View Full Version : Static pressure question
I thought this was interesting...
Infinity four ton furnace and performance air conditioner. Generally circuitous ducting.
With the infinity control on the "comfort" setting, getting 1400cfm, about 1200 rpm, .95 static pressure. On the "maximum" setting, getting 1600cfm, about 1270 rpm, .85 static pressure.
Intuitively I figured the higher cfm rate would result in higher static pressure.
Seems to not be...
I'm really curious as to how this works, any ideas?
beenthere
05-11-2008, 07:31 PM
Something moved out of the way of the air flow.
jrbenny
05-11-2008, 07:43 PM
Something moved out of the way of the air flow.
Could be less water on the coil.
Could be less water on the coil.
All I did was change the setting on the thermo, so I dont think anything got out of the way.
Coil wise, our area humidity has been running around 35-38% (california central valley near sacramento) and the in-home humidity is around 40-41%. The AC had been running for around 20 minutes (outside temp 84, inside temp 75, thermo set to 74 to get it to run), then I made the change, the reported airflow and static pressure reading change -.10 was immediate.
Its been off for >30 minutes and I just kicked it back on. Same .84-.85. When the system was reinstalled from the ground up with a clean infinity eac, the static pressure was .92-.93 and I imagine after being disconnected and sitting on the garage floor in 80 degree weather and ~35% humidity the coil was pretty dry on startup.
The ducts are tough though. There are 3 returns that all join to a plenum under the unit and take a hard right turn up into the upflow unit. On top of the coil there is maybe a 3" transition to a supply plenum box thats about 9" tall and 2'x3', which has three branch pipes off of it to each floor. From each of those is 4-7 room outlets.
The old system had a physically larger cabinet (about 6-8" wider and deeper) so the installer put a piece of metal wrapped plywood over the old return plenum and mounted the system to that. But with the huge coil and infinity air cleaner they installer, the distance from the top of the unit to the ceiling is about a foot less.
I was initially getting a lot less air flow to 3 of the rooms in the house from this setup (the old was a 5 ton, 17 year old 100k btu payne, this is a 4 ton 90k two stage carrier infinity). The installer put some dampers in a few of the lines to balance the air flow out a little to solve that, but that didnt change the static pressure any.
Adding the 3rd return (and its a big one, about 16x16) didnt change the SP either, and when I covered it with a piece of foam the noise and air sound from the other two went up, but the SP reported by the infinity control stayed about the same.
beenthere
05-11-2008, 08:00 PM
Would have to be a whole lot less water.
If it was just the difference of a wet to dry coil. Thats a very restrictive coil when its wet.
JBM1000
05-11-2008, 08:49 PM
Sounds if it walks like a duck, quakes like a duck, it is a duck. Infinity units are gossiped to be the bomb of units do to its ability to judge static in one tenth of a second during a coast free wheeling after the first 45 seconds of ramping to speed. This is done by algorithmic analysis by the control board software. Hard data is physical data and is carved in stone to be the real figure in dealing with static/velocity etc. Sure many times even taking EXT static(Infinity Control) reads within a tenth of hard data taken. but this is due to a normally good system at .50wc EXT static. You've got other issues from leaving the existing ductwork although I know one would be almost impossible to expect replacing the ducts to upgraded new. Sorry, but You cannot fix stupid applies to the ducts. Above .85wc starts burning the DC capacitors that rectify back to AC voltage, so your looking at maybe a year past install warranty before they may bake out and fail costing big bucks to replace with it being on you to pay or argue back and demand results!7206
7207
I put it back to the comfort setting (1400 cfm) and its back to .95.
So either the infinity control doesnt accurately calculate static pressure, or theres something strange about this I dont understand where more air being pushed into the system results in lower SP.
I sure hope things dont start burning out in this on me. The old contractor grade system lasted 17 years and was still ticking along.
So past that I guess the fun question is if it was your system, would you run it at the 1400cfm/reported .95/comfort setting or at the 1600cfm/reported .84/maximum setting? Unfortunately that only works for cooling, not for heating or fan-only.
Another interesting tidbit: this was originally installed with an AS accuclean on the bottom of the stack. The accuclean was physically larger than the furnace cabinet and had a metal plate screwed on top of it to mate it to the furnace. The accuclean was about 1" larger all around than the opening in the return plenum, and about 4" larger than the furnace on top.
The infinity control reported the SP at about .85 with that in place. The contractor removed the accuclean and replaced it with the infinity purifier which is the same size as the furnace cabinet, with the return plenum reduced under the infinity purifier. As soon as they were done and I fired up the furnace, the SP was .10 higher.
So either the infinity purifier (I think its 16x25?) is .10 SP higher than the accuclean (which I think was 20x25?), or the change in cleaner cabinet sizes and where the reduction was placed resulted in the .10 higher SP. Either cleaner running on the low fan speed gave me about a .25 SP @~550cfm.
But for some reason stuffing 1600cfm through it results in a lower SP vs 1400...
skwsproul
05-12-2008, 12:49 AM
I thought this was interesting...
Infinity four ton furnace and performance air conditioner. Generally circuitous ducting.
With the infinity control on the "comfort" setting, getting 1400cfm, about 1200 rpm, .95 static pressure. On the "maximum" setting, getting 1600cfm, about 1270 rpm, .85 static pressure.
Intuitively I figured the higher cfm rate would result in higher static pressure.
Seems to not be...
I'm really curious as to how this works, any ideas?How are you calculating cfm is this from the mfrs specs or a true reading. Mfrs spec may not be correct for your app. More air less static something else would have to have changed, leaks, ducts pulling apart, filter moving out of air stream. Something does not make sense. Dampers opening against the airflow, I do not know just stabbing at possibles.
beenthere
05-12-2008, 06:10 AM
He's reading it at the Infinity control.
Thats correct. The airflow, rpm and static pressure are from the extra advanced setup mode on the infinity control.
skwsproul
05-12-2008, 01:06 PM
Thats correct. The airflow, rpm and static pressure are from the extra advanced setup mode on the infinity control.I believe that I would confirm those readings with actual readings not calculations then call the mfr.
"With the infinity control on the "comfort" setting, getting 1400cfm, about 1200 rpm, .95 static pressure. On the "maximum" setting, getting 1600cfm, about 1270 rpm, .85 static pressure."
My guesses:
1.Wet coil difference.
2.Increase in duct leakage ,with the increase in cfm.
3.Control is not reading correctly
Plus I want to bet the static below .7 in wc.
docholiday
05-12-2008, 02:48 PM
well that and the more air you try to cram through a coil the greater the pressure drop and at some point you can actually cause airflow to lower.
What amps is that motor pulling at a .95 static? I'll bet more than a PSC.
skwsproul
05-12-2008, 03:01 PM
well that and the more air you try to cram through a coil the greater the pressure drop and at some point you can actually cause airflow to lower.Excellent point.
JBM1000
05-12-2008, 08:34 PM
You need to know on what side of the equation is the problem? Return Air side, or is it the Supply Air side? Hard data taken by using static testing instruments and the right static probes will gain the first base line information needed to know where to look. If the R/A side is .70wc then game over & you have either starved sizing or blockage or pulling only on one side of the heater uneven with eddy's hanging all about causing voids which will some day blow out the wheel fin crimping to the hub and get out of balance and self destruct.
If it is S/A side then you have to know the drop dry and then wet, and then you also need to know the TESP *this includes the total length of ducts and all fittings static drops or adds, and finally the ATSP or available total static that is still usable and inline with design factors. Manufactures list the units they make mostly as an accepted .50wc and is to represent the maximum allowed, not the minimum. Staring at your infinity T/Stat display and making decisions or asking for them here is right back to getting the contractor to get with it to fix your problems or get Carrier reps to work for there pay check finally. Be prepared for it to be your ducts all along and is now in your ballpark and they bail including upholding the unit warranties etc.
7228
JBM1000
05-12-2008, 08:47 PM
I thought this was interesting...
Infinity four ton furnace and performance air conditioner. Generally circuitous ducting.
With the infinity control on the "comfort" setting, getting 1400cfm, about 1200 rpm, .95 static pressure. On the "maximum" setting, getting 1600cfm, about 1270 rpm, .85 static pressure.
Intuitively I figured the higher cfm rate would result in higher static pressure.
Seems to not be...
I'm really curious as to how this works, any ideas?
Listed at the bottom inside the pdf as page 74 or in the header as page 7 Lower the static and velocity increases, reverse and the other increases etc. Figure #6
7229
skwsproul
05-12-2008, 09:34 PM
Listed at the bottom inside the pdf as page 74 or in the header as page 7 Lower the static and velocity increases, reverse and the other increases etc. Figure #6
7229
Maybe I misunderstood but: "As the air
passes at high velocity from the relatively small
blade passages to the connecting ductwork, the velocity is decreased, as at Station 2 in Fig. 6, and the
static pressure is increased." The drawing shows a small round duct transition to a larger round duct. Not exactly the same as in speeding up or slowing down the fan, as this is fluid properties for a duct transition. If I am wrong show me please but on large commercial equipment I generally program controllers to speed up the vfd in order to gain static until my static set point is met and we calibrate all sensors with analog manometers to the hundredth when possible. Recently quit using digital when the wonderful uei that I own was almos .5 off even after re-zeroing. Either way great pdf thanks I am sure it will come in handy. Do you have any more?
jrbenny
05-12-2008, 09:44 PM
Listed at the bottom inside the pdf as page 74 or in the header as page 7 Lower the static and velocity increases, reverse and the other increases etc. Figure #6
7229
That's true for a PSC motor. The OP has variable speed.
I have a killawatt, so I can measure the wattage draw of the motor in both modes. Of course the 1600cfm rate will be taking more since its spinning the fan at a higher rpm rate. But I can see if its proportional or if the higher rate is forcing the fan to work harder for its rpms, the same, or less.
I dont think it has anything to do with wetness of the coil. I can flip it on the control from comfort to max and the static will dutifully show .95 or .84 respectively. I can even turn off the unit for hours and then start it up and get those same numbers...the humidity is around 22% today, so not much to condense.
Since I blocked the 3rd return completely and the static didnt rise, I'm guessing the problem area is in the supply side. Not a whole lot I can do about that since half of it is inside the drywall and runs from the garage up 2 or 3 stories in the walls. From there the ducts arent bad, fairly straight and lots of registers.
Maybe I'll do the killawatt experiment tomorrow. I pulled four windows out of the house today and installed new ones. I'm a little pooped. Eighteen more to go.
" Not a whole lot I can do about that since half of it is inside the drywall and runs from the garage up 2 or 3 stories in the walls."
If there is a first floor with conditioned space adjacent to the garage,they can run sidewall supplies to those areas ,ducts being in the garage.Then cap the supply grilles that went to those areas.This will lower the static pressure of the supply ducts.
beenthere
05-12-2008, 10:31 PM
I think you'll find that at .95, your drawing amps closer to that of a PSC motor, then a ECM motor working at normal pressures.
I think you'll find that at .95, your drawing amps closer to that of a PSC motor, then a ECM motor working at normal pressures.
Actually higher then a PSC,but delivering more cfms,from what we have seen.
beenthere
05-12-2008, 10:59 PM
kinda whips out the savings then.
kinda whips out the savings then.
Not if you remediate the duct system to lower that static.
beenthere
05-12-2008, 11:16 PM
I meant continued running at .95 kinda whips out the savings potential.
catmanacman
05-13-2008, 12:00 AM
does the rpm on the motor seem kinda high to produce the proper air flow
JBM1000
05-13-2008, 12:14 AM
Here is more, but really is just a basic info file
7237
JBM1000
05-13-2008, 12:18 AM
That's true for a PSC motor. The OP has variable speed.
Please enlighten me if a variable speed motor changes fan laws or fluid dynamics.
beenthere
05-13-2008, 05:59 AM
Yes.
The VS motor will continue to increase its speed until it gets the right right torque feed back.
Your at about 90% of max RPM.
weber
05-13-2008, 07:11 AM
He left the door off after he changed the settings, static dropped, then he reset the settings and reinstalled the door, static increased again......;)
Nope, I never touched the unit...did it from the control inside.
Not much of a chance of changing the ductwork. The house is built into a hill. The garage is in front and sort of a "first floor", then theres a living room/kitchen "floor", then a floor with 3 bedrooms, then a fourth floor with the master bedroom/bath. The unit is in the garage which is adjacent to the crawl space for the first floor. Those ducts are fairly straightforward. Everything to the 2nd and 3rd floors goes through the walls behind the stairs and to two separate attic spaces.
I think the worst of it is that the returns all come into one pipe sideways from the first floor crawl space under the unit, then takes a 90 degree turn in a squared off box up into the furnace, then 3" over the top of the coil is another plenum box with the three different floor branches all tapped into it. No turning vanes or other goodies. The furnace is just blowing into a box a foot or so high and about 20x25 with three pipes butted into it.
I'd have to tear out a ton of drywall and tear out the return and supply plenums, and then I have almost no room in the garage to work with to do anything differently.
Funny part is, same builder put up about 400 homes in the neighborhood. All pretty much the same furnace design and ducting setup.
The RPM's are 1257 on high/1600cfm with a reported .84 SP. I did take a watt readin of the carrier furnace with the accuclean cleaner turned on and it was under 100 watts on the low setting, and around 500 watts on the high setting.
weber
05-13-2008, 09:57 PM
Have you checked your tesp, the old way?
jrbenny
05-13-2008, 10:16 PM
Please enlighten me if a variable speed motor changes fan laws or fluid dynamics.
It does neither.
However, if I increase ESP on a ECM, the motor will ramp RPM (increase torque) to match the fan curve for programmed airflow.
Increase in ESP = Increase Power Consumption = Same Airflow
Pretty basic ECM stuff.
Nope, I never touched the unit...did it from the control inside.
Not much of a chance of changing the ductwork. The house is built into a hill. The garage is in front and sort of a "first floor", then theres a living room/kitchen "floor", then a floor with 3 bedrooms, then a fourth floor with the master bedroom/bath. The unit is in the garage which is adjacent to the crawl space for the first floor. Those ducts are fairly straightforward. Everything to the 2nd and 3rd floors goes through the walls behind the stairs and to two separate attic spaces.
I think the worst of it is that the returns all come into one pipe sideways from the first floor crawl space under the unit, then takes a 90 degree turn in a squared off box up into the furnace, then 3" over the top of the coil is another plenum box with the three different floor branches all tapped into it. No turning vanes or other goodies. The furnace is just blowing into a box a foot or so high and about 20x25 with three pipes butted into it.
I'd have to tear out a ton of drywall and tear out the return and supply plenums, and then I have almost no room in the garage to work with to do anything differently.
Funny part is, same builder put up about 400 homes in the neighborhood. All pretty much the same furnace design and ducting setup.
The RPM's are 1257 on high/1600cfm with a reported .84 SP. I did take a watt readin of the carrier furnace with the accuclean cleaner turned on and it was under 100 watts on the low setting, and around 500 watts on the high setting.
Not saying your static can be lower for sure,but e haven't found on yet that can't be.Just need a Pro ,that thinks outsidde the box.
Add turning vanes,use better fittings,add another return duct and grille,add supplies,use better grilles,add a parallel return or supply trunk,etc.,etc..
Heres some data I collected this morning. Pretty interesting.
Speed CFM RPM SP Watts cfm/rpm sp/cfm Watts/cfm Watts/rpm
Low 597 615 0.26 97 0.97 0.16 0.16 0.16
Med 918 853 0.46 205 1.08 0.22 0.22 0.24
High 1400 1400 1248 0.95 620 1.12 0.44 0.44 0.5
High 1600 1600 1263 0.85 660 1.27 0.41 0.41 0.52
Also in a picture in case the formatting is screwy.
If the infinity control is accurately reporting cfm, rpm and sp, then it looks like in fact the SP lowers at 1600 vs 1400 and the watts/cfm is pretty similar.
I did all this at about 6 this morning and did the first 3 readings fan-only, compressor off, then changed the setting to maximum to get 1600 in the cooling cycle and turned down the temp to kick off the a/c. The whole thing took just a couple of minutes. Watt readings were done through a Kill-a-watt.
Humidity was <20%, and since there was just air running through it for a few minutes I'm guessing the coil wasnt wet.
When I had the larger accuclean on the unit I wrote down some of the readings and wattage. I'll look around later and see if I can find those but my recollection is that the medium fan was taking about 185 and the high fan 1400 was taking around 520. So the infinity furnace with the large size accuclean filter under it was less restrictive and drew less watts to spin the fan than the infinity furnace with the same size infinity cleaner under it.
When I discussed this with the installer, he said the SP was fine. "At least there isnt a '1' in front of the number! I've seen the infinity's run up to 1.4 and work okay."
Yeah, I know, they're a bunch of hacks.
I threw all my money into this "upgrade" so I cant afford to hire another duct guy right now. I just want to make sure I dont blow up my stuff until then.
Given these numbers, even though they dont make a lot of sense, I'm inclined to run the a/c portion on the "maximum" 1600 setting since the airflow is better, the watt draw and rpm's arent much higher, and the reported static is lower. I cant make the heater part run at higher than 1400 but thats okay...I wont need heat for another 7 months.
jrbenny
05-15-2008, 03:51 PM
The system is jumping off the fan curve, and it's dorking the calculation.
Actual ESP is likely above 1.
Doesnt the infinity control give a warning if the SP goes over 1?
jrbenny
05-15-2008, 04:10 PM
The UI doesn't think it's above 1 IWC. So, no warning.
I recommend someone actually measure the pressure.
When will you be by?
I have beer.
I'll be here all day!
;)
weber
05-15-2008, 06:33 PM
I dont care what is says on the control, test it yourself and see.
jrbenny
05-15-2008, 10:35 PM
When will you be by?
I have beer.
I'll be here all day!
;)
I doubt you could afford my rates. Bit of a trip charge. :)
When will you be by?
I have beer.
I'll be here all day!
;)
You might want to test to prove it's off the fan curve.
Remove the filters and run the same test at 1400 and 1600 cfms.
Yeah, I figured the trip charge might be excessive and bought an extra case of beer.
Dash...I had been thinking the same thing about pulling the filters and seeing what the difference would be. I'll see if I can try that out this weekend now that I'm done replacing 22 windows in 100 degree heat...
Okay, I did some extra homework, still interesting results.
I pulled the paper filter cartridge out of the infinity air cleaner and re-ran some tests. I left the safety grid and the charging wire array since they didnt look like big air flow stoppers.
The first thing I ran into was the unit not wanting to go into the 1600cfm mode. I set it to 'maximum' just like I did before but it still showed 1400.
I had it do a 'reinstall equipment' and after that it decided it'd do 1600.
The numbers all dropped, but the 1600 is still lower SP than the 1400.
During the 'reinstall' it did a static pressure check and came up with .62.
With no filter in the infinity cleaner:
1400 was 1146 rpm vs 1248, static was .73 vs .95 and the watt draw was 550 vs 620.
1600 was 1197 rpm vs 1263, static was .70 vs .85 and the watt draw was 643 vs 660.
So it looks like the infinity paper filter piece is worth about .15-.20 SP. And for some weird reason it still is reporting a lower SP at 1600 than 1400.
jrbenny
05-19-2008, 11:05 AM
Well, looks like they have an issue with their calculation. Definitely within the fan curve at that point.
beenthere
05-19-2008, 11:15 AM
Either its reporting wrong, or your not getting 1600 CFM.
That could be. I note a change in the pitch (to louder and more whistly) when I change it from 1400 to 1600. There was a 5 ton payne furnace in there before and I dont know how many cfm/ton is was set to produce, but it blew roughly the same amount of air as this does on high.
I do note that this stat is running software version 13 and I think thats a rev or two back from current. Not sure if there were any fixes in the newer revs to make this work better, but its either not working properly (which is a bummer because its not a cheap stat), or something weird....
key1cc
05-20-2008, 11:48 AM
I have a killawatt, so I can measure the wattage draw of the motor in both modes.
I looked at the Kilawatt unit and only saw the plug in type. Your furnace is a plug in type? Mine is hardwired. I am in the process of understanding how different filters effect my static pressure and ultimately the wattage draw on my ECM (new Trane XV95/XL15i). Since I am hardwired and my installer indicated he does not do actual static pressure checks (he makes assumptions based on duct sizing) I was considering checking my wattage draw the hard way by shutting all the power down in the house except for the circuit breakers on the HVAC and take my readings directly from the outside meter....except I don't have an outside meter.:o
Key1
Yeah mine plugs into an outlet. About all you can do is have an electrician come put in a receptacle and a plug. The kill-a-watt gives a lot of interesting info. You can leave it plugged in for a period of time, say a week or a month and see what the total KWH usage for the device was in that time period.
Its interesting to see what different devices in a similar class use. Sometimes its very counterintuitive.
My old CRT tube rear projection tv set draws significantly less electricity than our smaller flat screen set. So much for my excuse to replace it with something newer and more energy efficient.
A little 12" fan that I have uses more electricity than a 20" one thats moving twice as much air.
My dewalt 18v charger pulls about 40 watts when theres a fully charged battery in it.
My dishwasher uses such a small amount of electricity that its not worth overstuffing it. If I remember right its something like 21c worth.
key1cc
05-20-2008, 08:25 PM
Thanks,
That's a good idea and probally much more efficient than regularly having the static pressure checked (even if I could find a tech to do it)......since the goal of checking the static pressure (for me) in the first place was to see if the wattage draw on the ECM is excessive. This technique allows you to measure the wattage draw on the ECM directly.:cool:
In reference to your situation..... my trane installation manual has a table of relative static pressures versus airflows and system tonnage. I vaguely recall that the static pressure actually does go down at the highest air flow numbers (or visa versa). I'm out of town until friday but if you like I can post some details when I get back home.
Key1
beenthere
05-20-2008, 08:42 PM
Its not that teh static goes down at the highest air flow.
Its that you get the highest air flow at the low statics.
Wondering is the supply or return static could be so high that it "fools" the control.
What does the connecting supply and return duct look like,can you post so pictures??
Is the return connected to only one side of the furnace?
What model numbers are being used?
Thanks Dash, I'll get some pictures and draw some diagrams of the ducts this weekend and get the model numbers.
Its an infinity 80% variable speed four ton unit with a performance 14 seer a/c unit and infinity air purifier, if that helps.
Basically the returns all feed into the back of the plenum box the furnace is sitting on, and the plenum is a couple of feet deep and about 20" wide, and about 30" front to back. A piece of metal wrapped plywood is on top of that plenum to adapt it to the size of the infinity filter, which then has the infinity furnace on top of that, then a very large 5 ton ADP coil. There is no stepped reduction between the plenum and the smaller infinity filter. Then theres about 3" of insulation wrapped metal on top of the coil adapting the coil to the distribution plenum, which is a box around the same size as the bottom plenum only about 12" high instead of a couple of feet. There is no stepped expansion of the duct between the coil and the return plenum. There are three pipes coming into the top of that ceiling plenum, one to each floor of the house. Those each connect to a big sheet metal box that has all the flexi ducts connected.
I think having all the air come into the bottom plenum and have to take a 90 degree turn up into the furnace, then half of it having to take another 90 degree turn from the top of the coil through the supply plenum and into the two upper duct distribution pipes is the big culprit. One of the return lines (the biggest one) is also messed up...its a 20x24 on the front of a six inch deep sheet metal box in the wall that then goes down 2' and takes a 180 degree turn...thats air in the front, then down, then back out again in a U. The smallest one upstairs goes right into a flexi duct that immediately takes a 90 degree turn and really has that pipe flattened. The 3rd return the contractor added also takes a 90 degree turn immediately.
I cant see how much could be done in the garage with the supply and return plenums. The distance from the floor to ceiling barely accommodates the code height platform with the supply plenum, the furnace, filter and coil before hitting the return plenum which is mounted to the ceiling. The 180 degree turn return has the kitchen on the other side, so it cant be made deeper. The small one upstairs has the roof right on top of it so straightening out that pipe is problematic. Theres a water heater and then the wall to the left of the furnace although theres a few feet of space to the right of the furnace platform.
I think in short, the guy who built the house and installed the original duct layout was an idiot. I probably made the whole thing worse by putting in the infinity filter and having no room to gently slope the connections for the furnace/coil to the two plenums.
About the only thing I can think of doing thats cheap and easy is cutting a 4x4 on the diagonal lengthwise and putting that in the front of the return plenum to eliminate the hard corner up, and doing the same in the big return box that has the 180 degree turn to take out its corners and make both 90 degree edges into 45s. I doubt that will make a particularly significant change though.
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