HVAC523 Heat Gain

Heat First law of thermal dynamics states that HEAT TRAVELS FROM HOT TO COLD. 95 degree outside air will flow through the building materials to an indoor space at 75 degrees. 72 degree indoor heated air will flow through building materials to the outdoor space at 20 degrees.

The BTU BTU Stands for British Thermal Unit In the US it is the conventional unit of heat. It is the amount of heat required to raise or lower one pound of water one degree Fahrenheit.

Heat Loads There are several types of heat loads: Sensible heat is transmitted through: Walls Ceilings Floors Windows Doors Sensible heat is the heat you can measure.

Heat Loads Latent heat is transferred through: Ventilation Infiltration Duct Loss or Gain Latent heat is mostly in moisture, but it is heat you can not measure.

Ventilation Air that is being taken into a structure to supply air for heating systems. Air exchanges are done to remove odors, fumes, etc. Keep in mind all humans exhale Carbon Dioxide that in high levels can make people sleepy. Ventilation is planned infiltration.

Infiltration Infiltration is air that leaks in and out from cracks around windows, doors and spaces in the walls.

Additional Heat Loads Solar Heat Load The radiant heat from the sun through windows and doors. Solar heat load varies based on the time of the day.

Additional Heat Loads Electrical Heat loads from electrical devices Lights Motors Electrical heat loads five off BTU/Hr of heat per Watt.

Additional Heat Loads Gas Appliance Heat Loads Cooking ranges Pizza oven Clothes Dryer

Duct Loss or Gain The heat loss picked up from the air temperature around the duct. A duct passing through an unconditioned space will pick up heat or give up heat from the surrounding air.

Duct Loss and Gain Ducts running through unconditioned spaces must be insulated to prevent picking up heat. Air conditioning ducts will sweat where the dew point is high. Basements Attics Outside areas such as roofs and outside walls

Duct Loss and Gain Ducts run through attics will reduce the system capacity for both heating and cooling. Duct insulation allows some heat transfer or leakage. Multipliers (magic numbers) are used to compensate for duct leakage. A good rule of thumb is 10%.

Surface Film Factor Wind velocity has a wiping effect that removes heat from surfaces and is taken into consideration when calculating heat gain. Wind velocity can also cause evaporative cooling and increase infiltration.

People Heat Load The average person gives off 300 BTU/Hr. This is both latent and sensible heat.

Latent Heat Loads Humidity in the Air To remove one pound of water from the air requires 970 BTU’s

ACCA Manual J The Air Conditioning Contractors Association has developed formulas and guidelines for calculating the heat load. The outdoor temperature must be known. This outdoor temperature is called the “Design Conditions”.

ACCA Manual J To properly design a system you should know both the “Winter Design Temperature” and the “Summer Design Temperature”. In the Hartford area the summer design temperature is 95 degrees. In the Hartford area the winter design temperature is 0 degrees