I recently attended a training class by Alabama Power on Manual J training (Residential Heat Load Calculation). They taught the Manual J version Eighth Edition abridged version.  Tim Offord was the class teacher.  Here are the notes I took as well as the link to download a free excel spreadsheet to do the calculations.

In south 15% you can add to sensible load is not over-sized
In north 25% you can add to sensible load is not over-sized

A gas furnace can be over-sized by 40% to help airflow on cooling side

75 degree and 50% humidity is design specs by Apco standard
Basic principles of heat transfer:
Heat is a form of energy that moves from hot to cold
Load calculations are used to determine how much is gained/lost at the Design Conditions for the Location

Modes of heat transfer
1. Radiation. Heat transfer by waves or rays “solar”
Glass areas that collect the sun’s rays Canada significantly to the summer load
No credit is taken for solar radiation in determining the winter loads.
2.  Conduction. Heat transfer by and through a solid such as a wall. Components of the building envelope that conduct Heat are
Ceiling, walls, roof, doors, floors/basement, windows

3. Convection. Heat transfer by a fluid such as a liquid or as a vapor
Air infiltration through the cracks in the building adds to the load
Ventilation is the controlled movement of the air to building for better indoor air quality

3 biggest factors of load are:
Attic, ceiling and windows

An AC was invented to be a dehumidifier, the cooler air was a by-product

Load in btuhs cooling = area x TD x u value
Or area x Htm (Heat transfer multiplier)

U-value = 1/r value. U-value is the ability of a substance  to conduct or transfer heat

Heat gain is the load btuh cooling

Heat loss is the load btuh heating

Sensible load  plus latent load equals total load
Sensible is temp control and latent is humidity control
Apco recommends and Manual j setting
95 outdoor temp
75 degree indoor temp
50% / 63 degree wet bulb

Heating 70 Deere indoor temp

Humidity below about 25% will have static electricity starting to occur

Rounding is weird in manual j books. 1 degree round down, 2,3,4 degree round-up

Foam house and HVAC loads

A foam house is considered tight.  If moisture gets into the house, it has to be removed  by the AC even in the winter or cool days

Why is equipment sizing so important?
1. Needs run time to reach maximum efficiency
2. Moisture removal to control humidity

Basically there are two types of foam insulation
1. Open cell (we see this mostly) soft and flexible r3.6 per inch on average
Better Air barrier with less infiltration but not a vapor barrier
Max thickness of 6 inches recommended
More expensive than bat insulation but less than closed cell

2. Closed cell (more expensive) dense like a styrofoam cup.
Extremely tight home because it’s a air barrier and vapor barrier
R6-R7 per inch
Makes the home stronger and safer to high winds (tornado/hurricane)
However, it’s much more expensive

Attic temperatures are only 4 degrees summer and 6 degrees  in winter due to foam in homes
For ceiling use a u value of .408 which is the worst in the tables but the value the energy efficiency comes from the low Ptdh (6) and Ptdc (4)

When you run a load, normally there are three things that account for the largest part of the load
1. Ceiling load
2. Duct load
3. Glass load

Foam houses are great
Attic temp apps 80 degrees
If ducts are in the attic space – virtually no duct loss or gain
Tighter home

However, caution must be used with a furnace as the that uses house combustion air
1. Self-venting or 90% plus
2, mechanical room that is completed encapsulated

Download the free software from ACCA
Click on standards and codes then ACCA speedsheets
Then click on manual j
Then save as…

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