Running a blower in reverse

[heatingman]

Ive been told running a forward curve blower in reverse will yield roughly 50% flow

How accurate is that?


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[2sac]

I think it would be somewhat difficult to calculate. A forward curve fan is pushing the air towards the outlet of the squirrel cage. Were you to reverse the rotation, the fan would be throwing the air in the opposite direction of the squirrel cage outlet. The result would be the air moving thru the outlet because it has nowhere else to go. I imagine there would be a ton of turbulence making it difficult to accurately measure performance.

[Nuclrchiller]

For some reason, about 70% comes to mind. But I don't remember where that number came from.

[icy78]

Ive been told running a forward curve blower in reverse will yield roughly 50% flow

How accurate is that?


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That's an interesting question @heatingman. Where is this going?

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[heatingman]

That's an interesting question @heatingman. Where is this going?

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It came up in a conversation with a coworker about a burner I was having strange performance issues with.

With power burners, airflow capacity has a large influence over burner capacity. IE the more the airflow, the larger the volume of gas you can consume and still maintain combustion quality.

The issue I was having was that the gas manifold pressure was much lower then stated pressure, like .6” compared to 3.2 rating yet the combustion was running very low O2 with a wide open damper.

Its most likely the wheel is very dirty reducing air, and the burner had been derated on fuel to compensate.

But he meantioned that the blower could possibly be running reverse, then he said doing so would cut airflow in 1/2.


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[WAYNE3298]

Airflow with reverse rotation is around 60%. With the bigger fans you can normally tell by the sound if the fan is running backwards.

[Rovert]

I once replaced a furnace blower with a universal motor. Running in reverse, it felt like 25% or so. It was not much airflow. I wish manufacturers could agree on which end of the motor they use to determine rotation.

Anyway, it took a long, long time for the blower to come to a rest after running backwards. This amount of time vs the amount of time to come to rest after correct rotation, along with some math, would probably be a way to get an idea of percentage of output. It's not going to be simply a matter of division, though.

[2sac]

I once replaced a furnace blower with a universal motor. Running in reverse, it felt like 25% or so. It was not much airflow. I wish manufacturers could agree on which end of the motor they use to determine rotation.

I'm not implying this about you. If a technician doesn't know which way the fan blade is supposed to rotate, the motor rotation being cwse or cwle is a moot point.

[Rovert]

I'm not implying this about you. If a technician doesn't know which way the fan blade is supposed to rotate, the motor rotation being cwse or cwle is a moot point.

I knew very well which direction the blower needed to spin. The universal motor, however, only had cw/ccw settings, no arrows or indication as to shaft end or drive end as the reference for direction. It was quickly and easily remedied.

[WAYNE3298]

My comment was concerning commercial fans. I don't know how much air a backwards resi will move. Never measured airflow with one of them running backwards.

[2sac]

I knew very well which direction the blower needed to spin. The universal motor, however, only had cw/ccw settings, no arrows or indication as to shaft end or drive end as the reference for direction. It was quickly and easily remedied.

Again, wasn't implying my comment towards you. There are a number of techs that have issues discerning rotation, primarily on paddle type fans and backwards curve fans.

[bradluke0]

40 - 60 % when running backwards .

[WAYNE3298]

Fans running backwards have low airflow and high current draw.

[Guyinfield]

Fans running backwards have low airflow and high current draw.

40 - 60 % when running backwards .

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[Guyinfield]

Ghtukcyhmmy FBI gbbnb,9k, ,,!-ehdmRJEHHEHRRH3HEHAUaqh3j3h
1qfhHeJEEHh


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Excuse me on this one. My kid is getting pretty good with the phone but not with spelling.

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[heatingman]

Fans running backwards have low airflow and high current draw.

All types of fans?


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[pecmsg]

Excuse me on this one. My kid is getting pretty good with the phone but not with spelling.

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Then I wont type what my grandson says it means!

[WAYNE3298]

Not all types of fans. Some fans move about the same amount of air but the flow is reversed.

[Tom Thermo]

I haven't studied blowers much but centrifugal pumps put out full head when running in reverse at zero flow but when you try to let it flow then the head drops right off.

[bradluke0]

I have never tested a pump running backwards so this is a new one on me...propeller fans and the like are a different animal.