- Check all numbers for
consistency. For example, in typical construction, total
area of exterior walls facing north or east (including a
wall to garage) is usually equal to the total area of the
opposite south or west walls; ceiling area is usually
equal to the building footprint area; window area is
usually from 10% to 25% of the floor area; gross wall
area is bigger than the window area.
- Use design outdoor
conditions and daily temperature range exactly for your
location per Manual J . Otherwise, use the data for the
closest location with a similar climate.
- Use standard 75
degrees F design indoor temperature.
- Consider both location
and level of insulation of ducts.
- When selecting cooling
factors for roofs, floors, and walls consider their
R-value and type; for example, frame wall or masonry
wall. Partitions and knee walls that separate a
conditioned space from an unconditioned space like an
attic or garage should be treated separately from the
exterior sunlit surfaces.
- Always account for the
effect of the overhang shading. This is one of the most
efficient load reduction measures. When calculating this
effect, consider window height, overhang length and
distance to the top of the window.
- Pay great attention to
window type, material, and interior shading. An error in
this area can throw off the window heat gain.
- Calculate infiltration
rate based on blower door measurements.
- Calculate the latent
load based on the number of people and the outdoor air
humidity ratio. Do not use a "typical"
multiplier of 1.3 or any other to calculate the total
load from the sensible load. This implies that every
building has a latent load that is exactly equal to 30%
of its sensible load and that the quality of construction
and location are not important. It also means that if a
new source of the cooling load is added, for example
another window, the moisture gains will also increase.
This simply is not accurate.
- Consider ventilation
load if appropriate.
- Select equipment based
on the detailed manufacturer's performance data. Do not
rely on the nominal tonnage since different units may
have more than 10% capacity difference.
- Choose equipment based
on the ACCA Manual S without using any safety factors.
This method selects the unit that has the sensible
capacity at least equal to but no more than 15% greater
than the building sensible load, and the latent capacity
equal to or greater than the latent load at standard
indoor conditions of 75 degrees F dry bulb and 50%
relative humidity.
- Test the duct leakage
with a duct test rig and suggest that that the ducts be
sealed before the new air conditioner is installed. Use
the appropriate sealant and retest to ensure the quality
of the job.
- When installing a
complete system, design the duct work using Manual D.
Test for proper installation.
- Properly evacuate the
coil and refrigerant lines before releasing the
refrigerant charge from the outside unit into the system.
- After installation,
check the air flow across the coil and the refrigerant
charge using the manufacturer's suggested methods.
Correct any deficiencies.