Regular Member
I ask the question because I am in the process of having my ductwork enlarged. The 0.5 max water column static pressure is what is listed on my nameplate and from what I can tell is the same max for many systems.
I have a new Trane 4-ton XL15i/ 100K btu-XV95 system in a 2800 sq. ft. 2-story home in NJ. With some key dampers closed the system works great but the static pressure is sitting at 1.1 with a new filter (positive =0.8 and negative = 0.3). I decided to have some ductwork done to get the SP down to nameplate or below to save some energy cost and not shorten the life of the motor.
My Duct guy said it is unrealistic for people in most homes to think that they can get the 0.5 SP or below. He said some want to get to 0.1 so they will only be using the "75 watt light bulb" amount of energy that the salesman sold them on. He said when you consider ~0.25 for the coil, ~0.2 for a good filter that only leaves you 0.05 for everthing else. He said it is no way that with grills, elbows, turns, etc. no matter how gigantic you make your ducts you will ever get the 0.5 or below value.
Do you guys agree with that? Or is it that my guy just does not know how to get there? Why would the manufacturers make 0.5 the max if it is unacheivable in most homes?
Key1
Professional Member
I have a number of transitions that are creating higher static and branch dampers on all the branches. My 3 ton system has the following:
1,202 CFM (measured)
Supply .14
Return .08
Filter .125 (20x30 media at filter grill)
Total .345
I think it can be done.
God Speed, Jim. We'll miss you - 2-26-23
.5" is unrealistic for many existing homes. And higher tonnage coils/systems.
But, your can be lowered from tha 1.1", so you can get more use out of a media air filter.
1.1" is TOO high.
Regular Member
What size are your ducts?Originally Posted by mchild
Do you have an AC coil?
Key1
Professional Member
As all women supposedly say - size doesn't matter.
Your duct system will have to be designed for your situation and no two (or at least very few) are alike. Having said that, the trunk (in attic) is 20x8, it splits into two 8x10 to go around an obstacle and includes several tight 90s (the split and these 90s are the biggest static creators). That part is all metal ducting. Branch runs are six inch flex stretched tight and straight. Any turns required on the branches where they come off the truck or into the boot are done with metal adjustable ells. Return is 22 feet of 18" flex with two 90s and a somewhat funky return plenum (a bit of a static creator).
System is a heat pump - no separate coil.
God Speed, Jim. We'll miss you - 2-26-23
mchild.
Is yours a furnace, or an air handler?
Professional Member
Heat pump air handler.
Banned
If space is available,and ducts are not buried inbetween floors or walls,.5 should be doable.
Tell us more about your duct system.
Pics of the indoor unit and atteched ducts would be a good start.Often improving the fittings near the unit makes all the difference you need.
Regular Member
I'm sure it is possible but I don't think it is realistic (when a coil is involved). If the othe poster "Mchild" had a coil contibuting ~0.25, he/she also would be over the 0.5 max.
My duct work is already underway but when it started, the duct guy cut a 14x14 hole in the front of the plenum for a 14x14 additional trunk. We checked the static pressure at that point with no ductwork attached coming out of the front of the plenum (just a big hole) and a 12x9 trunk coming out the top (unchanged) and a 8x9.5 trunk coming out the back of the plenum (also unchanged). The static pressure was 0.79 (versus 1.1 when the 14x14 hole was not there). Next, we opened the filter door and the SP dropped to 0.57. So with a big 14x14" hole in the front and the filter door wide open only 0.57 was acheived. That is why I beleive the 0.5 max is unrealistic.
Key1
Banned
14X14 hole, 12X9, and 8X9.5 trunk isn't much in they way of supply duct,if that's what you have,for 4 tons.
What size system did you have before?
At what "points" on the equip./duct is the static being tested?
What's the supply and the return static reading?You are giving us the total of both,I assume.
God Speed, Jim. We'll miss you - 2-26-23
On 4 ton A/C systems installed on furnaces.
.8" static is usually not too hard to attain. Some times when a 5 ton evap coil is used, .9" is doing good on a swap out, with duct alterations.
It depends on what you have to work with. And how much money you are willing to spend.
Regular Member
Positive static is being checked with a magnehelic gauge after blower before coil. Neg static is beging checked in blower bay after filter. Before any duct work was started static was 0.8 pos and .3 neg. for 1.1 total. I do not know what is was on my 30 year old GE system because I and apparently neither did my installer know anything about static pressure.
My return duct is 24 x 10 entering the blower bay
Key1
Regular Member
I have room to also expand the 8x9.5 to 16x8. Do you think that will be enough or is more still required?
Key1
Banned
It would help,but can't say how much,or how it will effect the room by room air flow and resulting temperatures.If we were on site and tested,we could predict the performance,can't tell from here.
Pictures of the supply plenum and trunk take-offs could tell a lot.Often these can be improved to reduce the static.
Here's a sketch of a furnace,coil and plenum,showing how adding vanes or an angled take-off can reduce the static.This was for another thread ,a while back.In your case you'd want to improve all the trunk take-offs ,not just one.
God Speed, Jim. We'll miss you - 2-26-23
He posted pics in another thread, a couple weeks ago. I think you posted in that thread also.
Regular Member
The thread with all the pics and details is called "Finally getting Static Pressure checked" by key1cc.
I don't know how to link it but if you do a search-then advanced search- then search for poster key1cc you will find it.
Key1cc
Regular Guest
http://hvac-talk.com/vbb/showthread.php?t=172134
God Speed, Jim. We'll miss you - 2-26-23
Regular Member
Well the work has already started and it is divided into three stages.
Stage 1 is to add the new 14x14 trunk which transitions to a 16x 12 then a 16x8 before linking up with seven 5" round runs which will be converted to 6" rounds. After that is done The static and air flow will be assessed then....
Stage 2 will be considered which will widen the 8x9.5 trunk to 16x8 and all five of the 5" round runs will be converted to 6" round.
Stage 3 will be to convert the 24x10 return drop into a 24x16 return drop with a 5" media filter cabinet.
Hopefully all of that will get me down to my realistic target of 0.7 or less.
Key1
God Speed, Jim. We'll miss you - 2-26-23
Don't forget to install balancing dampers.
Posting Permissions
Reply