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Heat Recovery System

Engineering Inputs: Building System

Patrick Chopson avatar
Written by Patrick Chopson
Updated over a week ago

Heat Recovery System input defines the type of Air Side Heat Recovery used. Heat Recovery, or Energy Recovery is a common feature in HVAC systems and is required by many codes around the global. The core concept is to recovery energy (heat) from already conditioned air within a building to reducing the heating and cooling energy required. There are several types of heat recovery devices to fit various project types. Heat recovery should be considered on all projects as it is one of the most straight forward solutions for reducing energy use in the built environment.

cove.tool input options for Heat Recovery System are:

  • No Heat Recovery: No energy is recovered from the conditioned air within the building.

  • Run Around Coils, No Pipes: A dedicated fluid loop (typically water or glycol solution) transfer heat from exhaust air streams to supply air streams. The transfer of heat is achieved via finned coils within the air stream (very similar to traditional coil or heating coils) and a pump is used to circulate the transfer fluid. Run around coil systems are common for projects where the exhaust and supply air streams are in different locations, for example laboratories and hospitals. One advantage of a run around coil system is zero leakage between the air streams, however the addition of the circulation pumps uses electrical energy reducing the energy saving potential of these systems. Run around coil systems only recovery sensible heat. Here is a simplified diagram of a run around coil system.

  • Heat-Pipes: The constant circulation of evaporating and condensing of the working fluid is the principle of the heat pipes heat transfer. It is a vacuum-tight device consisting of an envelope, a working fluid, and a wick structure. Heat pipes can transfer heat with the minimum temperature difference between one end and the other. Heat-pipes transfer heat effectively between two air-streams that are directly next to each other. Like run around coils only sensible heat is transferred, however the working fluid is self-circulating an no additional pump is required. Heat pipes are best used in applications with high dehumidification needs as they can be configured to reduce reheat energy. Learn more about heat pipes here

  • Loading Cold with Air Conditioning: The loading cold with air conditioning is the most basic form of heat recovery. Two ventilation ducts are run next to each other, one carries cool, outside air in and the other carries stale, warmer air out. These air streams run through a heat exchanger to transfer heat from outgoing air to incoming air. There is no leakage between air streams, however the effectiveness is low due to the limited time the air comes in contact with the heat exchanger and the passive nature of the heat transfer. Loading cold with air conditioning is most common in residential where flows are small. Here is a simplified diagram of the two air streams.

2020 ASHRAE Handbook: HVAC Systems Plate Heat Exchanger Diagram
  • Sensible or Enthalpy Wheels: The enthalpy wheel utilizes the heat/cold in an exhaust ventilation process to precondition the make-up air to reduce the required energy load of the system. The wheel is placed into a make- up air or air handling unit to capture the heating/cooling energy from the exhaust air and transfer it to the supply air. Thanks to their high effectiveness and ability to recovery latent and sensible heat from the exhaust air streams wheels are very common in offices, education, apartments and other commercial projects. Wheels are not advisable for buildings where IAQ is a specific worry like hospitals and labs due to the small leakage which occurs over the rotating surface. Here is a sketch of a typical enthalpy wheel

2020 ASHRAE Handbook: HVAC Systems Rotary Air to Air Energy Exchanger

Each Heat Recovery System corresponds to an effectiveness. Higher effectiveness equate to more energy savings for the project. The effectiveness ratings used in cove.tool are provided in this table.

The Heat Recovery System input is located on the Baseline page, under the Building Systems tab.

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