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I. Functions of the buffer tank 1. Failing to use a buffer tank will cause the main unit to start and stop frequently. Due to the limited amount of water circulating in the loop, the main unit reaches its designed temperature in a very short time, after which it stops operating. Again, in a short period of time, the water temperature reaches the conditions necessary for the unit to start up, and it begins operating once more. Such frequent startups **reduce the service life of the unit and result in energy waste. Not only is a buffer tank needed, but it’s also best if the tank has a larger capacity. 2. Efficient defrosting with a short defrost time. When used in severe winter conditions, especially between -3°C and 5°C, frosting on the main unit becomes quite noticeable, and the impact of defrosting on indoor temperature poses a significant problem. Because the unit needs to absorb heat from within the pipes during reverse cooling, if there is a low volume of water in the water system, the defrosting time increases, and this also results in lower water temperatures inside the pipes, leading to poor defrosting performance. If a buffer water tank is installed, during the defrosting process, the presence of a certain temperature in the tank allows for rapid defrosting with less heat consumption. This prevents fluctuations in indoor temperature caused by the main unit’s defrosting process, which greatly contributes to stabilizing the overall performance of the system. 3. It ensures smooth water flow in the system, enables complete automatic exhaust, and prevents the unit from reporting faults. We know that when the water tank is placed in a return water system, the circulating water enters from the upper part of the tank and exits from the lower part; as a result, gases in the water accumulate in the upper space of the tank. The pressure within the closed system automatically forces these gases to be expelled through the exhaust valve at the top. Since there are no gases at the outlet at the bottom of the buffer tank, which connects to the inlet of the tank, this not only protects the impeller of the pump but also ensures that there are no gases in the system, thus allowing the main unit to function properly. If there is no buffer tank, the presence of gas can cause problems such as malfunctions in the flow switch and high-pressure alarms in the main unit. 4. It enables more thorough waste discharge from the system, preventing blockages. There are a large amount of impurities in our system, and these impurities gradually accumulate at the bottom of the buffer tank over time. As the water passes through the Y-filter, its quality improves, which in turn reduces the need to clean the Y-filter. II. Method for selecting the size of the buffer tank: Method 1: Buffer tank capacity = Water volume required for system stability – Actual water volume in the system. First, calculate the amount of water required for system stability, denoted as M1; thus, M1 = QT ÷ (C△T), where Q is the heating capacity of the unit in KW ； T is the defrosting time, in seconds; C is the specific heat capacity of water, taken as 4.187; △T is the maximum allowable decrease in the supply water temperature, taken as 3°C. Actual water volume in the system = water capacity per meter of the underfloor heating pipes × total length of the system pipes. If the end is a radiator, the water capacity of the radiator must also be taken into account. Taking a machine from a certain brand in Beijing as an example, for a space of 150 square meters equipped with a 7P low-temperature heat pump heater with a heating capacity of 20 KW, the amount of water required by the system is 20×4×60÷(4.187×3) = 382 (L). Actual water capacity of the system: Using the 20-pipe system commonly used in the north, the water capacity per meter for 20 pipes is approximately 0.314 L; considering that each square meter corresponds to about 5 meters of pipes, the actual water capacity of the system is around 235 L. Therefore, the volume of the buffer tank required is: 382L – 235L = 147 (L); 150L will suffice for actual installation. Method 2: If it’s your first time installing a domestic heating system and you’re not sure how to calculate the required capacity, you can simply use tanks with capacities of 100L or 150L. Regardless of the size of the system, use either a 100L or 150L buffer tank; having these two sizes on hand will be sufficient to meet the needs of most projects.