I want to replace a noisy furnace fan with something better

[Stainless]

Hi,

I have an old tent trailer that I'm going to use as a source for appliances for my project, and among its parts is an Atwood furnace. I don't have the trailer in front of me, so I don't have the model number, but it's a 1990 vintage. The furnace works, but the fan squeals. Sounds like old, dry bushings. It's very annoying in the middle of the night.

So, I was thinking, why not replace it with a much more energy efficient and quieter computer fan? It's pretty tough to figure out what kind of CFM fan the OEM unit moves, but it seems to be ~100. If that's true, there's a long list of computer fans that would be as good or better. Plus, I boondock only, so battery life is paramount in all my planning.

Now, I understand that the OEM fan is probably a squirrel cage style, so the retrofit would need some pretty significant fabrication to make a new fan format work. I have no problem doing that, but I'm wondering if anyone has ever seen this kind of conversion done, or can think of any reasons why it wouldn't work.

Thanks!

S

[bobhenry]

I had an O2Kool fan that ran on 120 AC , 12 volt DC and on board batteries. When the circuit board died I simply stripped out the fan and wired it direct to 12 volts. I was able to reverse the polarity with a double pole double throw switch and had intake as well as exhaust. You may want to wire it backwards. In this mode it moves less air but still works very well.

I was in Goodwill Saturday and found 2 of them for $1.25 each These are $25.00 +/- fans new, so I was a happy fellow.

[Trackstriper]

Stainless,

I have no first hand experience with matters about your question. I did pick up an Atwood furnace from a forum member.....it's not installed yet....but I have wondered about the fan efficiency too. My pure guess on the matter is that the Atwood furnaces haven't changed dramatically over the years, probably the same basic shell for manufacturing efficiency, but with small updates as the years progressed. Here's a good place to start: http://www.atwoodmobile.com/manuals/furnaces/MPD%2032072%20SP%201.08.pdf

For a tent camper my guess would be that you'd have the small 12,000 btu unit (7912-11....perhaps an older variant). There may be some model identification within the case. The fans (assuming the control circuitry doesn't draw much current) pull 3.4 amps, which has been the standard RV owners complaint in cold weather if operating on battery. They do make a more efficient 8012-11 which pulls 1.8 amps, but has no provision for running external ductwork.....which you don't need anyway. The higher-draw fan motors will support the additional pressure required for pushing air through ducts. But, Atwood does have a lower-draw, maybe higher efficiency, motor which can be used in some applications.

You'd have to ask Atwood for proper tech support, but the smaller motor of the 8012-11 might be able to be retrofit into your older furnace if it is 12K btu, maybe as a simple bolt-in. One kicker in the mix is that the motors are double-ended and run two fan wheels simultaneously, one to feed the burner air and the other to circulate room air over the heat exchanger. Dollars to donuts they operate at different static pressures. You might be able to replicate this with two smaller squirrel cage computer type blowers, but then you are changing the logic in the safety controls. What if one motor fails but the other continues to operate?

The root of the problem is that these furnaces are actually pretty well engineered, and have a lot of operational history behind them. There are safety components that help make the unit fail-safe if a component fails. Remember, we are playing with fire here....maybe fire while we are asleep. I would be very, very hesitant to experiment with a home-brew mod. Having said this, Atwood might allow for the retrofit that I mentioned above.

It would be dollar wise and safest to buy a new 8012-11 furnace if efficiency is very important to you. You could unload your older, functional furnace on eBay.

[lancew]

my concern would be that if you put in a computer fan that it would not produce enuff air movement and you would burn up your heat exchanger then produce carbon dioxide. you need good return of air as much as forced air out.

[Wobbly Wheels]

Yup, as long as you have the same CFMs or better.
FWIW, PC fans are available as squirrel cage as well. The one I'm using was pulled out of a projection tv I was scrapping.

[Shadow Catcher]

I have been able to prolong the life of sleeve bearing fans for many years by simply taking them apart cleaning and greasing the bearings. I have revived sealed ball bering fans by simply drilling a hole in the cover of the race and injecting grease or oil. The diagram I see looks like a sleeve bering.
For parts check out http://www.rvdealership.com/rvdstore/in ... ber=885555

[Stainless]

Yup, as long as you have the same CFMs or better.
FWIW, PC fans are available as squirrel cage as well. The one I'm using was pulled out of a projection tv I was scrapping.

Oh, you've actually done this, have you? If so, excellent -- you're the first person with actual experience that I've found. Any downsides so far?

I've been thinking that I'd buy a regular (non-squirrel) PC fan, then make a shroud out of sheet metal or plastic that funnels the air into the same passage that the OEM furnace fan uses to send air over the heat exchanger. By reducing the PC fan's output diameter like that, I'd also increase the air velocity (which would be nice). I hadn't thought of PC squirrel cage fans, so that's something to consider as well.

I have been able to prolong the life of sleeve bearing fans for many years by simply taking them apart cleaning and greasing the bearings. I have revived sealed ball bering fans by simply drilling a hole in the cover of the race and injecting grease or oil. The diagram I see looks like a sleeve bering.
For parts check out http://www.rvdealership.com/rvdstore/in ... ber=885555

Yeah, I've done that before as well (I've raced electric R/C cars for years and years), but these bushings sound like they're hogged out and the motor shaft is vibrating in addition to the bushings just being dry. I suspect she's been squealing for a long time. Thanks for the reminder -- I'll test the motor with some additional lubrication before I dive into a fab project.

Anyone know how many DC amps a typical Atwood furnace fan draws? The trailer is stuck in a snowbank, so I won't be able to actually dig into this until we either get a good melting spell (hello mountain chinooks... where have you been this winter?) or I develop some ambition with a shovel.

S

[Wobbly Wheels]

I haven't done the furnace mod, but I've replaced squirrel cage fans with PC fans in heat exchangers in older boats built when PC fans weren't available. As long as there's enough airflow, they don't care where it comes from....so far, anyway
It might take some tinkering with a few fans to work out to be able to reduce a PC fan's output (square inches). When I've tried to install bigger fans (more CFMs) than what was installed, the fan speeds up - I assume the blade is cavitating from pressure because the air can't get out fast enough. That was going ridiculously oversize from what was there originally though....

I'm using the 12V squirrel cage to duct in fresh, cool air from under the trailer. It's just a vent fan.
It has a ~1 1/2" square output, so you would be enlarging it to mate with a 3"square (IIRC) inlet on the furnace.
This is my salvaged 12V fan and the diffuser that will go into the galley cabinet. The pipe it's sitting on is a scrap of 3" ABS just for fitting.