Sizing central air system


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Old 07-05-14, 11:24 AM
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Sizing central air system

I am going to be installing an air conditioning/heat pump system in my house soon. I need help sizing it. I will try to give as much info as possible. I am also installing a pellet stove in my basement, I plan on having it in the same room as the air handler and a cold air return to help cycle warm air from the basement to the rest of the house. The rest of the heating will be picked up by the heat pump.

I have a one story brick ranch house with a finished basement , located in south central PA. On the main floor I have a 23x16 living room. This room faces the west, it has a 6x4 picture window and on each end are a 6x2 window. I have an 8000 btu window unit in this room and it has no problem keeping the room cooled.

The living room opens up to an entrance way for the front door, roughly 7x13, that is open to the dining room which is roughly 13x13, it has two 5x3 windows. The kitchen is 21x12, and faces southeast. The kitchen has a 5x3 window, and two 4x2 windows. It also has a door.

The hallway leads to the master bedroom, spare bedroom and bathroom. Master bedroom is 15x13 with 2 5x3 windows, spare bedroom is 10x13 with two 5x3 windows. These face the east. The bathroom is roughly 7x10, and has one small window. The ceilings are about 10', and the house is around 1200 sq ft, insulated walls and brick on the outside, the ceiling is also insulated with 4" fiberglass insulation.

The basement, consists of a 23x14 family room and a 14x17 bedroom. I am more concerned about heating vs cooling for the basement.

From what I calculated I should need a 2.5 ton to 3 ton system.
 
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Old 07-05-14, 11:36 AM
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You can try doing the sizing yourself or your children can do it using this free online load-calc
 
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Old 07-11-14, 07:37 AM
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I have done a load calculation on my house upstairs and the basement. For air conditioning I need a 2 ton system by those results and for heating I would need a 3 ton. The plan is to use my heat pump only as a supplement to my pellet stove.

Question is should I get a 2 ton system, a 3 ton system or something in between?


Also I have done a good bit of research on duct work, I have a decent understanding of velocity and cfm. Rule of thumb is 1 cfm per sq ft, then added percentages for master bedroom, kitchen living room and reduced for bathroom, closets etc. By rule of thumb I need roughly 2100 cfm to supply the correct amount to all of my rooms. So 400 cfm is roughly a ton, that would leave me at a whopping 5 ton system which doesn't seem correct at all. I hope someone can steer me in the correct direction.

1200 sq ft upstairs and 900 finished basement.
 
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Old 07-11-14, 08:07 AM
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Usually you need very little cooling for the basement...

Normally you size for cooling not heating; a 3-Ton will probably be 'way oversized' for cooling.
Forget about your airflow numbers, U size cooling around 400-CFM per/ton; 2-Ton 800-CFM.
 
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Old 07-11-14, 08:26 AM
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Thanks. The only reason I am adding ductwork to the basement is to help move the heat from the pellet stove to the rest of the finished basement. I am going to shut the vents off in the summer to keep from cooling down there a good bit.

I do however want enough airflow to keep the basement warm and comfortable in the winter. It usually hovers in the 60s down there with little heat, but to keep it comfortable with my hot water baseboard, the furnace ran a good bit, and I don't want that anymore.

Ultimate plan is to use the heat pump + warm air from the room that the pellet stove is in to heat the house, with the oil furnace as backup.
 
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Old 07-11-14, 09:55 AM
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I'd, size cooling for main floor & for heating let the backup handle anything the 2-Ton heat, pump doesn't handle.
 
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Old 07-11-14, 01:22 PM
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Thank you very much. I think between the pellet stove in my basement and the heat pump I should very rarely have to use my oil furnace. The basement is fairly open so I am thinking of not even bothering installing any vents in the basement, I believe the warm air will travel through out with very little issues.

If an issue does arise however, I could fairly easy retrofit a duct and vent off of the main trunk to supply to the basement, but I don't think I will have an issue.

The 2 ton Ac was load calculated for my main floor and my basement, but I think even if I excluded the basement I would still be around 2 ton.
 
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Old 07-11-14, 06:58 PM
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I think the very first thing I'd do is add 8 to 10 inches of cellulose insulation over the fibreglas in the attic.

the ceiling is also insulated with 4" fiberglass insulation.
 
 

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