CNERG: Study of temperature in and around septic systems for feasibility as heat pump heat source

Karen Kokkelink, P.Eng. School of Engineering & Technology

This project investigated the thermodynamic feasibility of installing ground-source heat pump (HP) heat exchangers alongside or inside septic systems to leverage higher soil temperatures during the heating season. Ground-source HPs, known for their higher efficiency compared to air-source HPs due to better thermal conductivity in moisture-laden soil, face high capital costs from the need for deep vertical boreholes or wide horizontal installations. The research aimed to address these cost issues by exploring the potential thermal benefits of integrating heat exchangers with septic systems in rural Ontario. 

The research objectives were to quantify the available heat, calculate the performance effect on a heat pump system, and assess whether the cooling effect on the septic system would harm its performance. Ground temperatures were measured at varying depths and distances from operational septic systems, and the resulting temperature profiles over time were analyzed. The study determined that integrating heat exchangers with septic systems could potentially enhance heat pump efficiency and reduce installation costs by shortening the heat exchanger loop or downsizing the compressor. Additionally, combining septic tank work with heat pump installation could further economize the process, making high-efficiency ground-source heat pumps more viable for smaller properties and remote, cold northern Ontario applications.