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Old 11-30-2010, 02:42 AM   #1
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Furnace Directly Over Ceiling Return-Noisier? How Much?


Hi,

My house is 120 years old. It uses non-dimensionalized lumber. This means that at best, there is exactly 14 inches between the studs. A 14" filter grill really won't quite fit. This is a new installation, currently the flat is served by a pair of old gas console heaters, 20K btu input each. This in in San Francisco, mild climate very seldom below 40 degrees. About 1500 square feet. 40K btu furnace, 95+%. A bit on the low side but the pair of gas heaters worked pretty well, as long as all the doors are wide open.

I'd like to use filters readily available locally: that more or less eliminates 30 X 14 or 30 X 12. I think 30x20 is my best choice. I plan on making a little frame out of 2x4 that hangs from the ceiling. The ceilings are 11' high so the little drop won't be noticeable.

I could use a pair of 12X24 but don't like that idea. Not sure it would work, anyway.

The return will be in the central hallway, with which all rooms communicate. For whatever reason all the doors are cut about 2" clear of the floor already, gonna try relying on that before I start cutting jumpers or pass-throughs.

Now here's the question for you. At that point my choices are:

1) To make a pan with a 14" round hole, use flex duct back to the furnace. 2) To make a pan with an oval hole for a bit less flow restriction, flex back
3) To place the furnace directly over the return grille, piece in to seal.

No. 3 obviously has minor cost advantages. Also, it allows for least restriction back to the furnace: about 14X24 (336 in^2) vs. 14" diameter (~150 in^2).

The question is, which config can I expect to be quieter? I figure I'll get less "whoosh" from the direct-over config, but greater chance of direct noise from the motor: bearing and blade noise.

It's a bit complicated (or simplified?) by the fact that it's an ECM motor which I believe is CV as well. In fact, I think even with a traditional thermostat it continuously varies the blower speed by monitoring return air temperature. Do you think I can expect lower blower speed given the wider return path?

It's rated about 800 cfm "high heat" and 400 cfm low. So 14" D return duct is right on the edge. I'm probably going to go with qty. 2 10" and 1 6" supply ducts, perhaps qty. 2 8" and 1 6"

Anyone have experience/horror stories to share on this one? Am I missing an obvious, better solution? It may be possible to give it a try and if it's too loud, go with flex.

Thanks in advance, and take care always!

-Jeff

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Old 11-30-2010, 05:53 PM   #2
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Furnace Directly Over Ceiling Return-Noisier? How Much?


You'll get more noise if you put it directly over the filter set up you said you were going to make.

You flex, it will absorb some of the mechanical noise and make it quieter.

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Old 11-30-2010, 10:00 PM   #3
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Furnace Directly Over Ceiling Return-Noisier? How Much?


Well today I laid it all out and decided at least %80 of the time and materials could be "recycled" if I had to switch schemes. So I think I'm going to try it directly over if only as an experiment. I'll post my results, complete with dB figures.

We've had a heat wave here followed by a cold snap. It was too hot to work up in the attic 2 weeks ago, now I'm very eager to finish.... basically I want to turn the key Wednesday evening. At this point any hour I can save or even simply defer is precious.

-Jeff
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Old 01-19-2011, 06:00 AM   #4
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Furnace Directly Over Ceiling Return-Noisier? How Much?


Well, I went ahead with mounting the furnace directly over the vent and the results were fantastic. I was shocked at how quiet it is. 60 db directly under the return vent. Least noisy installation I've heard in a long time. The oversized filter has a lot to do with it I think.
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