Balancing Duct Air Flow?
I am replacing an existing furnace/AC with a new HP/furnace of pretty similar size. I have been wondering about the significant difference in flow to various registers. The current duct work basically has a major duct that goes left from the furnace and another that goes right - both in finished soffits. Each of these major ducts feeds one side of the house or the other in this 2 story house there are mini-trusses between the floors.
Initially this didn't make much sense to me. However, on closer examination I notice the the duct with greater apparent flow appears to come directly out of the furnace/coil and feeds the right side of the house. The other duct comes out at 90 degrees to the first and within a couple feet makes a 90 degree turn to pretty much go 180 degrees in the opposite direction of the first spur as I would expect.
Is it probable that this 90 degree turn has caused a major imbalance in the air flow to the weaker, left side of the house? I would appreciate it if somebody could help me to understand if it is possible with out a great deal of add-on work and damage to finished areas to at least balance the flow in each major duct.
There are 2 things I try to do on any indoor unit change out...
1) Improve the airflow by reducing the equivilent length of the near unit ducting.
2) Provide a varriable speed blower in place of a standard ( this is a " given " ).
The ducting can be improved by the way the " main branches " take off from the " plenum " ( usually a " box " ). Instead of leaving the duct just " cut in " , cut it back and add a transition at this point ( the more area cut into the " box " the better ).
As far as the 180 degree turn...is there a better way to take off from the plenum ? Again...transition take-off's in this area make a world of difference.
Not to be used as a " crutch " ...manual volume dampers in these main ducts can help direct the air as well.
And if you have enough room....fabricate a " plenum " tee with throat radi so large that your coeffecient is a negative and you actually " gain " volume and velocity . just kidding you know
Last edited by tech45; 06-12-2009 at 07:36 PM.
tech45, appreciate your input. Not sure I understand 1). Are you possibly suggesting moving the coil ducting closer to the 90 degree turn?
Originally Posted by tech45
The new unit is a York Affinity 4-ton HP with 100,000 Btu, 80%, VS, modulating furnace, so 2) looks OK.
Not too up on air flow or duct work but are you suggesting that a coil "box" with different dimension feeds to affect flow might help?
The 90 degrees out of the coil and 90 degree turn to left seems a necessity due to the tight area for the furnace/coil and hot water heater/circulating pump plumbing immediately to the left.
I was wondering if dampers would help but afraid it might upset the whole flow out of this 4-ton unit. Is this solution viable and still in the running after I have provided additional information?
The current furnace chimney and the coil drain are in the space in front of the current coil, but there is an additional 8" from the current coil to the dry walled surface where the 90 degree turn occurs. However, I understand that the new, more efficient, 5-ton coil is larger. Not that I understood your suggestion but maybe I have flunked the "And if you have enough room" criteria, right?
Any others have suggestions?
I had " added " the ..." just kidding " as an edit , lol. But yeah...when your out of room it's all it can be I guess. With all other options eliminated just remember that duct size and resistance determine how much air will flow through it. Sometimes the unit can be " turned " , or some other " obstruction " moved ...to benefit all. Sometimes the only obstruction is the mind. ( edit : wording )
I've been watching the original " Kung Fu " ...too much perhaps , lol
Last edited by tech45; 06-12-2009 at 08:43 PM.
I found the VS fan didn't make any difference. It took adding manual dampers to each supply banch to get my home balanced out.
Thanks, motoguy. Can manual dampers overcome 90 degree turns like I described? Did you find that they hampered the blower in any way by restricting flow?
Originally Posted by motoguy128
Turning vanes in the elbows, along with the manual dampers, assuming you can't get the duct straightened out.
Originally Posted by jerryd_2008
The new furnace has a pretty robust 1 HP VS motor (2000CFM capable) and the vents are delivering lots of air. The contractor did say he could look at balancing but cautioned that manual dampers would be about the best that he could do. unfortunately the supply ducts are in finished soffits and this would require some dry wall destruction. We'll look at what's coming out of the registers and the new system capabilities before going there.
Manual dampers can overcome anything... but yes, fundamentally if you are restricting a duct, the air is no longer taking the path of least resistance... so static pressure will increase. It takes the added pressure to force air to longer or undersized branches.
If would be handy if someone would design a restrictor that could be installed in the boot of a supply register to reduce airlfow without creating noise... in cases where the start fo the branches are not accessible... such as Jerry's.
Perhaps simple sheet metal orifice plates with different ID's that you could slide into the round end of the boot. Maybe have 3 tabs on the outside diameter to keep it in place. An orifice plate would be the same as using an iris valve.
Stamped metal or plastic, they would be very cheap. Or could be made by hand with a round sheetmetal cutter.
You should be able to use some HVAC calc software to determine what ID each plate needs to be for each register... or you could use simple trial and error.
Manual dampers are not a cure-all for bad duct design and installation.
Originally Posted by motoguy128
It's also inaccurate to state "the air is no longer taking the path of least resistance". It is far more accurate to state that any increase in static pressure seen at the blower outlet reduces the volume of air delivered by the blower wheel. The wheel has a performance curve, and will follow the fan laws accordingly.
Check out the fan laws...here's a link:
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.