TN 75: A design guideline of energy recovery ventilation to improve actual contribution to energy saving in buildings

In this design guideline, key points are explained to succeed in reducing energy use for HVAC systems with energy recovery systems. It covers the reduction of the heating and cooling need (load) due to ventilation and infiltration as well as the reduction of energy use for fans and heat generators. As background of the key points, performance indices and testing standards of energy recovery systems are explained.

To depict the key point in a quantitative way, a building energy calculation program integrated with algorithms for energy recovery systems is used for case studies. At a maximum, 18% and 13% reduction of total energy use for HVAC systems have been shown for central heat generator and rotary wheel energy recovery units and for variable refrigerant flow systems and fixed-plate energy recovery ventilators, respectively.

The following are the key points:

1. It is necessary to maintain the air balance between total supply airflow and total exhaust airflow for each floor or section in buildings in order that the mechanical ventilation does not induce infiltration or exfiltration through the building envelope. The air leakage may deteriorate energy saving intended by using energy recovery systems.
2. To maintain the air balance, adjustments of mechanical airflows and/or replacements of exhaust-only ventilation with balanced ventilation are necessary in the design and installation. Those adjustments and replacements should be made taking specific fan power (SFP) of ventilation equipment into consideration.
3. When energy recovery systems are used, the airflow ratio (the ratio of supply airflow to exhaust airflow) should be equal to or closer to 1.0, so that efficiency of the systems is maintained higher.
4. When choosing energy recovery systems, net efficiency should be paid attention rather than just gross efficiency. Unit exhaust air transfer ratio (UEATR) included in the performance data of energy recovery systems should be used to calculate net efficiency.
5. The energy recovery system is one of the methods to reduce heating and cooling need (load). To reduce the energy use of heat generators, it is indispensable to carry out the sizing of the heat generators according to a standardized sizing protocol. Without sizing, the part load ratio of the heat generators may fall in the range, in which no reduction of energy use can be expected.