booster fans in 6" metal ducts

[rkthomas13]

Does anyone have any actual experience trying to increase air flow in a 6" hard metal duct using booster fans? My dining room gets air from a 25' long 6" duct, but it only as two bends, one at the start above the master duct and the other coming up through the wall. The actual gas blower is 4 tons and should be enough, but the force at the end is weak. Were you able to get a better air flow?

[DavidInAustin]

Duct design is critically important - On your plenum (if a spider duct system) - is there a supply duct at the opposite end of the furnace output? By this I mean if there is a supply duct at the direct opposite end of the output of the furnace (whether it be a spider system or supply trunk line) - move it to the side of the ductwork and cap the end of the duct work. You could be losing a significant amount of static pressure that would be critical loss in the longer runs of a duct system (in a properly sized duct system). If you eliminate the obvious problems first, as a last resort, add the fan.

[BaldLoonie]

We've done it, to no success.

[hvaclover]

They don't work. Expierenced the same prob. Turned out I had a bryant frnace that could be equipped with a 1/4, 1/3, or a 1/2 horse power motor. Blower)

I have 2500 sq ft and when I removed the the duct booster I happen to check my blower. It wa on a 1/3 so I bumped it too a 1/2. Got better airflow but still not as good as rest of house.

[Kevin O'Neill]

Does anyone have any actual experience trying to increase air flow in a 6" hard metal duct using booster fans? My dining room gets air from a 25' long 6" duct, but it only as two bends, one at the start above the master duct and the other coming up through the wall. The actual gas blower is 4 tons and should be enough, but the force at the end is weak. Were you able to get a better air flow?

The propeller fans are nearly useless. The backward inclined booster fans work well.

But it sounds like you have a bad duct design.

If the ducts are accessible enlarging the trunk and branch duct will improve the air flow.

[craig1]

I installed a propeller type booster fan in my house about 6 years ago and it worked great. this was for a small bathroom that had about 20 CFM coming out of the duct. fan boosted it to around 50. still awful, but the bathroom is plenty warm now. i think the duct separated in the wall, but there was no way to find or repair it.

i don't know how much of a difference it would make to a room like your dining room that has a bigger load

[docholiday]

20-50 is certainly possible if the duct's problem was low supply static or the take off was in an eddy of sorts. Increasing one with a propellar type fan that actually blocks as much volume will hardly get you from say 70 cfm to 100.

[rkthomas13]

Would anyone be willing to look at a fax of the diagram of my ductwork and suggest where I might try to add booster fans, maybe spider cage fans or whatever to get more airflow on the other side of my house? I just can not find any hvac tech here who is willing to take on this kind of trial and error work. I can add the fans, but I need a pro to tell me what size and type to add and where.

[dash]

Sure 352.795.6601

[rkthomas13]

Thanks. I shall post the diagram, (which I did myself, so not very pro), today. I also have digital pictures of the ductwork itself as well as the furnace, the steel supports, the joists, etc., which I could email to you so you have a better picture of my situation.
I have tried to find a pro here with real experience of using dampers, booster fans, etc. to augment air flow and even out the heat, but so far have no really found anyone. I found plenty of guys who want to put in a new heat pump in the attic, but no one who has real experience making the existing system work better.

[classicrock4you]

If you call tech support for the blower manufacturer they will tell you where to put the fan for best performance. I did it and got 50% more air when I placed it where they suggested ( 2/3 of the way down the line) compared to where I placed it the first time (1/4 down the line).

[kwilcox]

Make sure you get a real duct fan if you go that route. Here's two good ones:

www.fantech.net/fkd.htm
www.atmosphere.com

[rkthomas13]

Thanks for the help. I shall call the tech support of both fan makers tomorrow. Why would their fans be better than the propeller type sold at HD?
Can you suggest any other path? Local pros have recommended a new 2 ton heat pump in the attic with flex ducts to each room on the top floor, cap off the existing ducts and then experiment with the existing ducts for the basement and lst floor. This seems like a lot of duplication. And they do not want to work on the existing 150 btu system until the new one is in. But I blocked off the top floor vents and it only improved the air flow to the other side by a little. I still have rooms that are 12x14x8 serviced by one register that has to travel 36 ft. plus 3 right angle bends (or more than 66 ft. with each bend equal to 10 ft.).
I want to try dampers and booster ducts first unless someone has a better idea. Any suggestion would be much appreciated.

[dash]

The fans suggested have more static capability (able to overcome more resistance to air flow),then a propellar type blade.You seldom ever see a propellar type fan blade conected to a duct.

You saw little improvement when closing off the upstairs grilles,but we are not sure your existings system is clean,and moving all the air it should be capable of,could be dirty coil,blower wheel,etc..

[rkthomas13]

Thanks. The machine is brand new, so it is unlikely to be a weak motor or dirty coils and we cleaned the ducts out when we were working on the house. At the time I knew little about ducts and simply assumed a system that had been there 30 years must work okay.
Now I can see partly why it does not work. One of the trunk lines starts after the first bend from a 12 x 6 and then goes to a 10 x 8 rather than the other way around. Then it has to do 3 90 degree bends and go another 25 feet before exiting in one of the 2nd floor bedrooms. I am thinking of dismantling this part of the trunk and keeping it 12 x 6 the whole way. Does this seem like a good idea? It would be costly, because I would have to tear out new drywall, but somehow I have to solve this.
I bought a $30 Equalizer from HD and put it in the 12x6 register in the worst bedroom. The cfm went from about 23 to 55 which is still not enough to heat a 12x14x8 room, but still it was an improvement.

[dash]

12X6 is the same resistance to air flow as a 10X7 ,so 10X8 is less resistance.

I'd look at installing turning vanes in those turns(90° elbows,I assume),they would reduce resistance to air flow.

You could try running flex duct from the larger portion of the trunk,to a point as far as you can go in the 10X8.That's called a helper duct.

Posting the diagram of the duct would help a lot.

[rkthomas13]

I did not know one could post on this website anything but messages. I shall post the diagram (as soon as I figure out how) but need to add the location and direction of the joists and main supporting beams, because they are probably the cause of the duct design in the first place.
I also have digital photos of the system that could be added. Would these help?

[dash]

I did not know one could post on this website anything but messages. I shall post the diagram (as soon as I figure out how) but need to add the location and direction of the joists and main supporting beams, because they are probably the cause of the duct design in the first place.
I also have digital photos of the system that could be added. Would these help?

post the pics!

[rkthomas13]

These photos are not of my house, but a neighbor's, which has exactly the same system. Mine are unfortunately covered in drywall.
1. photo shows the central support beam and joists which are 16" on center. In the background you see the rear trunk line that turns up to split and feed the kitchen and, above that, the back rear bedroom.
2. photo shows the furnace, plenum and one trunk line before it makes a right angle turn to go down the length of the house (30x36').
3. photo taken under trunks just outside plenum shows beams plus the large return ducts.
4. shows a branch of the trunk plus one 6" that service the living room and the masberbedroom on second floor. These lines have plenty of force.
5. Shows rear trunk line coming over the return just behind it and coming out to go to kitchen duct.
This is the maximum 5 files I can send on this post. I shall try to send others on the next post. If I can find a way to digitize my diagram, I shall post it also. That would probably do the most good.

[propmanage]

Everything you are doing is just a guessing game. You need to have a balance person check this thing out and determine if you do or do not have the numbers necessary for supplying the building.