Cutting down on fan noise from inline booster fan

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Old 07-09-08, 06:19 PM
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Arrow Cutting down on fan noise from inline booster fan

I added a 330 s.f. addition with vaulted ceilings. So now I only have one register servicing about 500s.f. and it's the farther part of the house from the return. It wasn't very effective, so I installed an inline boost fan. It works alright, but it's pretty loud and it's pretty distracting trying to watch TV when the A/C is on.

Does anyone know what I can install to cut down on the noise coming out of the register? (Links appreciated) Any other suggestions?

Thanks!
Mitch
 
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Old 07-09-08, 07:24 PM
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You could install a properly sized duct system with the appropriate registers. That would also remove the necessity of the duct fan.
 
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Old 07-09-08, 07:45 PM
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Furd gave you the only correct way to do this!
 
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Old 07-10-08, 11:53 AM
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:/

Thanks guys, very helpful. <sarc>

I know it's not the Right Way To Do It. Does anyone have any useful advice?
 
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Old 07-10-08, 12:23 PM
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I didn't think that you would like my answer but you have now learned a valuable lesson; that there are ALWAYS little details on every job that a professional will (or certainly should) understand that few DIYers will have a clue about. The time to discover your potential mistakes is BEFORE you start swinging the hammer. That's one of the reasons that Internet forums like this one exist, to help people before they make expensive mistakes.

When I designed the heating system for my previous house I had a living room of about 216 square feet. I installed three 4x12 floor registers, each fed by a 7 inch diameter supply duct. I also had an independent return air duct in the ceiling. You, my friend, had a room slightly smaller fed by one supply duct with no return duct. Then you proceeded to almost double the size of the room with no thought to the HVAC system. Any competent builder, engineer or architect would have caught this error in an instant.

Unfortunately, there is NO easy fix for your problem. You could install electric baseboards for heating and you could install windowbangers for cooling. Neither is what I would call a satisfactory solution.

Sorry.
 
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Old 07-10-08, 10:56 PM
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Yes, well, note that I didn't ask how to get enough air there... I asked if anyone had any tips on deadening the noise....
 
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Old 07-11-08, 06:01 AM
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Remove the van and install the right duct.
 
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Old 07-11-08, 10:14 AM
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Originally Posted by mopo999 View Post
Yes, well, note that I didn't ask how to get enough air there... I asked if anyone had any tips on deadening the noise....
No one here is trying to be sarcastic or rude, just trying to help everyone learn the correct and most effective way to get the job done. Although it is a pain to have to fix the ductwork, it is truly the route to avoid the noise and insufficient heating and cooling.
 
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Old 07-11-08, 01:14 PM
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Originally Posted by mopo999 View Post
Yes, well, note that I didn't ask how to get enough air there... I asked if anyone had any tips on deadening the noise....
The noise is because of too high a velocity of air flowing through the duct and register. The only way to reduce the noise is to reduce the air velocity and the only way to do that is to either reduce the volume of air (which will reduce the heating / cooling capability), increase the size of the duct and registers or increase the number of ducts and registers.

It is a fatal design flaw.
 
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Old 07-11-08, 02:11 PM
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Is there a way to tell whether the noise is from the ducts/registers or the fan. My impression is that it's the fan, but any advice? Like if I split the duct in two and add another register, will that help ?
 
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Old 07-11-08, 08:42 PM
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Remove the register and see if it is quieter. If it is, and you can live with this noise, you can take a branch off of this duct and then install two registers. I don't know where you installed the booster fan but the farther it is from the room the quieter the fan noise will be. If you want to branch off this duct as I described then I would suggest a Y fitting and that its inlet be no less than ten duct diameters (assuming you have round duct) from the fan discharge. You want the two branch ducts to be as close in developed length as possible.

You already know that the duct without the booster fan cannot serve the room so the other option is to either add more ducts from the main plenum of the furnace / air handler or increase the size of this existing duct. You may have to increase the blower speed on the furnace / air handler and you will most likely need to "balance" the air flow through all your ductwork.

Do you have balancing dampers already installed in all branch ducts? It may be possible to increase the air flow through this one duct (without the use of the booster fan) by slightly closing down the air flow to the other branch ducts by slightly closing the balancing dampers. This will be a time-consuming process and it may severely upset the way your system responds to the thermostat.
 
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Old 07-12-08, 09:53 PM
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Thanks. Well, the booster is pretty far (25') from the register, and it still makes noise without the register. I think maybe the airspeed with the booster is just way fast for the duct. What if I switch to a bigger duct?

In general the system doesn't work too well now because the front half of the house has too much air and the back doesn't have enough. It probably like this:

-front two bedrooms, and living room, all about 10' of 6" duct (ea.)
- master bedroom, about 40' of duct. it works pretty well, so it's probably 8". Also there's a register in the master bath which is probably helping.
- dining room, 20' of 6"
- new family room, 40' of 6" incl. booster... not working so well, and loud.

So, what would fix this? Say I re-ducted the family room with 8" and two registers.... does that seem like enough? How do I do the math?
 
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Old 07-13-08, 09:14 PM
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How do I do the math?
That's why engineers make big bucks!

Seriously, the proper method is to do a complete heat loss/heat gain calculation and that is rather involved. It will tell you how much heating or cooling is needed on a room-by-room basis and then you would need to do a duct design calculation to determine the proper airflows in each room and the number and size of the necessary ducts and terminal registers.

There are a number of such computer programs available and they all cost big bucks if you purchase them outright. There is at least one company that will sell a "trial" session of their program for about $50. that has a limited time period of use. This one is recommended by some of the members here:

http://hvaccomputer.com/

This would keep you busy for several hours measuring your walls, doors, windows and trying to make an educated guess as to the type and amount of insulation, weatherstripping and such. It would give you some real definitive answers but it may also point out that your entire system has some serious deficiencies. You definitely need a larger duct to your family room. Your current 6 inch duct has a cross-sectional area of about 28.27 square inches and an 8 inch duct will have about 50.27 square inches, an increase of about 80% so you can see that it will flow a great deal more air at a lower velocity.

Would a single 8 inch be enough? I don't know and it has been close to thirty years since I did duct design and I would have to hit the books (which I no longer have) and I would have to have a lot more info on your house and the local environment before being sure of my answer. However it would certainly be better than the 6 inch you have now.

Since you have stated the house is already unbalanced in its airflow I would start by installing the 8 inch duct to replace the existing 6 inch duct. I would install balancing dampers at every point where a branch duct leaves the furnace plenum or main trunk duct. Start with all the dampers wide open and see which rooms heat (or cool) the fastest and then close down on the dampers to these rooms about 25%. Let the system run for a while and then see how things have changed. Continue to close down individual dampers until the rooms all come to an even temperature in about the same time. You may need to go back and open some of the dampers a bit if the rooms they serve become too slow to respond in sync with the others. Ideally you want the dampers to be mostly open and only those rooms that respond too quickly will have their dampers more towards the closed position.

You may need to do this with both the heating and cooling cycles separately. If so then you will need to mark the damper positions and change them manually when you change between mostly heating and mostly cooling seasons.

I know this is a time consuming method but it is also inexpensive. You will need a good thermometer to carry from room to room to check the temperatures, I would recommend an electronic type for about $50.
 
 

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