Results 1 to 7 of 7
  1. #1
    Join Date
    Jul 2009
    Location
    Dallas, TX
    Posts
    42

    What are the effects of indoor coil sizing and CFM adjustment?

    I am looking at getting a new split heat pump system this fall. I wanted to be a little more educated as to the effects coil sizing and CFM have on my comfort so I can better voice those concerns to the professionals who will be coming to talk to me about my system install. As no one will better know what I define as comfort, I want to understand the effects changing these system parameters has on my comfort.

    Cooling:
    For a given system, if everything else were equal, as I understand it, a smaller indoor coil will get colder than a larger coil, increase latent cooling, decrease sensible cooling, lower EER, increase delta T, and increase cycle time.

    As I understand it, these are the exact same results as reducing CFM if everything else remained equal, but that also adds the effect of reducing air noise.

    Please let me know if I am not correct in my understanding here.

    Heating:

    I do not have a grasp at all on what effects changing the indoor coil size has when in heating mode. That is what I am trying to understand. Any thoughts?

    As far as CFM, it seems to me that a reduced CFM in heating mode would increase delta T.
    __________________

  2. #2
    Join Date
    Nov 2004
    Location
    NJ
    Posts
    1,205
    How are you going to change indoor coil size? Going with a duel fuel?

    You have the cooling part down with the exception of noise. The "noisy" air comes from velocity (FPM) not volume (CFM). The proper sizing of supply and return ducts, grills and terminations will dictate this.

    Heating, ∆T means squat if you have the items listed above correct. Lower airflow on a heat pump will never give you a supply temp that you will be happy with if it blows on you.
    Ed J

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,784
    Generally using the smallest matched coil. Will increase latent and decrease sensible under all cooling conditions.

    A larger coil with the CFM reduced. May not reduce sensible enough to remove as much moisture.

    Best to match the indoor coil to meet both requirements. Then to hazzardly just choose one method over the other.

    Smaller coil on heat pump will reduce BTU.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Jul 2009
    Location
    Dallas, TX
    Posts
    42
    Ed,

    Thanks for taking the time to reply. Regarding FPM vs CFM and noise, I was assuming that nothing else in the system changes. If that is the case lower CFM would equates to lower FPM as well I would think.

    As for Delta T, I was not trying to get a "supply temp that you will be happy with if it blows on you." I am just trying to gain a greater understanding of the concepts. I assumed that speccing a smaller coil size for increased latent heat removal in cool mode would also have an effect on the heat mode of a heat pump. I was wondering what that effect would be.

    Beenthere,

    Thanks, I guess lower BTU with smaller indoor coil makes sense, I just could not get there without a little professional help. Thanks for taking the time to educate me on some of my more technical questions.

  5. #5
    Join Date
    Sep 2005
    Location
    Texas
    Posts
    2,793
    Quote Originally Posted by Illusion View Post
    I am looking at getting a new split heat pump system this fall. I wanted to be a little more educated as to the effects coil sizing and CFM have on my comfort so I can better voice those concerns to the professionals who will be coming to talk to me about my system install. As no one will better know what I define as comfort, I want to understand the effects changing these system parameters has on my comfort.

    Cooling:
    For a given system, if everything else were equal, as I understand it, a smaller indoor coil will get colder than a larger coil, increase latent cooling, decrease sensible cooling, lower EER, increase delta T, and increase cycle time.

    As I understand it, these are the exact same results as reducing CFM if everything else remained equal, but that also adds the effect of reducing air noise.

    Please let me know if I am not correct in my understanding here.

    Heating:

    I do not have a grasp at all on what effects changing the indoor coil size has when in heating mode. That is what I am trying to understand. Any thoughts?

    As far as CFM, it seems to me that a reduced CFM in heating mode would increase delta T.
    __________________
    I don't mean any offense and I'll try to be nice but you have just enough info to be dangerous. Don't reinvent this industry, it's been reinvented enough. You need a VALID manual J performed on your house, then you need a manual D performed to assure you have the proper duct work as well as selecting the proper blower. Next you will need a manual S performed for equipment selection. The equipment should be ARI matched and once installed it should be performance tested as well as a air balance. A a blower door test and a duct blaster test (if keeping existing ductwork) wouldn't be a bad idea either. Find a contractor that knows what ther're doing and I wish you the best of luck.

  6. #6
    Join Date
    Nov 2008
    Location
    Chapel Hill, NC
    Posts
    1,070
    Quote Originally Posted by air2spare View Post
    Find a contractor that knows what ther're doing................
    Therein lies the rub!


  7. #7
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,618

    Unhappy

    Also make sure what you select will have an ARI match-up number if you plan on applying for any federal tax credit, or Utility company rebates. It makes no difference what you aim for if it loses the ability to qualify!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event