Heating and Air Conditioning Loads

Heating and Air Conditioning Loads provide the design engineer with rates of heat transfer through the building materials and surfaces – walls, floors, roofs, windows, etc. – and the contribution to the heating or air conditioning systems by various building components – people, lighting, equipment, etc. With this information solidly in hand, the engineer can proceed to design the multiple components of the building’s HVAC system.

Heating and Air Conditioning Loads

Central Plant:

  • Chillers
  • Cooling Towers
  • Boilers
  • Pumps

Air Side:

  • Air Handling Units
  • Ductwork
  • Exhaust Fans
  • Outside Air Systems

Summer Heating and Air Conditioning Loads

The peak – or highest – cooling load during the summer is that amount of heat that must be removed to maintain the room’s design temperature. Factors that contribute to a building’s cooling load are:

  • The sun which heats up the building exterior. This heat is then transferred into the building through walls, roofs, windows, etc.
  • Items within the building that produce heat such as people, lighting, equipment
  • The buildings geographic location
  • The interior design temperature that needs to be maintained

Winter Heating and Air Conditioning Loads

The peak – or highest – heating load during the winter is that amount of heat that must be added to maintain the room’s design temperature. Factors that contribute to a building’s heating load are:

  • Heat lost to the outdoors through walls, roofs, windows
  • The buildings geographic location
  • The interior design temperature that needs to be maintained

Heating and Air Conditioning Loads

The Heating and Air Conditioning Loads calculated for both heating and cooling are necessary to accurately size the equipment – heating and cooling systems, the air distribution systems – ductwork, diffusers, terminal boxes – and the ventilation system. A Heating and Air Conditioning Loads system that is incorrectly undersized will not be able to keep the building at the desired indoor temperature as required in the design. And a Heating and Air Conditioning Loads system that is incorrectly oversized will constantly cycle on/off and will be unable to maintain the proper humidity levels within the building. This can lead to larger problems such as equipment damage, occupant discomfort, and mold growth.

Calculating Heating and Air Conditioning Loads

Heating and Air Conditioning Loads are mostly calculated using computer programs such as Elite Software’s CHVAC and RHVAC programs, Carrier Corporation’s HAP program and Trane’s TRACE program. These and many other programs on the market are highly specialized, can require an extreme amount of data input, produce large quantities of output, are very accurate in terms of Heating and Air Conditioning Loads, and are usually quite expensive. These programs are often necessary when it comes time to perform the actual equipment sizing, selection, and specification. A design engineer who didn’t spend the time manipulating wall construction, roof construction, occupancies, equipment loads, and lighting configurations would not be providing the building owner with professional service. It is imperative that the most realistic and accurate computer model of how the building will function in both heating and cooling environments and at different times of the day, week, month or year be used in designing the building’s HVAC systems.

But there are times when this accuracy and time/effort investment are not fiscally reasonable. For example, in the early stages of the design process, when the owner, architect, and engineer are meeting to determine the best course of design to pursue, it is important to understand the impact of the HVAC system on the overall design especially as it relates to space requirements. It becomes convenient to have a method by which the engineer can estimate these space requirements by quickly sizing the equipment required based on the current architectural design and extrapolating out to come up with mechanical room sizes, access into/out of and overall location within the building.

Heating and Air Conditioning Loads Calculation Worksheet

The Heating and Air Conditioning Loads Calculation Worksheet is one-page “snapshot” design tool based in MicroSoft Excel utilizes psychrometric equations to solve and provide data for heating and air conditioning designs. The simplicity and unique compactness allows the design engineer to input, change and manipulate multiple HVAC load variations which can be altered and adjusted on the spot with the output immediately available on screen. The benefit to the engineer, architect and owner is in the speed of decision making. If the design team can quickly arrive at the most beneficial building design with respect to building materials, site orientation, occupancies, hours of operation, etc., this saves the overall project budget in terms of time – meetings, phone calls, design – money and frustration.

Air Handling Unit Selection Worksheet

The Air Handling Unit Selection Worksheet was designed to be used in conjunction with the Heating and Air Conditioning Loads Calculation Worksheet although it can stand alone. The Air Handling Unit Worksheet uses parameters such as sensible load, latent load and total load to provide the remaining variables needed to select and specify air handling units. The worksheet also allows the design engineer to input known static pressures to get a more realistic estimation of the overall Internal and Total Static Pressures for the units.

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