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  1. #1
    Join Date
    Nov 2011
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    boston,ma
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    Default static pressure issues

    #5 Today, 10:41 AM
    jled96
    Junior Member Join Date: Nov 2011
    Location: boston,ma
    Posts: 1

    Re: Return Air

    --------------------------------------------------------------------------------

    I think I have the same problem with not enough return air flow. I had a carrier infinity 59k btu 3-stage ics furnace and 2-ton ac installed last year and kept having the change filter message pop up everytime we use a,c or high heat is activated. All brand new ductwork and My only return duct is 20x6 runs 15 feet to common hallway and the grill size is 20x8. The suppy is 14x8 runs 10 feet and goes to 7 six inch flex ducts with 4x10 otlets and grills.The house is only 1,080 sq. feet.The original installer has left the state, and now the new contractor i hired has found a few issues, he wants me to add another return off the return plenum 8"pipe up inside wall to second floor hallway, where there are 2 bedrooms to a new 8x14 grill and says to only use a merv 8 filter not the carrier merv 11 that i bought 4 off the original installer. he says the staic pressure .89 is to high on high speed and i will burn out the esc blower motor, he has also turned the cfm's on the infinity controller down to 800, it was at 950. He says its ok for now but I should do the return duct before the next cooling season, does any of this sound correct? the original installer states he does theses every week and has not had any issues for the last 15 years and my 1 year install warranty is up, I paid a lot for this system i cannot believe it would not be installed cooectly. any advise would be much appreciated. Thanks!

  2. #2
    Join Date
    Jul 2009
    Location
    Northeast
    Posts
    661

    Default Re: static pressure issues

    Your 14x8 supply is only good for 500-600 CFMs, your 7 six inch supplies are good for 700 CFM's. How long are your flex ducts and are they pulled reasonably tight? I believe there is a code in MA. says that Flex duct should be no more than 4-5 feet, the resistance is much greater than metal duct. If your supply and return are not sized right as is your problem it will make the system think you have a plugged filter, loss of air flow. You should have a 24x8 supply and return. Adding another return isn't going to do you any good as your supply is way undersized, so your only going to be able to fit so much air in that space at one time bringing even more back will do no good.
    He was right to turn down the blower motor because it's working to hard trying to move the air into a space that's to small, the blower will fail eventually.
    A 24x8 supply and return will give you 1000-1100 CFMs, your 14x8 supply is only good for 500-600 CFMs. If your using this system for Heat as well as A/C you may have problems with it going off on high limit. Once again this is cause by to small of ductwork and not moving the heated air the way it should be moving. The good news is your Furnace seems to be sized right for the square footage you posted. Good luck.

  3. #3
    Join Date
    Nov 2011
    Location
    boston,ma
    Posts
    114

    Default Re: static pressure issues

    Thanks for the reply, I knew there was someting up with the air flow as you said it seems to heat ok ,but the cooling was another story, I had to take off one of the six inch supplys to the upstairs master bedroom and put a new collar right on the plenum to get cool air upstairs in the summer as it was 15 f hotter in the upstairs, thats why the new carrier tech said to put an 8"oval return duct upstairs with a 14x8 grill,I am stuck with this system after dishing out 16k. The new tech sid it looked like it was pieces from another job to do this job, as all the duct work and flex was pieced together and lots of tape everywhere. What gets me is this was a 5 permit job, and it all passed, the building inpector even went around with tech with the air meter to each went and said it was great, thats probably why it was set at 950 cfm on the fan. is the 20x6 return duct, and 20x8" grill ok for this cfm of 800, and ok for this system? I think i am going to move the other upstairs 6" vent to the supply plenum and add that 8" return pipe to the upstairs in the spring,when its easier to work on the system.The carrier tech said this is the best solution and the static will come way down and the fan noise. Let me know if this sounds feasable. Thanks!

  4. #4
    Join Date
    Nov 2011
    Location
    boston,ma
    Posts
    114

    Default Re: static pressure issues

    I just measured the flex duct and the longest run is 11 feet,he said code is not to exceed 12 feet with they did not, they have angled take-offs on the supply trunk all 6" with dampers all taped wide open inline with the ductwork, some sections is metal pipe where it goes over the beams and the flex is all zip-tied and taped with metal strapping, there are some tight bends here and there ,but it looks like it should flow ok. The city is no help they said it passed and should work properly according to there balance report, the new tech said a different story. Well its installed now so I guess I will try to make the changes and see if it helps at all. Thanks!

  5. #5
    Join Date
    Nov 2011
    Location
    boston,ma
    Posts
    114

    Default Re: static pressure issues

    I have just got a call back from the very irrate original installer, he states that on infinity models supply and return airflow must equal each other. He states thats 20x6 return and 14x8 supply both with 16x25x16 plenums are the same and are sufficient for this carrier comfort system. He said that I dont really need more than 750-8oo cfms as this is a small system only 59k btu with 2 ton a,c. He says the controller must have been in the test mode thats why it was at 950 cfm's, He says I am going crazy trying to figure this out, he states it is all correct according to manual d and carrier requirements, he states the house is very old and thats the issue with the duct design. He says change the filter to merv 8 like it is now, change it every 3 months and call it a day. He says he will drive 4 hours and replace it if it burns up within the 10 year warranty period. I don't believe any of this, any take on this? Thanks!

  6. #6
    Join Date
    Jul 2009
    Location
    Northeast
    Posts
    661

    Default Re: static pressure issues

    If you want to know if something is put together wrong don't go to the person that originally said it was right. I'll post later, kind of busy right now

  7. #7
    Join Date
    May 2011
    Posts
    251

    Default Re: static pressure issues

    You are a little short on return. Maximum velocity should be 700fpm. You return grill duct is 1.11 sqft, so that's barely enough for even the 2 ton AC. The furnace on high fire will need around 890CFM for a 60F temp rise. On top of that, 700fpm on a grill taht size will be noisy.

    The supplies are even worse. Maximum supply velocity is 900fpm, at that velocity, you're only good for 700CFM.

    If you are using use a 1" pleated filter.... add it all up and you WILL have high static pressure. That will cause you blower to use a LOT more electricity (almost as much as a standard eff. motor, and can shorten it's life. IT alos makes it noisy.

    Easist fix is t increase return grill size. To the supplies, you best bet would be to install better quality registers and possibly increase the register size to 4X12" for less restriction. They cost about 5X more than basic cheap registers.

    IF youi have one room that underconditioned (cooler in winter and hotter in summer, than the others, you may want to add a new "home run" supply branch form the furnace to that room if possible.


    You service tech is simply wrong. They cut corners and unsersize your system. IT would only be sized correctly for a 1.5Ton AC and 50k BTU furnace.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  8. #8
    Join Date
    Nov 2011
    Location
    boston,ma
    Posts
    114

    Default Re: static pressure issues

    Thanks Motoguy, I have gone over this with the original installer again yesterday and he states the system is correct, the air filter is a 16x25x5 and is the infinity filetr housing, i looked to see if there was a smaller track in there to take a 1" filer but there is not, thats why he told me to switch to a merv 8 and good for 500 fpm filter of the same size , not the merv 11 that was originally installed which is only good for 400 fpm, he states that the system only runs on low or med most of the time thats why you can use minimum duct sizes, he states this will be 350-360 cfm, and then 500-550-cfm on med heat, he said on hign heat it would be what they limit it at on the contrroller wich is now 800 cfm, and yes for noise it sounds like the grill and the ductwork are going to take off like a jet plane with the a/c running on high stage, the grill and the ductwork are dancing like crazy, which is why I was going to add 1 more 8" return to up upstairs hallway, I need to take out the walls and cut floors and studs to do this , which i just finished boxing in like 3 weeks ago. I just read in the manual that the staic on high a/c is suppossed to be.50 max and it was at.89 on high, it also states that the high heat can be at 1.0 static press. max which doesn't make sense to me, but this is in the carrier manual. Well i guess i will add that return in the spring and see what happens, as it is cold now and i would hate to start tearing everything up again, thanks for the reply!!

  9. #9
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: static pressure issues

    One of the largest problems -- a single return.
    There's no way a single return will effectively aid in proper air distribution --- especially if it's located on a main level and with rooms on a second level.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  10. #10
    Join Date
    Nov 2011
    Location
    boston,ma
    Posts
    114

    Default Re: static pressure issues

    Thanks, I asked about a return for the upstairs when they were designing the system and they said its only 300 sq. feet upstairs, and it would draw the air down the hallway to the first floor,I figured the extra return would help especially since the second floor ceiling is basically the roof minus the insulation, I already had to switch the run for the upstais master bedroon to the supply plenum to get enough airflow to cool it somewhat for sleeping, The contractor said it was impossible to get a retun up there withot tearing the ceilings and walls down and panning them out.all my outside walls are stuffed full of cellulose insulation and he said the inside walls are all 2x3 construction and were too small for a proper return, I figuered any return from upstais would help somewhat, He said the first floor return next to the stairs was fine, but I guess now thats why it doesn't work very well!!!!

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