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figuring CFM required

Hi guys, I am attempting to figure out the CFM required to heat a 300 sq ft room using the formula CFM=btu / (1.08 x delta T)

What is the typical delta T used in this calculation?

Thanks !

Comments

  • Techman
    Techman Member Posts: 2,144
    CFM

    1st you need the btu load of the area,then compare the options for the btu heat/cool needed,as compared to a hot water coil(cfm)/furnace/ ductless split, ptac.  
  • hws
    hws Member Posts: 43
    edited May 2014
    have the btu

    Hi  Techman,

    I already have the btu of the room. The home is already heated by a furnace, but I need to install a duct into a currently unheated room. Want to make sure it is sized properly. I have all the parameters except for the temp differential typically used in this formula.
  • Techman
    Techman Member Posts: 2,144
    edited May 2014
    CFM.

    The btu/cfm for the 300s.f. room is being tapped off from the duct system for the main house? Does  the main house have AC? Flapping jaws here, that's about 6000-9000btu of cooling and that's about 1/2-3/4 ton of cooling , depending! Can the main house afford both h/ac cfm?
  • Spence
    Spence Member Posts: 316
    Delta T

    Your temperature drop is based on the sensible heat ratio from your load calculation. A high SHR allows for a warmer coil, while a low SHR is the opposite. Once you have the CFM value, you have to determine if your blower can meet this requirement at an acceptable ESP using the performance chart from your unit.
  • hws
    hws Member Posts: 43
    no AC

    The system in question is a forced air, oil-fired furnace. Definetly no AC plans for the future, main system is designed for heating only.
  • NJ, Designer
    NJ, Designer Member Posts: 53
    edited May 2014
    CFM?

    Forget the math, You got to check the manufactures Blower data for that specific furnace and then got to figure out the resistance of the ducts (a.k.a. manual D)
  • Spence
    Spence Member Posts: 316
    CFM

    This is a wonderful practice; one of which our industry should have more followers. However, it tells you what you have yet doesn't address what you need (load). With these two bits together, we learn how closely the CFM required by the load and the field conditions (as your point states) can meet. The more narrow that gap is means everything to the occupants and the equipment.
  • Eugene Silberstein_2
    Eugene Silberstein_2 Member Posts: 349
    Delta T

    The Delta T value is the temperature rise across the appliance. With the furnace running, you take temperature readings of the return air coming into the appliance and the temperature of the supply/heated air coming from the appliance. The difference between the two is your delta T.



    Now, if the area to be conditioned is far from the furnace, or the ductwork is not adequately insulated, the cfm value may very well need to be increased to take the duct losses into consideration.



    You also have to have reliable numbers with regards to the actual heat loss of the room, so be sure that you are using a Manual J, or similar, method to determine the needs of the conditioned space.
    Eugene
  • hws
    hws Member Posts: 43
    Thanks Eugene!

    n/t