I am not a fan of those fans either. Smaller doesn't equal quiter,quility equals quiter and quility equals more expensive. If when you mention sizeing 2 ducts to room in question,I assume that is a supply and a return in which case I would check carefully to make certain the return is working. My first thought is that if the origional installer went to the trouble of a return,they probibly engineered the system to avoid the type problem you are seeing. Could the return be smashed,leaking or partialy restricded by sharp bend? I'm also assuming flex duct here. A couple of things to put in the mix is that air circulating all the time will pick up heat from the attic so I would connect the booster to the air handler fan relay so it only runs when the main blower runs. Since you are spending money for a fan anyway,you have the option of a wall intake near floor in main room and out flow vent near ceiling of playroom using stud space as a duct.

I installed one on a vent at the farm because the living room was cooler with the wood furnace going than the rest of the house. I ran a 6" booster fan with a dimmer control rated to handle the current and it only runs when the blower is active on the wood furnace and only at enough speed to keep the room the same temp as the other room. I know others that have used an old baseboard heater stat in the affected room also and wired it so the stat only has power when the blower is active.

The booster fan has been in place for 20 years and still operates fine. Controlling the speed with the dimmer makes the fan barely audible but makes a large difference in airflow. With no dimmer it roars. The dimmer is located in the basement by the furnace and after the initial adjustment has not needed to be re-adjusted.

I think adding more return air and supply is the best solution but when that is not possible a booster is a solution and with some additional planning can be nearly silent.

I installed one on a vent at the farm because the living room was cooler with the wood furnace going than the rest of the house. I ran a 6" booster fan with a dimmer control rated to handle the current and it only runs when the blower is active on the wood furnace and only at enough speed to keep the room the same temp as the other room. I know others that have used an old baseboard heater stat in the affected room also and wired it so the stat only has power when the blower is active.

The booster fan has been in place for 20 years and still operates fine. Controlling the speed with the dimmer makes the fan barely audible but makes a large difference in airflow. With no dimmer it roars. The dimmer is located in the basement by the furnace and after the initial adjustment has not needed to be re-adjusted.

I think adding more return air and supply is the best solution but when that is not possible a booster is a solution and with some additional planning can be nearly silent.

Klem1

the duct booster fans are mid range, would be expensive
but I get contractor discount @ supply house.
the 6" is smaller, we installed it today. the 8" you
could hear downstairs...the 6"..not until you are in the room where the supply with the duct booster is located on the other side (small kneewall attic) of the wall.

ducts are all hard pipe. two supplies in the room...with
duct located inside small foam encapsulated attic @ rear of this room.

the return is in the middle room where the heating system closet is located. furnace is in closet, supply plenum penetrates into another foam sealed attic,
return is below furnace under heating platform.
no supply or return restrictions...

I would never ever use a stud bay as supply or
return. too much leakage.
there is no heat gain from attics...as they are
all foam insulated on roofline.

that said..when we got on the job this morning.. I
took temps of all three rooms, playroom was 4
degrees warmer. on & off during the morning...it held
the same temp difference.
by lunch the 6" booster fan was installed.
we turned the unit on...and took an hour and a half
lunch.
when we got back...temps were within a degree of each room.
homeowners will be back tonight and will keep me informed as to if this holds true for a longer period of
time.

countrybomo...good to know. I've always been against
these fans, mostly because people add a booster fan
to a 30' run of flex. but in this house to just even out temps...I"m hoping it will hold within a degree or two of
the other rooms.

this has been a growing project. you'd love it countrybo...
started out just sealing ducts, supply boxes & heating closets. my signature seal of returns..caulked, then ductboard on walls & floors of return...and mastic seal
of ductboard. caulking platform to seal heating closet from return...

we had to cut access to the attics...between dormers..
and supply plenum of upstairs unit.
once I got into the small attic with supply plenum there
was an ugly 30+ year install problem.
the top of the supply was held on by 4 pieces of
duct tape. the top cap had 2 supply ducts taken off
...the ones for the hot upstairs room btw..
the hard pipe duct was 1" away from the plenum with
tabs bent over to put a screw or two in to keep the duct
somewhat attached to plenum.
I could put my hand inside the top to side of the plenum..
and at each duct take off...2 fingers gap all the way around.
inside was the worst buildup of mold that I have seen in
15 years. gross stuff.

I spend a morning inside the plenum after pulling the
insulation out, scraping fg & old glue off the inside
of the plenum. getting in & out of the plenum was almost
as much fun as squatting on the coil to scrape the plenum...LOL! thought I'd cut myself in half!

once the inside of the plenum was clean...I painted mastic inside the plenum to encapsulate any fg particles
& glue residue. then we unwrapped each hard pipe duct,
screwed the pipe to the ells @ boxes..paint mastic again.
end of that day.

the next morning...I wrapped all ducts with R-8 stapled,
taped seams with fsk...and stapled fsk tape. externally insulated all supply boxes & supply plenum with duct wrap.

my rough estimate was that 1/3 of the capacity of the unit was lost in the small attic due to the shape of the plenum & supply ducts.
returns were also leaky...and the walls with the returns
(each system has 2 returns @ bottom of heating closet
as they are on corners...1 return in 2 different rooms)
the walls were open stud bays above the retrurn air filter grills. so I used ductboard & hardcast 1402 to seal the stud bays..and then replaced the r/a grills.

those filters POP when you put them in now.
the difference that the 2 days of sealing & insulating made was really noticable today...after system running last night. for the first time it was actually chilly!

sadly...the first floor supply had its own major issues.
furred down hall way with mold where plenum joined @ coil, and first bend in fur down.
had to take out insulation there too and replace with
ductboard. luckily it was at an area that I could lay over
the coil & scoot into the 8x16" supply duct. this was probably the tightest area I've ever worked in...
took me 2 days to do the work & get the plenum reconnected to the coil.
the essen drives only caught one section of supply
plenum...so these areas had leaked also since day of
install...30+ years ago.
if there hadn't been 2 of us on the job...I'd probably be stuck inside the supply plenum until the homeowners return!
(bet it would make the newspaper!)

2-3 years ago I did some diagnostic testing for the homeowner.
blower door tested the house & tested for duct leakage at that time, recommended foam sealing attics, sealing duct system, etc. homeowner is now acting on those
recommendations. his mother told me that the july utlilty bill was ...$700. wow. so...I'm estimating cutting that in
half.
the bulk of the air infiltration was from attics.
duct leakage was high. 5.5 tons of air
(two ton upstairs, 3.5 ton downstairs)
with 825 cfm of duct leakage.
I don't see where I seperated the amounts
of duct loss per unit in my paperwork...but
I know I did...just didn't break it down for the
software.

how is your attic project comming?
keep me informed as to where you are with it.

thanks to both of you for your replies.

WOW thats going to make a huge overall difference. My attic project is on hold for the time being because my brain will not stop.