespan. You should reinstall things just as you found them.

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While those answers may apply to some degree, I know the plate does create a restriction, which causes a slight vacuum in the blower section. This vacuum is used on some furnaces to actuate the diaphragm pressure/vacuum safety switch. If you have a diaphragm switch hose leading from the blower housing, then you'll find the furnace will not start gas flow unless that plate is in-place and creating the vacuum signal for the diaphragm safety switch. Ask me how I know... duh.

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  • I replaced mine without the plate and it worked fine. I may, like the first post points out, be losing money but the draft is sufficient to open the vacuum safety switch and the burners fire up fine.
    – user49173
    Commented Feb 14, 2016 at 12:03

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