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  1. #14
    Join Date
    Aug 2002
    Posts
    1,647
    The size of the blower whether 1400cfm, 1200cfm etc has nothing to do with the high speed heating airflow.. If the second stage airflow is waking you up either you are a light sleeper or the velocity of the airflow for the 2/stage is high due to a undersized ductsystem or the unit is too large for the home or atleast the ducts..Load calculation??? Duct calculation???

    A variable speed motor will try to move the correct cfm whether or not the ductwork will carry it, if duct can not carry the cfm it can make the velocity of the air thru the duct to become too high which can lead to objectionable airflow noise...

    If you have a infinity control the TESP can be checked to see if it is running too high at that cfm..

    This objectional airflow noise can also happen in cooling if the cooling cfm needed is the same or high than the cfm needed for the high speed heating..

    As stated the TESP need to be checked and if too high, the ductwork needs to be checked and upgrades are needed to lower the TESP to an acceptable level which will also make the airflow go thru the ducts at the appropriate velocity, this will allow the unit to be as quiet as possible..

    Also when the TESP is high the VS will ramp up to a high rpm which will cut any savings advertised with a VS,, this may also lead to premature VS failure..

    I hope some of this info may help..
    Goodluck
    J

    P.s. A totally correct TESP would be .5 at the highest cfm the system will operate at.. However, I have found that these unit are still quiet in most instances if the TESP is below .7, anything over that the airflow starts to get alittle noisy..


    [Edited by Advanced Response on 02-19-2006 at 12:16 AM]

  2. #15
    Join Date
    Jan 2006
    Posts
    32
    Originally posted by fortressofcomfort
    Low vs high fire operate at roughly the same efficiency. I believe any efficiency in burner output is offset by the increase in blower current.

    Yes, that's what I meant. It's not the gas efficiency, but the total furnace effficiency that changes. Power required by a blower motor goes up as the cube of the cfm requirement.

    In our Lifebreath air handler, we only ever run the EC motor on low and it uses 60W at 350 cfm during heating season. It uses 1.4kW at times during the cooling season.

    So if you have a second stage that blows hard and long, that 1.4kW is going to bite you in the pocket book. If you have a PSC motor, then you aren't getting the low speed savings you'd get from an ECM motor, but it is less. Typically 400 W at 400 cfm comparing Carrier specs with and without ECM motors.

  3. #16
    Join Date
    Jan 2006
    Posts
    32
    Originally posted by k_fridge
    Rules of thumb for gas furnace setback.


    Clear as mud????
    Actually, really clear. And makes sense too. SImilar reasoning to my earlier post, but you have give some good guidelines for setbacks.

    Thanks

  4. #17
    Join Date
    Jul 2005
    Posts
    717
    Originally posted by odcman
    I'd agree. It's really a comfort thing. Assuming you have a 60000 btu furnace, that 600 mins is 586 cu ft of gas, which is about 16.6 cu meters, which is about 8 bucks including all taxes and tranportation charges under my current gas contract. That will buy my morning's TIm Hortons coffee for a week.
    **********************************************
    the set back savings for me are; oil fired,70K, .65 nozzle, saves approx = 10 hrs @.65 = 6.5 gals. = 30 ltrs of oil @ .80 = $24. per month.
    not too shabby,eh,? and that's with only a 4 degree set-back for only 7 hours per day.

  5. #18
    Join Date
    Feb 2006
    Posts
    108
    I tested the system using the Infinity Controler checkout.Filter is dynamic 16x25 and is clean. A-coil has not been installed and I have a large by-pass humidifier.
    House is 2800 sqft not including fully finished basement.

    Plenum 19.5"x 19.5"x46"
    1 PTO 16"x20" into 8"x27" Y-Branch , Two Branchs are 8"x16"
    Return air drop 14"x23"x75" with round boot.
    Ruturn air duct 10"x22" and only 65" long.
    2 8"x30" r/a grills main level
    2 8"x14" r/a grills second level
    1 6" round r/a in bsement
    19 heating ducts 5" and 6" including the 5 in the basment.
    Carrier MVB-80-14

    Low Heat
    air flow : 669
    inducer rpm : 2140
    blower rpm : 492
    static press : 0.17

    High Heat
    air flow : 1331
    inducer rpm : 3390
    blower rpm : 945
    static press : 0.56

    How do these numbers look? Any idea why high heat fan or just high fan is loud?


  6. #19
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    the sp being 3.25x more jumps out!
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  7. #20
    Join Date
    Aug 2002
    Posts
    1,647
    cd,

    You stated your high speed heating is moving 1331 cfm at .56 static sound good but you also stated your airconditioning a-coil has not been installed.. This concerns me.. The a-coil will probably have a pressure drop of .2,, give or take.. This will make your static pressure higher..

    It sounds like you need to have your a-coil installed and see what the static pressure will be.. But it will definately be higher.. I hope you have no larger than a 3ton a/c system needed..

    Your ductwork needs to be evaluated to see what needs to be changed to make it compliant with noise considerations..

    You need a tech familiar with manual D and static pressure measurements.. It could be your return drop is the wrong size or your main extended plenum is too small or transitions are too retrictive,,,etc.. Need to have a tech that can measure return static and supply static separately so it can be determined where your greatest problems are..

    It may be possible to remedy your situation with minor ductwork changes or all may need to be corrected if possible.. As stated only a quality hvac tech that is well versed in Manual D will be able to advise you in person after taking measurement..

    You are really going to have higher TESP once the A/c coil is installed if I am understanding you correctly that currently you have no a-coil installed in your plenum..

    Search some of my previos post to see what your tech needs to check if ductwork changes are needed.. I typed a long enuough reply..

    Goodluck
    J

    Sorry, CDN, no disrespect meant.. I did not realize you are in hvac.. My appologies.. Check out Manual D and Check out NCI... It will help with more understanding or Sign up as a Pro and post any questions in the pro forum and you will get more answers..

    [Edited by Advanced Response on 02-19-2006 at 08:10 PM]

  8. #21
    Join Date
    Feb 2006
    Posts
    108
    Thanks AR.
    Seems I never have time to look at my own heating or cooling system!

    I think if I moved the beer fridge from the garage into the furnace room maybe I would spend sometime working on it It would make more room in my garage.

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