Combining High-Performance HVAC Energy Recovery and Heat Pumps

Rudolf Zaengerle, Konvekta USA Inc.

High-Performance Runaround Energy Recovery Fundamentals:

High-performance runaround energy recovery systems with advanced control software are operating at efficiencies of net 70-90 percent (based on annual energy consumption for heating and cooling). Advanced control software provides continuous recovery and efficiency reporting and verification.

It's critical that high-performance systems operate at optimum performance under varying operating parameters. With several variable input parameters, controlling and optimizing a system requires a numerical-simulation-based controller that allows variable amounts of heat transfer fluid to be circulated throughout the system. In multi-functional systems, additional heat and/or cold is introduced into the glycol circuit, either to boost the heating/cooling capability of the energy recovery system from waste heat/cold sources, or to control the supply air temperature to the building to eliminate the need of separate heating/cooling coils. These features add yet another level of complexity to the controller function.

Heat Pump Fundamentals:

Heat pumps are designed to move thermal energy opposite to the direction of spontaneous heat flow by absorbing heat from a cold space and releasing it to a warmer one, using mechanical or chemical power to do so (most commonly known example: chiller). Performance of a Heat Pump is defined by the Coefficient of Performance (COP), which is thermodynamically limited by the ideal Carnot process. In words: the larger the temperature difference between the two (hot and cold) spaces, the smaller the COP.

This fundamental determination of the COP requires careful design of a process where a Heat Pump is applied to heating a building: the heat source should be as warm as possible (e.g. ground source heat 30-50F, not outside air at 0F or below) and the heat sink as cool as possible (e.g.70-90F hot water for heating, not 160-180F).

Combining Runaround Energy Recovery & Heat Pump:

Building exhaust air seems to be an ideal heat source for a Heat Pump to heat the building. However, the Climate Zone a building is in and the supply air temperature are the main parameters that determine whether exhaust air should be used as heat source for a Heat Pump or whether it should be used as heat source for a high-performance energy recovery system. It will be shown that:
1. The combination of Runaround Energy Recovery and Heat Pump with exhaust air as common heat source makes the most sense in moderate Climate Zones (4) and if the supply air temperature is relatively high (>60F, neutral air ventilation)
2. In cold Climate Zones (5/6), the combination only makes sense if an alternate heat source (e.g. ground heat) is available for the Heat Pump.

Learning Objectives

  • Identify the thermodynamic fundamentals of a high-performance runaround energy recovery system.
  • Define the thermodynamic fundamentals of a heat pump.
  • Under what circumstances (climate zone, building parameters, heat sources) should the combination of a runaround energy recovery system and a heat pump be considered.
  • How should an energy recovery system and a heat pump be combined for the highest possible energy efficiency.


Rudolf Zaengerle is the President of Konvekta's North American operation, a subsidiary of the Swiss based Konvekta AG.

He holds a Master of Mechanical Engineering degree and a PhD in Business Administration, both from the Swiss Federal Institute of Technology, Zurich.

He was an Assistant Professor at the Swiss Federal Institute of Technology's Energy Sustainability & Urban Planning Institute and has managed Swiss technology businesses in the US for the past 20 years.


Note: I2SL did not edit or revise abstract or biography text. Abstracts and biographies are displayed as submitted by the author(s).