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Southern California Manual J load calculations – Residential Manual J HVAC Design around the United States.


A Manual J for Southern California can be tough if you do not perform Manual J HVAC designs on a regular basis. Most Residential HVAC designers have only worked within one climate zone - experience limited to one area.

At Savoy Engineering Group, we have performed HVAC Manual J, S & D designs in all the climate zones within the USA.

The major issues to consider for Southern California Manual J report:
•Title 24 errors that are carried over to the Manual J
•The Manual J design criteria can change drastically just going 25 miles in a different direction. Many different climates in such a small area
•As autumn arrives, temperatures can climb, not fall, in Southern California
•Required by CalGreen but not enforced

Title 24 Issues:

Often we are given Title 24 reports to use as inputs for the Manual J load calculation. However, these reports often don’t match the section cuts or have errors. The top 7 errors we see in California Title 24 reports are:
1.Glazing areas and U-values miscalculated.
2.Doors with glass modeled as solid wood.
3.Floors over unconditioned space omitted.
4.Shading coefficients incorrect.
5.Vaulted ceilings modeled as attics.
6.Ductwork location on plans and report not in agreement.
7.Inaccurate mechanical equipment sizing.

How does Title 24 inaccurately size residential HVAC equipment? ACCA Manual J may calculate a house heating load to be 44,000 BTUs. The Micropas program used by Title 24 calculates a 50,000 BTU heating load. This isn’t a huge difference but then, they added a 30% safety factor, for extreme weather fluctuations bringing the load to 65,000 BTUs. The HVAC contractor will recommend a 100,000 BTU furnace. This is TWICE the Manual J load calculation result!


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Design Temperature changes based on location:

The typical new California home in the central valley and the desert has a gas furnace and a split system air conditioner. In some areas, a heat pump provides both heating and cooling, eliminating the furnace. In coastal climates and in the mountains, air conditioning is rare and most new homes are heated by gas furnaces.

Coastal cities have a Mediterranean climate with a summer design temperature of 85 F but humid while desert areas such as Needles, CA where it’s dry and 115 F in the shade. Knowing what design criteria to use is essential to a correct Manual J result.

Autumn can be warmer than summer in Southern California

Warm ocean waters and muggy nights result in potentially record-breaking heat. These are typically the tell-tale signs of summer but in Southern California this is extremely common in the fall months.

The hot days are fueled by unusually warm ocean waters, which limit the cooling effect from ocean breezes at night. The warm-water phenomena has been dubbed “the blob” by experts, and its origin is unknown. After sunset, temperatures can still hover in the upper 60s

Neither CalGreen nor Southern California has addressed this issue so it has to be handled on a case by case basis.

Mandatory Manual J reports not enforced:

California has required Manual J load calculations since 2008. Here's an important fact for HVAC contractors to remember: Manual J calculations are code-required; they are not optional. If an HVAC contractor isn't performing a Manual J calculation (or another load calculation using a method that is approved by code authorities), then the contractor is probably breaking the law.

Permit offices do not enforce compliance so the majority of HVAC systems installed in California are not properly designed using the recognized industry standard of Manual J load calculations and Manual D duct design. Please contact Savoy Engineering Group if you have any questions.



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