Heat pump technology is an emerging heating technology with advantages such as energy saving, high efficiency, safety, and maintenance-free operation. In today's environment that advocates energy conservation, it has excellent development prospects and broad market demand. Currently, most people in China are still observing and learning about it, and there is not much practical experience in its application. As a supporter of energy-saving technology, I was fortunate to be among the first to design, invest in, and construct the largest heat pump heating system in the country at the time, with excellent practical application results. I would like to share the design and technical scheme for your reference. 1. Project Overview The student apartment building has a construction area of ​​88,000 square meters, with a basement, a ground floor, and 24 floors above ground, at a total cost of 1.8 billion yuan. It will house 7,200 students in 1,800 rooms. The building requires a hot water supply. The original design used an oil boiler located in the ground floor, which was neither safe nor economical for the more than 7,000 students. Given such a large demand for hot water, what heating method is most appropriate? Oil boilers, solar energy, electric heating, etc., were all considered, and ultimately, heat pump heating was chosen. 2. Reasons for Choosing a Heat Pump The main reasons for ultimately choosing heat pump heating are as follows: Initial investment is comparable to solar energy, but 40% lower than oil boilers; operating costs are more than 50% lower than traditional heating methods; it is safe and environmentally friendly, requires less space, and is easy to install, control, and maintain. 3. Heat Pump Main System Design Based on hot water hydraulic calculation principles and actual hot water demand, the average daily hot water demand per person is calculated at 60L, resulting in a total daily hot water supply of 420m³. The system uses 20 18kW modular units with a total power of 360kW. Based on the building layout, the units are divided into 4 groups, with 5 units in each group and 10 units facing east, south, west, and north. Forty 11m stainless steel prefabricated water tanks are installed, corresponding to the 4 groups. Each group of 10 tanks is located in the 4 directions corresponding to the units. Since each unit supports two sets of tanks, to save costs and prevent the lack of backup units in case of a single unit failure, all tanks are connected by connecting pipes. This allows the 20 units to serve as backups for each other while also saving money. The unit design scheme is shown in Figure 1. 4. Water Piping Network Design This building has a large area, numerous units, and a complex piping network, with a total length of over 10,000 meters. It consists of multiple pipes for cold water, hot water, and return water, forming a combined hot water supply network. Cold water is supplied by a high-zone variable frequency pump to a rooftop water tank, which is equipped with a level gauge and float valve to control the start and stop of water intake. Hot water is heated by the heat pump unit and supplied to each water point through various risers. The hot water pipes are made of special hot water pipes that can withstand 90℃ high temperature and 20KG pressure. Due to the multiple floors in the building, to prevent excessive water pressure on the ground floor, a pressure reducing valve is installed on the 15th floor to reduce pressure once (the ideal pressure reduction would be twice, on the 16th and 8th floors, but considering the return water issue, only one pressure reduction is installed; otherwise, the return water pipe pump head would be too large, which would be detrimental to long-term economic operation). The return water pipes are divided into 4 groups and matched with the unit. Each return water pipe is equipped with a pipe circulation pump on the 14th floor to pump the return water to the rooftop water tank to solve the problem of hot water coming out immediately when the user terminal is turned on (if there is no return water system, the hot water pipe will gradually cool down due to the lack of hot water circulation for a long time when the hot water pipe is first turned on, so cold water will flow out. Hot water will only flow out after the cold water in the pipe is drained. The longer the hot water pipe, the longer the draining time. If the draining time is too long, it will cause dissatisfaction among users. This floor has multiple floors and long pipes, so a return water pipe must be installed). 5. Automatic Control This hot water system is a timed supply system, providing hot water from 5:30 PM to 10:30 PM. Therefore, the system has three main automatic control components: timed heating; timed start of the return water pump; and timed water temperature detection. To achieve these functions, a solenoid valve, timer, and signal relay are installed on the rooftop hot water main pipe. These are used to automatically start the solenoid valve to supply hot water at set times, and simultaneously send a signal to the return water pump to start circulating return water. The rooftop water tank is equipped with a temperature control probe; when the temperature drops to a set value (generally 3°C below the standard value), the unit automatically starts heating. 6. Circuit Design This project is currently the largest heat pump hot water project in China, with a large number of units and a high power output, totaling 360kW. To avoid interference with other power supply equipment, a dedicated cabinet and line power supply are used, with four ZR-VV (3x95+1x50) mm² insulated cables leading to the distribution boxes of each rooftop unit. The distribution boxes are equipped with molded case air switches and disconnect switches for easy maintenance and disconnection. 7. Water Tank Main Unit Structural Treatment The roof supports 40 11m³ water tanks, increasing the total load by 440t. The roof's conventional design load is 150 kg/m². Without specialized structural treatment, the water tank load would far exceed the design load, leading to structural safety issues. Therefore, reinforced concrete beams were constructed using the reinforcing steel of the load-bearing columns, and the water tanks were placed on these beams, resolving the safety problem. 8. Problems and Discussion The project performed well after commissioning, marking a successful first phase for such a large-scale heat pump heating project. However, there are some shortcomings, mainly the following two problems: 8.1 The Problem of Hot and Cold Water Source Co-supply The best way to adjust the hot and cold water output ratio is for both hot and cold water to originate from the same source and be at the same pressure. This prevents sudden temperature fluctuations during use and makes it easy to achieve the ideal water temperature. However, this project was originally designed with an oil boiler for heating, with cold water supplied by a downflow, upflow variable frequency system. After the hot water was changed to a heat pump system, it became an upflow, downflow system. Therefore, the hot and cold water lacked co-supply from the same source and at the same pressure, affecting the performance to varying degrees. 8.2 Return Water Issues The project uses a central return pipe, but the return water effect is not ideal in actual use. There are two main reasons: First, the pipes are too numerous and long, resulting in slow return water flow from a single main pipe and incomplete flow in hot water risers far from the main return pipe. Second, water remaining in the indoor horizontal branch pipes cannot return, resulting in a small amount of cold water when the system is turned on, with hot water only appearing after the flow is completely drained. The best approach is grouped return water, with 3-4 risers grouped together for one return cycle. This is much more effective and economical than having a separate central return pipe for each riser. 9. Conclusion This heat pump system has been running stably and effectively for the past six months, saving significant operating costs compared to previous boiler heating. It is a truly energy-saving technology worthy of promotion.