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 3.414 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