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  1. #1
    Join Date
    Sep 2008
    Posts
    8

    Am I sizing Correctly - Please Help

    I need some help with the following:

    I am trying to size a central air system in an older part of Philadelphia. The unit is only going to service the 1st floor of a 3 story row home w/ a basement. The total square feet is 1000 sqft. (68' x 15'). The entire front of the home will be windows w/ a door, assuming all glass (6mm lowE, 3 mm argon, 6 mm lowE). Since it is on the first floor and a row home, there are no roof, and side wall losses, only basement losses. There is an addition on the rear of the house that is about 182 sqft (11' x 16.5') that has R-19 insulation throughout the walls & roof. with a large door & window leading out the rear and side. Same material as above and about another 150 sqft of the same glass material as the front. Both the front and the rear addition are exposed to sun light (Faces W & E) respectively.

    After I sized the system, I found a heating load of only 24,000 btu/hr and a cooling load of 3,500 btu/hr. I understand that since the 1st floor of this building is like barely exposed to the exterior environment, but am worried that the loads are too light. I would appreciate any assistance or advice anyone can offer.

    Thank you,

  2. #2
    Join Date
    Dec 2000
    Location
    NW Ohio
    Posts
    516
    Something does not appear correct. Design temperatures? Is the second story also cooled? Are you conditioning the basement? What is the infiltration rate that you used?

  3. #3
    Join Date
    Oct 2004
    Posts
    167
    Also are the adjacent houses cooled? If not then there would be marginal gain there. But you're right that the cool load seems awfully low. Kitchen appliances need to be considered if there is one on that level too.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,120
    Sounds like you missed infiltration.
    Kitchen load.
    People.
    If the second floor isn't cooled, you have heat gain through the ceiling.(treat as partition wall)
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  5. #5
    Join Date
    Oct 2004
    Posts
    37
    * Post deleted *
    Last edited by rru2s; 09-25-2008 at 07:20 PM. Reason: *deleted*

  6. #6
    Join Date
    Sep 2008
    Posts
    8
    Sorry for the delay in responding and I appreciate the interest. I used an infiltration rate of .0736 m3/s which is equivalent to .1 air exchanges/hr. The neighbors are assumed to be conditioned/heated. The 2nd & 3rd floor is heated/cooled and will be 70-75F. Design temp for the 1st floor is 75 for both heating & cooling. Included appliances like the stove for 1000 w/hr for 2 hrs/day, lighting at 200 Watts/person & 400 watts for fridge & appliances. Designed occupancy is 5 people. Not conditioning the basement. Neighbors also have basement so heat loss is through floors only. Basement is unfinished. Heat loss through hardwood flooring was considered as well.

  7. #7
    Join Date
    Sep 2008
    Posts
    8
    I would agree with your comments relating to city temps relative to the burbs. The building is pretty much enclosed on all sides besides the front & rear, that is what adds much confusion, I figured the load would be rather small, but didnt think it would be that small.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,120
    5 people x 450 BTU = 2250
    5 people x 200 watts lighting per person = 3413
    1 fridge x 400 watts = 1365
    Infiltration = 540
    Sub Total = 7568 BTUs.

    I don't have anything in for doors or glass.
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  9. #9
    Join Date
    Sep 2008
    Posts
    8
    Based on the thermal properties of the window, I am getting a loss of 2.83 btu/hr/ft2 in the summer and 14btu/hr/ft2 in the winter. Does that sounds right to you?

  10. #10
    Join Date
    Sep 2008
    Posts
    8
    same properties for doors since they will be glass. I am ignoring frames.

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,120
    Your HTM is picked from your framing.
    A wood frame for a winter temp difference of 70° is 18.4.
    For a metal frame its 26.5.

    Unless your windows have a lot of shading, your summer HTM is way low.
    More like 30 plus per sq ft for east and west windows with no external shading.
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  12. #12
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by markj View Post
    Something does not appear correct. Design temperatures? Is the second story also cooled? Are you conditioning the basement? What is the infiltration rate that you used?
    OD desgn temps for Philly: 90/14.

    AM

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