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Fireplace Fan Physics
Old 11-27-2016, 09:37 AM   #1
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Fireplace Fan Physics

Out woodstove-like fireplace has a fan (in the bottom) that blows air around the firebox and out, like this:



Note that it does not get very hot where the fan is.

The fan is going bad, and I'd like to replace it with a much smaller, quieter unit.

I'm thinking that even if there's much less airflow, the air that flows through will be hotter. Therefore the efficiency of the heating will not decrease as much as expected.

For example: Airflow cut by 50%, efficiency cut by only 20%.

Is my reasoning faulty?
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Old 11-27-2016, 09:49 AM   #2
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I think that's generally correct.
If the air is moving slower it will spend more time in contact with the firebox.
I posted in the other thread but if you have a variable speed fan you have some control between more speed/noise vs. running quietly. I envision running the fan faster until the room warms up and dialing it back for continuous operation.

It would be helpful to figure out if the smaller fan is really less volume. You might be able to maintain the flow with a quieter fan.
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Old 11-27-2016, 10:59 AM   #3
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smaller fan does not necessarily correlate with more quiet fan. If your focus is noise reduction, then look at what makes it noisy.
Is it the fan itself?
Or is it the mounting? (loose or vibrates with the fan)
Does the fan need lubed?
What type of fan is being used? (some are just noisy)

You may find a larger fan run slower may be more quiet.

Noise and air flow could be effected by back pressure in the system.
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Old 11-27-2016, 11:28 AM   #4
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I'm currently considering this exact replacement for $103.



versus this for $17 (it's only 3 inches high):



I'll probably go with the former.
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Old 11-27-2016, 11:36 AM   #5
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Old 11-27-2016, 11:42 AM   #6
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Yes, a squirrel cage blower is capable of much better performance (flow with static pressure present) than a fan. That's why the engineers use them.

Get the part that was engineered to work in your stove.
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Old 11-27-2016, 12:00 PM   #7
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Those little muffin fans are very inefficient. Don't waste your time with one.
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Old 11-27-2016, 03:47 PM   #8
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Technically, it might heat less. The lower air flow would generate less turbulence at the hot metal surface. Higher turbulence increases heat transfer from the metal to the air. The air coming out would have a higher temperature due to higher contact time at the lower velocity, but still carry less heat since there are fewer cubic feet of air moving and the heat transfer rate was lower. Because of the higher temp it might "feel" warmer near the vent, but still heat the room less.
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Old 11-27-2016, 04:56 PM   #9
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If the unit was designed to be operated with the fan running and at a certain airflow, by reducing the airflow could you over heat the internals of the unit ?

That would be my concern.
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Old 11-27-2016, 05:13 PM   #10
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Quote:
Originally Posted by frayne View Post
If the unit was designed to be operated with the fan running and at a certain airflow, by reducing the airflow could you over heat the internals of the unit ?

That would be my concern.
The fan doesn't have to be run at all so overheating is not likely to be an issue.

I have a similar unit that uses a similar fan. Last year after 18 years the fan became too noisy to use. I am told that the unit can't be repaired. I just pulled it out and let convection take care of things and didn't really notice a dramatic drop off in heat output.
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Old 11-27-2016, 07:11 PM   #11
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I think it's a lot more affordable to get variable speed control on small AC motors now. Buy the original and add variable speed to the motor.
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Old 11-27-2016, 07:18 PM   #12
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Your original looks very much like the one in my Osburn 1100. The motor is a cheap shaded pole 2 pole offset. Mine has a on/off speed control and a temp sensor, only runs when it's hot enough.
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Old 11-28-2016, 10:44 AM   #13
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Thanks for the advice. Yes, I'll go with the one that matches the original.
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Old 12-01-2016, 10:18 AM   #14
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Got the squirrel-cage fan that was an exact match.



It's much quieter. At low speed, I can hardly hear it, which means that now we'll be able to have the fan on while watching TV.
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