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  1. #27
    Join Date
    Apr 2007
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    623
    how old is the systm,how long since it was serviced?

  2. #28
    Join Date
    Nov 2004
    Location
    NJ
    Posts
    1,200
    A little knowledge can be dangerous.

    You set your blower correctly?
    Based on what calculation?
    Did you determine the sensible heat ratio of the load and try to match it with the airflow?

    That 530 CFM per ton might have been where the system was supposed to be are you trying for a nominal number like 400 CFM per ton now?
    Ed J

  3. #29
    Join Date
    Jan 2009
    Posts
    386
    Quote Originally Posted by aintitfun View Post
    how old is the systm,how long since it was serviced?
    The system just turned 2 years and has never been serviced. I've cleaned the fins out on the condensing unit outside every Spring, and I replace the filter every 30-45 days.


    Quote Originally Posted by Ed Janowiak View Post
    A little knowledge can be dangerous.

    You set your blower correctly?
    Based on what calculation?
    Did you determine the sensible heat ratio of the load and try to match it with the airflow?

    That 530 CFM per ton might have been where the system was supposed to be are you trying for a nominal number like 400 CFM per ton now?
    I do not have a load calculation for my home, and will be looking into getting one when I call a tech out. I used tonnage, temp split, TESP, and the Furnace Spec Sheet to calculate the blower speed. As I mentioned in a previous post, the HVAC company that installed the system said they leave the cool and heat blower speed taps set at the factory settings. The heat speed is probably set correctly since the blower is a component of the furnace, and I doubt the mfr. would ship the system out with the blower speed set incorrectly for heat (although it's always good to have someone check). However, the mfr. has no idea what size A/C unit with be paired with the system, so the cool speed would definitely have to be set by the tech (which it was not).


    Quote Originally Posted by Ed Janowiak View Post
    Did you determine the sensible heat ratio of the load and try to match it with the airflow?
    I didn't realize that is how it should be done. If the unit is not sized correctly, setting the blower speed based solely on what you mention could cause several problems, right? If the unit is oversized, you might not be moving enough air across the coil. If the unit is undersized, you might be moving too much air. I thought there were several other factors that went into calculating blower speed. The cooling load (sensible and latent) should be used to size the equipment in the first place, right?

  4. #30
    Join Date
    Nov 2004
    Location
    NJ
    Posts
    1,200
    Your putting the cart before the horse.

    You need and accurate load done before you can determine what the total air flow required is.

    Lets start out slow.

    More air flow (higher sensible heat ratio) = more efficiency.

    Less air flow (lower sensible heat ratio) = lower efficiency

    More air flow quicker to satisfy the thermostat.

    Less air flow longer to satisfy the thermostat.


    What is you ultimate goal? As I read ti your not happy with the amount of run time you have (too much), lowering the fan speed will increase run time.
    Ed J

  5. #31
    Join Date
    Jan 2009
    Posts
    386
    Quote Originally Posted by Ed Janowiak View Post
    Your putting the cart before the horse.

    You need and accurate load done before you can determine what the total air flow required is.

    Lets start out slow.

    More air flow (higher sensible heat ratio) = more efficiency.

    Less air flow (lower sensible heat ratio) = lower efficiency

    More air flow quicker to satisfy the thermostat.

    Less air flow longer to satisfy the thermostat.


    What is you ultimate goal? As I read ti your not happy with the amount of run time you have (too much), lowering the fan speed will increase run time.
    The unit is only capable of removing a certain amount of heat from the air. It wasn't satisfying the t-stat any quicker when it was set for 530 CFM / ton (1850 CFM), than it is now at 440 CFM (1550 CFM).

    My ultimate goal is to be able to cool the house lower than 78 degrees when it's 95 (or hotter) degrees outside (and not do a set and hold at the temp I want at 4:30 AM ).

    Quick question, are you saying that your "more air flow = more efficiency, and less air flow = lower efficiency" applies to heating as well?

  6. #32
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by skizot View Post
    The unit is only capable of removing a certain amount of heat from the air. It wasn't satisfying the t-stat any quicker when it was set for 530 CFM / ton (1850 CFM), than it is now at 440 CFM (1550 CFM).


    Quick question, are you saying that your "more air flow = more efficiency, and less air flow = lower efficiency" applies to heating as well?
    Generally speaking yes. But to a point. Much above 450CFM, I think you'll just be spending money to move air. Also, depending on your ductwork, as the static presure increases, you're spending money to compress and thereby heat the air.

    Also, above 450CFM/ton, the fan itself is putting out more heat, so when cooling, it must be subtracted from the total system output. There is a point where getting another 170 BTU's will cost you another 50 Watts... so you've gained nothing.

    If you have very restrictive ductwork, the point where your system is most efficient, might be closer to 400CFM/ton or even lower.

    At some point you might gain more capacity, but lose efficiency as well. it really depends on each system and installation.

    Also keep in mind that improving latent capacity, could be more efficnt if it allows you ot keep you home comfortable at a higher temprature because hte humidity is lower. This is something that has gotten missed in the rush to obtain ultra high SEER systems... that sometimes have inadequate latent capacity when compared to older lower SEER sytems.

  7. #33
    Join Date
    Jan 2009
    Posts
    386
    Thanks, moto. I was just getting at this; the way I understand heating is that the you're adding a fixed amount of heat to the air inside your home (determined by the size of the furnace), and you're not going to be able to add more heat to the home by increasing the fan speed when heating. If your output is 80,000 btu/h, then you're adding 80,000 btu/h to the home, regardless of whether you're moving 1200 CFM across that heat exchanger, or 1600 CFM. You're just going to end up with different DeltaT's there (63.7 and 47.8, respectively). Obviously you run into problems when you don't move enough air across the heat exchanger (cracking), or coil (freezing). But, you can only add as much heat to the home as your system can output.

    I assume the same is true about cooling. You can only remove as much heat from the air as your tonnage allows (42,000 btu/h in my case). Running the fan slower will remove more moisture, and yield a larger temp split. All increasing the fan gets you is a smaller temp split, less humidity removal (and good point on the added heat from the blower). If my total heat gain (latent + sensible) is more than 42,000 btu/h, and I want a comfortable temperature of less than 78 in the house, then adjusting the fan speed up or down gets me nothing (well, aside from extra humidity removal).

    Am I correct with these assumptions?

  8. #34
    Join Date
    Nov 2004
    Location
    NJ
    Posts
    1,200
    Your assumptions are not correct.

    Heating is a 100% sensible process, cooling is (amost allways) both sensible and latent.

    All Cooling BTUH's are not created equal, your sensible heat ratio changes with the amount of air you move.

    This would be easier for me if I knew what part of the country you live. Designs will differ from desert to a green grass climate. A load calc would help too .
    Ed J

  9. #35
    Join Date
    Jan 2009
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    386
    Quote Originally Posted by Ed Janowiak View Post
    This would be easier for me if I knew what part of the country you live. Designs will differ from desert to a green grass climate.
    Closest city to look at for design temperatures and normal humidity is Topeka, KS.

    Still, I'm not understanding why you think setting the blower to 1850+ CFM would be correct for a 3.5 ton setup, even if I had a load calc sitting right in front of me to give to you the sensible and latent. I've never seen that much airflow recommended for cooling in all the posts I've read on this board. But maybe I'm not understanding things. :-p

  10. #36
    Join Date
    Nov 2004
    Location
    NJ
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    If your SHR is a 90 10 split, the faster you move the air the better.

    I don;t know what your air flow should be with out a load calc, but I also know that I never automatically think less air flow is better unless noise is the major complaint.
    Ed J

  11. #37
    Join Date
    Jan 2009
    Posts
    386
    Quote Originally Posted by Ed Janowiak View Post
    If your SHR is a 90 10 split, the faster you move the air the better.

    I don;t know what your air flow should be with out a load calc, but I also know that I never automatically think less air flow is better unless noise is the major complaint.
    That's cool. Sorry if I come across as a smart-ass. I'm here to learn as much as I can, and posing my assumptions, and questioning answers is a good way to do that for me. Having people correct me and explain why is a good thing. :-)

  12. #38
    Join Date
    Nov 2004
    Location
    NJ
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    1,200
    You don't come across like a smart anything LOL.

    Kidding, I looked up your area and you have both latent and sencible loads, design temp is 95 and if you can't maintain not recover but maintain 75 inside when it's 95 out with blinds draw and such something is wrong.

    Could be lots of things, but from what you have listed I'd be confidant in saying it's not an air flow problem. I've been out on a lot of 3erd party complains and finding units too small almost never is the reason.

    I'm sure you have read it here, if not in this thread, but look @ the temp of the air @ the return grill(s) and the temp of the air entering the coil and see if it's the same.
    Ed J

  13. #39
    Join Date
    Jan 2009
    Posts
    386
    Quote Originally Posted by Ed Janowiak View Post
    You don't come across like a smart anything LOL.
    Interesting. I guess my employer is paying me the money that they are, to do the work that I do, for nothing. j/k

    Quote Originally Posted by Ed Janowiak View Post
    I'm sure you have read it here, if not in this thread, but look @ the temp of the air @ the return grill(s) and the temp of the air entering the coil and see if it's the same.
    I will do that when I return home. Given the return temps I was seeing at the AH yesterday, I doubt much unconditioned air is being drawn into the return side of things.

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