I. Functions of a Buffer Tank
The buffer tank is an important auxiliary device in air conditioning heat pump systems. Its functions mainly include stabilizing system operation, improving energy efficiency, protecting equipment, and optimizing user experience. The specific functions and underlying principles are detailed below:
1. Prevents Frequent Compressor Cycling: Without a buffer tank, the circulating water volume in the loop is limited. This causes the heat pump unit to quickly reach the set temperature and shut down. Shortly after, the water temperature will rapidly drop to the restart condition, causing the unit to start again. Such frequent cycling greatly reduces the unit’s service life and wastes electricity. Not only is a buffer tank needed, but a larger tank capacity is often more effective.
2. Enables Efficient and Short Defrosting: In harsh winter conditions, particularly at temperatures between -3°C and 5°C (26.6°F to 41°F), the outdoor unit’s coil is prone to heavy frosting. The impact of defrosting on indoor temperature is a major concern. During reverse-cycle defrosting, the unit draws heat from the water in the piping system. If the system water volume is small, the defrost time increases, and the water temperature in the pipes can become quite low, resulting in poor defrost performance. Adding a buffer tank ensures that there is a reservoir of warm water available. During defrost, this stored heat allows the defrost cycle to complete quickly while consuming less heat energy, avoiding indoor temperature fluctuations that would otherwise occur. This greatly stabilizes the thermal performance of the terminal units (e.g., radiators, underfloor heating).
3. Ensures Smooth Water Flow and Complete Air Elimination, Preventing System Faults: When the buffer tank is installed on the return water line, the circulating water enters from the top of the tank and exits from the bottom. Any entrained air bubbles rise and accumulate in the upper space of the tank. The pressure in the closed system then automatically forces this air out through an automatic air vent valve at the top of the tank. Since the water exiting from the bottom of the tank is free of bubbles, it protects the pump impeller from cavitation and ensures an air-free system, allowing the heat pump unit to operate normally. Without a buffer tank, the presence of air can cause flow switch errors and high-pressure alarms on the unit.
4. Facilitates Thorough System Purge and Prevents Clogging: The system inevitably contains dirt, debris, and other impurities. As water circulates, these impurities gradually settle and collect at the bottom of the buffer tank. By the time the water passes through the Y-strainer (the filter before the pump), its quality is much better, reducing the frequency of cleaning the Y-strainer and preventing blockages.
II. Sizing the Buffer Tank
Method 1: Calculation Formula
The formula is: Buffer tank capacity = Required system water volume – Actual system water volume
Step 1: Calculate the required system water volume (M1).
M1 = Q × T / (C × ΔT)
Where:
Q = Heating capacity of the heat pump unit (kW)
T = Defrosting time (seconds). Typical value: 3–8 minutes (convert to 180–480 seconds).
C = Specific heat capacity of water. Constant value: 4.187 kJ/(kg·°C)
ΔT = Maximum allowable drop in supply water temperature (°C). Typical value: 3°C.
Step 2: Calculate the actual system water volume.
For underfloor heating: Actual water volume = Water volume per meter of pipe (L/m) × Total pipe length (m)
Note: If the terminal units are radiators or fan coil units, their internal water volume must also be included.
Example (using a specific brand from Beijing):
A 150 m² house equipped with a 7 HP low-temperature heat pump unit (heating capacity = 20 kW, defrost time = 4 minutes or 240 seconds).
Required water volume: M1 = 20 kW × (4×60) seconds / (4.187 × 3°C) = 382 Liters.
Actual system water volume: Assuming a typical 20 mm diameter pipe (common in Northern China) with a water volume of approx. 0.314 L per meter. Using a rule of thumb of 5 meters of pipe per square meter of floor area: Total pipe length = 150 m² × 5 m/m² = 750 m. (Wait, the original article says 235L based on 0.314L/m * 750m = 235.5L, yes). So actual volume = 750 m × 0.314 L/m ≈ 235 Liters.
Required buffer tank capacity: 382 L – 235 L = 147 Liters. In practice, a 150 L tank is selected.
Method 2: Rule of Thumb for Beginners
If you are new to residential heating projects and do not know how to perform the calculation, you can use common standard sizes. For most applications, simply choose a 100L or 150L buffer tank. Keeping these two sizes in stock will cover the majority of project needs.