I don't think my HVAC system is sized right - Anyone know if I can calculate if it is good?

[cjchaps]

In the summer when it's really hot in Chicago my A/C doesn't seem to be able to keep up. Heating works fine. I have had the system checked and everything is functioning correctly so that's not the problem. I believe it is sized wrong because my house has two rooms with cathedral ceilings that were an addition before we bought the place and I don't know if the A/C was resized to take those rooms into account.

Anyone know of a place I could do some research and see if I can figure out if things are sized properly?

 

[quikah]

Just search google for air Conditionaer BTU sizing

A quick search revealed the following:

Square Feet Approximate BTU'sNeeded
100-150 5,000
150-250 6,000
250-300 7,000
300-350 8,000
350-450 10,000
450-650 12,000
650-850 13,000
850-1100 18,000
1100-1400 24,000
1400-1800 25,000
1800-2500 29,000

 

[SarcasticDwarf]

Note that while the above chart is important, the airflow is critical. How the ventilation is laid out makes a huge difference.

 

[FreedomGUNDAM]

residential HVAC systems can only cool to a delta T of 20 degrees. Thus if it is 100 degress outside, the lowest indoor temperatures is 80+ degrees. I

If you size increase the HVAC in your house too big, you will have humidity problems that will lead to mold growth.

BTW: I work in the commerical HVAC industry for over 10 yrs

 

[brandonbull]

Originally posted by: FreedomGUNDAM
residential HVAC systems can only cool to a delta T of 20 degrees. Thus if it is 100 degress outside, the lowest indoor temperatures is 80+ degrees. I

If you size increase the HVAC in your house too big, you will have humidity problems that will lead to mold growth.

BTW: I work in the commerical HVAC industry for over 10 yrs

What?

I think you mean that the A/C works by having inlet and outlet temps of the refrigerant with a delta T of 15-20 degrees. The evaporator and condenser changes the state of the refrigerant to cool the air. The"outside" air does have an impact on the lowest temp a unit can cool to but not the example you gave.

While going to college, I helped design A/C systems for most of the mobile home manufactures in the US.

There can be several reasons why you are not getting the air temp you wish. It is correct about oversizing an A/C unit will lead to "short cycling". Your system should reduce the humidity and cool at the same time and if it runs for short periods it can't remove excess moisture. If you live in a dry climate, then you should go with a swamp cooler.

You need to see if some of you rooms are getting cooler than the rest of the house. If that is the case, you need to reduce the airflow to that room so it can go to other rooms. It depends on the R value of the insulation, type of house construction, zone you are located in, windows, how much window surface area, wall surface area, how the house sits relative to the sun, how big you house is, and other factors.