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.
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?
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.
Sounds like you missed infiltration.
If the second floor isn't cooled, you have heat gain through the ceiling.(treat as partition wall)
Last edited by rru2s; 09-25-2008 at 07:20 PM.
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.
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.
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.
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?
same properties for doors since they will be glass. I am ignoring frames.
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.
OD desgn temps for Philly: 90/14.
Originally Posted by markj