Heat Transfer

We humans build structures for safety and living that also undergo heat transfer processes. The purpose of any shelter is to provide privacy, security, and protection from natural occurrences. The temperatures inside these structures are impacted by environmental conditions, construction materials, and the humans living in them.

Heat must be released in these shelters for humans to achieve thermal comfort levels. We are walking around as bio-chemical combustion chambers that turn food and oxygen into mechanical work and heat. We are constantly putting out heat. And that’s the way you want it.

Factors that impact thermal comfort are the rate at which heat transfer processes through human skin, the ability of heat to transfer through the structure and the actual indoor temperature.

Heat as Energy

Heat is a byproduct of energy creating factors at the cellular level. The burning of a fuel must take place and this combustion action creates a transformation of the energy, not an eradication of it. Energy is never lost, only changed by outside factors.

Systems and therefore structures tend toward the conservation of energy. A structure’s heat gain or loss is impacted by the rays of the sun, the number of occupants, and the source of the energy such as electricity or fuel being used to create heat. This heat capacity can be lost through the envelope of the structure, the insulation being used, and the entire surface area of the building

Reducing Heat Loss

According to theory and as substantiated by experience, there are essentially three ways to reduce heat loss in a structure. First, the set point on a thermostat can be reduced during the winter. Second, the insulation depth and quality can be increased to reduce heat loss. Third, the surface area can be reduced without changing the volume of enclosed space.

Methods of Heat Transference

Heat energy is transferred by conduction or by molecules interacting with one another. It may also be transferred by the principle of convection, which is air or a liquid being heated. As the air or liquid is heated, it travels away from the heat source carrying the heat energy with it. Radiation is the transfer of heat through infrared or visible light.

HVAC Heat Load Calculation

So, the reason the understanding of the principles surrounding heat transfer is important is because any installation of an HVAC system will require an accurate assessment of how much heat a space is producing and how much it will lose in any 24 hr. period. This calculation will also influence and inform how much tonnage will be necessary to cool and heat a space.

In the world of HVAC, this is usually called a Manual J HVAC load calculation.

There are softwares that will make these calculations, and the basic factors to consider are:

The size of the area. This is done by measuring the total space and squaring it. Then, multiply this by 31.25. This generates the Area BTU.

The number of occupants. This calculation is done by multiplying the number of people by 600 BTU.

The size and placement of windows. This is done for each side of the structure where windows are present. The North windows with shading are measured in m. sq x 164. The North windows with no shading are rendered BTU x 1.4. The South windows with shading are m. sq. x 868. The South window with no shading are rendered BTU x 1.4. These results are all added together.

For the machinery and equipment, you must find the watts for each item, total them and then multiply this by 3.4.

Heat generated by lighting is done by adding together the wattage for each light and multiply this by 4.25.

The final calculation results in the total heat load. This is an adding together of the Area BTU + Total Window BTU + Occupant BTU + Equipment BTU + Lighting BTU.