Mech/Elec ALS Lesson One - Basic Thermal Processes: Heat Load Calculations - Conduction Below Grade, Infiltration Load, Cooling Load, Etc.

Mech/Elec ALS Lesson One - Basic Thermal Processes: Heat Load Calculations

- Conduction Below Grade, Infiltration Load, Cooling Load, Etc.

It is difficult to determine what value to use below grade for the outside temperature since it varies with depth and moisture content

Loss through basement walls and floor slabs is comparatively low.

below grade wall values are taken from a table based on ground water temperature which is assumed to be same as the average annual air temperature

below grade loss = total area X factor from table

Infiltration = flow of air leaking through cracks in building skin, window frames, walls, sockets and switches, etc.

Outside air leaks in, replaces internal air and needs to be heated

Heat required is called "infiltration load" Qi

Calc'd in two steps:

1. determine amount of air infiltration

Two methods, "air change method" OR "crack method"

Air change method = need to know air changes per hour, measure or estimated, good method for tight buildings

Crack method = number of linear feet of crack or joint in all the windows in space, then look at result on table

2. use infiltration amount from step 1 to solve infiltration load (qi)

Infiltration load calc's

Qcfh = LF x CFH/lin.ft.

Qi = .018(Qcfh)(T-inside - T-outside

Total heating load = sum of all loads considered

Heat Load Calc = Qtotal = Qc + Qs + Qi

Where

Qc = conduction(expressed in Btuh's

Qs = Conduction below grade

Qi - infiltration

Temp. Gradients = usefull to calc. temp within wall to avoid freezing and bursting pipes for example.

Cooling load calc's =used to size cooling equipment, many factors must be included

Sources of heat gain to be considered:

Occupants(Qp) - number of people & activity, 450btuh's at rest, 2500btu's for heavy activity,

number of people X btuh per person = people load

Lighting(Ql) - heat in proportion to wattage, 3.4 X Wattage = lighting load

Equipment(Qm) - many different ways to calc. btuh's, wattage like lighting, motor horsepower

Cooling Load Temperature Differential (CLTD) - heat gain through walls and roofs

1. determine wall/roof classification or group

2. determine base CLTD from the time of day, wall type, wall orientation and wall or roof color

3. adjust CLTD based on the temp. history of last 24 hours.

Radiation through windows: (qr or SHGF-Solar Heat Gain Factor)

insolation = radiant gain through glass

several methods to calculate: Intensity of direct sun X glass area X percentage transmitted = SHGF

Percent transmitted compared to clear glass is the Shading Coefficient (SC).

There are tabulated SC values

Best to stop direct sunlight OUTSIDE the building, using fins, overhangs or awnings.

Prevents heat gain on interior, IE. interior blinds would block light but heat up and radiate heat. If required, lighter colors are better since they absorb less.