Thermal Loads Part-2

Autodesk Support

Apr 26, 2018


Continuation of Thermal Loads Part-1

Heating and Cooling Loads

Internal and external thermal loads translate to heating and cooling loads. This is how much heat energy you need to heat and cool the building, and control moisture within the building.

Loads are usually calculated as the amount of energy that needs to be moved into or out of the building to keep the temperature at a specified point (setpoint).

  • If heat gains are greater than envelope and ventilation losses, the building or space has a net cooling load(the building is too hot).
  • If heat losses are greater than the internal gains, the building or space has a net heating load (the building is too cold).
  • The heating thermostat setpoint is often different than the cooling thermostat setpoint both to save energy and because of human preference. The distribution of heating and cooling loads is climate dependent.
     

The heating and cooling loads below provide a break-down for what drives the heating and cooling energy demand.

Monthly heating and cooling load charts tell you where heat energy is being gained and lost.

The charts list heat conduction through windows separately from solar radiation heat gain through windows, as well as separating heat transfer through roofs, walls, and floors or underground areas.

An explanation for how to interpret heating and cooling load charts.
 

When interpreting energy load charts, pay attention to whether the biggest heat losses and gains come from internal or external loads. 

Also note that it is the PEAK heating and cooling loads that are used by engineers to size HVAC equipment. These energy analysis graphs are meant to help understand energy flows, not size equipment. However, using energy analysis tools can allow you to better understand and calculate energy use so that you can avoid oversizing equipment and look past the typical “rules of thumb.”
For guidance on how to make these calculations, look at the links provided below and reference texts like Mechanical and Electrical Equipment for Buildings By Walter T. Grondzik, Alison G. Kwok, Benjamin Stein, John S. Reynolds.

Using Energy to Meet Heating and Cooling Loads

The values in the heating and cooling load charts above represent the amount of heating or cooling required, not the amount of energy a HVAC system would actually consume to generate the required load.

Passive systems reduce the energy demand or meet it naturally. Active systems move heat and moisture using gas or electricity. How much and what type of fuel the HVAC system will consume depends on the system type and efficiency.

When using active systems, it usually takes more energy to meet heating loads than it does to meet cooling loads. Heating systems based on combustion of a fuel are approximately 75%-95% efficient at converting the chemical energy in the fuel to heat delivered to the building. The efficiency of cooling systems (and heat pumps in heating mode) is not measured in percent efficiency because they do not convert potential energy to delivered heat, rather they use energy, most commonly electricity, to move heat either into or out of a building. The Whole Building Design Guide provides ranges of efficiency values and sizes that are typical for various types of cooling systems – see WBDG. Heat pumps and air conditioners use energy to move heat, they do not generate coolth – see Heat Pumps). The cooling effect that we feel is the removal of heat rather than the addition of coolth.

Also, when you put cost into the equation it brings another level of complexity because heating fuel is much cheaper per unit of energy than electricity. Building owners often spend more on energy to cool their building than to heat their building. There are many reasons for this, but the easiest to understand is that electricity typically costs three to five times more than heating fuel per unit of energy.

Balance Points

The concept of a building’s balance point can help designers determine when heating or cooling is required in the building. The balance point is the outdoor temperature at which the building makes a transition from a heating need to a cooling need. It is calculated by comparing internal heat gains (from people, equipment, etc) with external heat losses (from building infiltration, etc). It is not the ideal comfort temperature inside the building. It is the temperature at which the building's heat gains equal its losses.

  • If the temperature is BELOW the balance point, heating is required.
  • If the temperature is ABOVE the balance point, cooling is required.
  • If the temperature is AT the balance point, no heating or cooling is required, because the building is gaining as much heat as it is losing.
     

For example, if the balance point of a building is 65 degrees Fahrenheit and the outdoor temperature is 75 degrees Fahrenheit, a passive cooling strategy like shading would be helpful at that time.
Buildings that have high internal heat gains (like offices), and low rates of heat loss (well-sealed and well-insulated), will have a lower balance point.

Links and References



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