Woodburning stove fan relocate project

[gabeNC]

Howdy gents,

I've got an idea I would like to run by some of you DIY experts.

I use wood as a primary heat source and I have these noisy fireplace fans (well... one died see this thread)
They probably blow around 200cfm each and they cost around $150 a pop. It has been suggested to use large 80mm computer fans, the upside is that during ice storm power outages I could run them off of a battery but those types of fans do not throw much air.

So my idea is to add some duct work in the crawl space, relocate the fan there and tap into the hvac return since I don't want musty dirty air blown into my living room. I would have to cut a hole in the area where the old fan used to sit (nice thick piece of steel) and then continue down through the decorative concrete on the hearth. I should be able to mask the pipe coming up right under the stove with either a shroud or just paint it black.

I hope that this would be an improvement in several areas like, allow me to use a much larger fan at a slower speed, and it would be quieter not having the blower motor in the room.

The crawl space has about four feet of vertical space and the hvac return is probably less than 6 feet away. The fan in the picture is a 120v 15" single speed attic fan capable of 1200cfm, which is probably more than twice what I really need.

The pictures close up on the stove show the dual thermostats, controlling the three speed fan. I could see this project getting complicated with the electronics and possibly a PIC being involved .

So... any thoughts or advice?

 

[pilko]

A couple of things to consider:-
1 Installing fans in ductwork several feet away does not reduce the noise very much. The noise is transmitted through the duct.
2 Does the attic fan have a motor that is variable speed capable.

pilko

 

[cr0sh]

I would also be concerned about any possibility of the duct acting in some manner as a chimney for fire or such getting to the attic. Maybe the air simple circulates around a completely enclosed firebox (you didn't mention whether this was true, but it seems likely). I'm not sure, though, whether such "customization" done by a homeowner (instead of a licensed contractor) would invalidate any portion of your homeowner's insurance policy. You might want to look into this further (even if the whole thing is completely safe, such modification could still invalidate it; should a fire occur, and these modifications come to light - it could be a factor in whether you get your claim approved or not).

 

[gabeNC]

Maybe the air simple circulates around a completely enclosed firebox

The stove has a heat exchanger. I sweep my chimney three times a year so the odds of a fire are very low.

A couple of things to consider:-
1 Installing fans in ductwork several feet away does not reduce the noise very much. The noise is transmitted through the duct.
2 Does the attic fan have a motor that is variable speed capable.

I see a tremendous amount of PWM circuits out there for these types of fans. I was hoping that a larger fan run slower would help. There seems to be some different types of hvac silencers out there also.

thanks for the replies, I'm just trying to get some feedback or better ideas.

 

[tcmtech]

Thanks for the heads up PM. I probably would have skipped this thread otherwise.

First off from what I can see is that your fan motor that is bad more than likely could be replaced with a generic one that has the same shaft size and approximate speed. I have done that many times with peoples, and my own, small blower motors that are just like yours that had cheap bushings instead of actual ball bearings.

A little on line searching will probably find you a compatible motor for around $20 - $30 thats of a better but still physicaly compatible design. The other option is to go to a larger AC powered muffin fan types that are in the same or similar size as your original blower. I have pulled used ones out equipment before that have been in 6 inch up to 12 inch (150 to 304 mm) diameter ranges which have can havevery high air flow ratings despite being surprisingly quiet!
I have a 10 inch one I have been using as a crawl space ventilator for about 4 summers now, 600 - 800 CFM I think, for my house that runs 24/7 and I can barely hear it in my master bedroom that is right above it.

The other advantage with those fans is those AC motors tend to be rather forgiving to speed control with common dimmer units. At most a small power factor correction capacitor may be needed to get a more stable speed control of them but nay have that built in and don't wont require it. Lastly they are ball bearing motors and sell for around $ 60 - $100 new with typical service life ratings exceeding 50,000+ running hours.

As far as ducting your wood burner into your present air handling system I don't see any problem with it. Many wood and coal fired furnaces are designed to work specifically that way but it mostly comes down to proper blower capacity needed to make up for the additional air restriction that the ducting may impose.

I don't see why a DIY conversion would change much if you did it correctly.

That my thoughts any way.

 

[More Coffee]

I use wood also.And Ive been fortunate,my fan is going on 4yrs old ,All i do is use a paint brush to clear the fan and put it back on ,

It doesn't have bearings ,just bushings.One thing Ive noticed is ,people who have to replace their fans almost yearly to bi-annually are the people who insist on oiling the bushings.It only collects dirt and drags the motor out to the point it just fry's. Leave them dry,but some people cant be told.

When I was on natural gas and had a new furnace installed..I had to get the guys back to install a booster fan to get the warm air up stairs to the 3rd floor.

Just a thought..booster fans are available it may be cheaper than trying to DIY it in the long run .

 

[gabeNC]

Thanks for the suggestions guys. I think for now i'll go with the muffin fan option. I posted a similar question on a hvac forum and was crucified. I wonder if I failed to articulate exactly what I was trying to accomplish considering some of the reaction I received. In any case, I think the original idea has some merit but would require more R&D.

As far as ducting your wood burner into your present air handling system I don't see any problem with it. Many wood and coal fired furnaces are designed to work specifically that way but it mostly comes down to proper blower capacity needed to make up for the additional air restriction that the ducting may impose.

I wondered if that was an option but considering the layout of my house and proximity of the stove, ducting the heat would be unsightly and probably cause a negative pressure problem. Course you DID say "if done correctly"

Just a thought..booster fans are available it may be cheaper than trying to DIY it in the long run .

That's a good idea too, i'll look into that.

Well... here's to the difference between a plan and an idea. It's good to have people offer advice or shoot down bad ideas.