Abstract: Resent years, air-to-water heat pump products have developed a variety of using conditions, which can be integrated with a variety of heating sources and applications with a strong flexibility. The double temperature zone of ATW includes radiation branch and floor heating branch. The operation requirements of different inlet water temperatures can be realized in the two branches, and the water mixing unit is the core component. The water mixing mode can be divided into two types: the manual valve located on the parallel bypass branch of the water mixing valve is opened or closed. Furthermore, the relationship of water flow in double temperature circuits can be divided into several cases. The results are shown as follows: when the manual valve on the parallel bypass branch of the water mixing valve is closed, it is easy to cause large water flow fluctuation of the radiation branch, which is not recommended. When the manual valve on the parallel bypass branch of the water mixing valve is opened, the water flow fluctuation in the double temperature zone is small, which is max. 9.72% of flow variation rate in the test. During the circuit design, the circulating water flow of floor heating is equal to the water flow of floor heating branch is preferred. While for the complex situation of on-site pipeline, a larger water pump of main circuit can also be selected appropriately.

Key words: ATW, Double temperature circuits, Mixing unit, Water flow rate