Examining a Non-Mixing Concept for Hydronic Heating Systems: A Simulation Study

While buffer tanks are common to address the short-cycling concern of hydronic heating systems, they involve water mixing, where the boiler’s hot water is mixed with the system’s return water. This water mixing condition brings concerns of the performance of the boilers and the in-space heating equipment (e.g., radiant floors and terminal units) due to the reduced temperature differences (ΔT) through these devices. Then, a non-mixing concept is introduced by putting a movable separation plate in a buffer tank, which can store supply hot water and return cold water separately. To examine this non-mixing concept, a simulation study is conducted by developing two hydronic system models with mixing and non-mixing concepts. After simulating these models under continuous heating loads, it is observed that the non-mixing model can achieve better system efficiency (e.g., 86.7% versus 82.0% for low heating loads) due to better boiler’s efficiency and less system cycles. In addition, it is noted that the non-mixing concept allows better control of supply water temperature, which can better support the outdoor reset control of boilers.