Need Blower Design Help for Wood-Burning Stove

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KineticoUser

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This is not your normal HVAC question, but I figured you’d be most qualified to help me design what I need.


I have a FABCO Signi-Fire wood burning stove (the company no longer exists). I find the fans to be too noisy, so I want to replace them with blowers. There are 7” holes with 3 ¾” space below and 5 ¾” space above. The motor needs to be installed below, or it will get too hot, and I was thinking of extending the shaft of the motor and attaching a metal squirrel cage with the open end facing downward. I’m not sure if this needs to be surrounded by a case or if it will pull air without being enclosed. This is the opposite of most squirrel cage assemblies, as they would have the motor in the heated area. I’m not sure if the squirrel cage should be slightly under the diameter of the 7” hole, fitting slightly below the opening or if it should be larger and fitting above it. I’m thinking a bearing supporting the motor shaft above the squirrel cage would be helpful. I’m attaching photos, diagrams and information from FABCO and would appreciate any suggestions you may have. Due to the fact I need 4 of these fans, suggestions for locating used/surplus parts would be appreciated, also.
 

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Dana

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With sheet metal ducted fireplace inserts it's not clear that you'd get much noise abatement no matter what you installed for a blower, but having multiples guarantees there will be "beat notes" as the speed / phase of the units shift relative to one another.

My guess is that this beast is something like 25-30 years old(is there a copyright date on the manual?), and at the end of normal service life for a sheet metal wood burner. If you burn wood primarily for the ambiance you can probably just pull the blowers or turn them off. If you burn wood as a large fraction of your heat, it's worth scrapping the thing and installing a new EPA-rated fireplace insert, which will use about 1/3-2/3 the fuel for the amount of heat it delivers, and emit about 1/5 of the particulates out the chimney. When the EPA first put limits on particulates back in 1988 most wood stoves & inserts were running in the 35-50% efficiency range. But the improvements over the past couple of decades designed to limit emissions have also boosted efficiency of these appliances to the 75-80% range and higher.

You may not see "payback" on a new wood burning insert in short years unless you burn a lot of wood, but the local air pollution issue may make you breathe easier. The blowers on the new units tend to be a lot quieter than those from the 1980s & earlier too. A new insert is not a cheap way out, but it's not a hack, and closer to a "right" solution, if you have the budget for it.
 

KineticoUser

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This isn't a fireplace insert. Though it looks like a fireplace, it is actually a 80,000 BTU/Hr wood burning stove. I did a lot of research to find a wood burning stove that looked like a fireplace, and I installed it new when I built my home in 1992. It was fully tested by Pacific Inspection and Research Laboratories in Redmond, WA and passed all UL and EPA requirements. It is very well built and puts out a lot of heat (enough to heat my large home). My issue is not with the unit itself, but I was looking for ways to reduce the noise level of the fans. It is very quiet when I run the fans at a slow speed, but higher CFMs spread more heat (sometimes too much). People have told me that I wouldn't see a reduction in sound by switching to a squirrel cage, so I guess that is out. I am replacing the motors, as they are showing their age, with AO Smith 9208F2 motors. I'll just continue to use the same fans, though I am now thinking of insulating the back of the fan compartment doors as a means of reducing the sound. Do you have any suggestions to use for that purpose? The doors are 6” X 17” and there is a 1 3/8” space between the inside of the door and the fan.
 

LLigetfa

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I don't think there is enough room for smooth airflow around squirrel cage blowers. You would need close tolerance around the lip of the blower to keep air from short circuiting.

If the old motors are getting noisy due to hum or sloppy bearings, you might try new DC motors. If you think air turbulence around the fan blades are creating noise, you might consider a cowl around them.

Are the motors speed controlled or single speed? Speed controls can introduce hum.
 

KineticoUser

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As I mentioned, I'm replacing the motors with AO Smith 9208F2 motors, but the noise I hear is from the fan blades cutting through the air. The motors are speed controlled and make no noise at lower speeds.
 

LLigetfa

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...the noise I hear is from the fan blades cutting through the air.
As I mentioned, putting a cowl around the fan might help. As they are now, in the open, the air can short circuit.
The motors are speed controlled and make no noise at lower speeds.
The old ones or the new ones? Not all motors are the same when speed controlled.
 

KineticoUser

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Can you suggest a cowl I could use? I wouldn't know where to start looking for one.

The new motors haven't arrived yet, but I don't anticipate the motors making much noise.
 

LLigetfa

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You mention 7 inch holes but the fans look to be a bit smaller so not sure if a coffee tin would work as a test. If it does then you could probably make some cowls out of round duct or sheet stock.
 

DonL

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I think you need to re-think the fan motors that you plan to use.

They will not move near the air of what you have now.

Are you trying to heat the whole house ?
 

KineticoUser

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The fans are 7" in diameter and the holes are no more than 7 1/8" in diameter. Are you saying to just create a tube around the opening? If so, how high above the fans should the tube extend? I have 5 3/4" above the base before I run into the support bracket. Should I make the tube exactly 7 1/8" diameter or a little larger?

How is it that the new motors won't move as much air as I presently move? Both rotate at 1550 RPM at 115V. If the rotation is the same and the fans remain the same, how would the CFM drop? I understand I'm dropping from .75A (and 27% efficiency) to .58A (and 65% efficiency), but I don't see how that would lead to any significant drop in CFM. According to Century/AO Smith “The iMotor refrigeration motor uses less wattage to achieve the same airflow as a shaded pole motor, so it produces less heat." I ran this by my son (who is actually a rocket scientist), and he would like to have more information from you to understand the logic of a drop. I'm not looking to heat the whole house with the 2 wood burning stoves. I only heat the back half (about 1800 sq.'), and they are more than adequate.
 

DonL

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Well your son should know that it is not a good idea to use a Evaporator fan motor for your application when it is cooled in a different way.


Please do report on how well it works. You may want to buy a few spares.


Good Luck on your project.


Happy Holidays.
 

KineticoUser

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Don, please back up your statements with factual reasoning. I can't consider statements that aren't explained. The motors will be sitting in ambient room temperature with cool air being drawn in and around them, not within the heated box. I see nothing in the literature stating these fans must be used in refrigerated conditions only.

These are cooler running motors with "wide temperature range oil for use in evaporators and condensers." Condensers get very hot, so I'm not following your thought process.
 
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DonL

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Don, please back up your statements with factual reasoning. I can't consider statements that aren't explained. The motors will be sitting in ambient room temperature with cool air being drawn in and around them, not within the heated box. I see nothing in the literature stating these fans must be used in refrigerated conditions only.

These are cooler running motors with "wide temperature range oil for use in evaporators and condensers." Condensers get very hot, so I'm not following your thought process.


Where do you see them used for condenser cooling ? The model you posted is to move Cold air, not ambient room temperature, unless I misses something.

What type of fan blade will you be using ?
 
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