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Heat Pumps Design Problems: Oversizing, Buffer Tanks & DHW | AGELOPOULOS GROUP

Heat Pumps: Why Many Systems Fail to Perform as Expected Oversizing, Missing Buffer Tanks and Domestic Hot Water Without Proper System Design Technical analysis based on real-world system evaluation by AGELOPOULOS GROUP. Heat pumps are widely considered one of the most efficient and future-proof solutions for space heating and domestic hot water production. However, real-world performance often falls short of expectations. In practice, a significant percentage of heat pump installations exhibit high energy consumption, frequent on/off operation, reduced seasonal efficiency and premature component wear. The root cause is rarely the heat pump itself. The real issue is system design. At AGELOPOULOS GROUP, through the evaluation of numerous residential and commercial installations, we consistently observe that poor performance is almost always linked to incorrect sizing, inadequate hydraulic design and the absence of sufficient thermal mass. This article explains the most common design errors in heat pump systems, based on international technical guidelines and field experience, and highlights why proper system architecture is critical for long-term efficiency and reliability. 1. What “Short Cycling” Really Means in Heat Pump Systems Short cycling refers to a condition where the compressor starts, reaches the target temperature very quickly, stops, and restarts after a short interval. This behavior is not normal inverter operation. Reduced seasonal efficiency (SCOP) Increased electrical consumption Unstable indoor temperatures Increased mechanical stress on the compressor and inverter electronics Important: An inverter heat pump does not eliminate on/off cycles by default. Inverter technology only provides a wider modulation range — it does not compensate for poor system design. 2. Oversizing: The Most Common and Costly Design Mistake Oversizing remains the number-one mistake in heat pump installations. “Let’s install a larger unit to be on the safe side.” Engineering reality tells a different story: modern, well-insulated buildings have low thermal loads. Typical heating demand often ranges between 40–60 W/m². When a heat pump is significantly oversized relative to the actual load, the unit cannot modulate low enough, satisfies demand too quickly, and is forced into repeated start/stop cycles. International standards and design guides (CIBSE, ASHRAE, EN standards) are explicit: Oversizing does not increase reliability — it reduces efficiency. 3. Minimum Water Volume and the Role of Buffer Tanks All major heat pump manufacturers specify minimum water volume, minimum flow rate, and operating conditions under which a buffer tank (or volumiser) is required. The reason is fundamental physics: low water volume results in rapid temperature rise; rapid temperature rise triggers compressor shutdown; the cycle then repeats. In systems with fan coils, multiple zones, and compact pipe networks, the absence of sufficient thermal mass dramatically increases the likelihood of short cycling. From extensive system evaluations conducted by AGELOPOULOS GROUP, buffer tanks play a critical role in increasing thermal inertia, stabilising system operation, protecting the compressor, and supporting stable defrost cycles. 4. Fan Coils and Heat Pumps: A Demanding Combination Fan coil units are characterised by very small hydraulic volume, fast thermal response, and rapidly changing loads. When combined with an oversized heat pump, no buffer tank, and no hydraulic separation, the heat pump “sees” abrupt load changes and reacts aggressively. Frequent compressor cycling Reduced efficiency Misleading conclusions about the heat pump’s actual performance 5. Heat Pumps and Domestic Hot Water (DHW): A Frequently Overlooked Risk One of the most underestimated issues in heat pump design is domestic hot water production without proper hydraulic strategy. In poorly designed systems, the heat pump raises supply temperature significantly to charge the DHW tank. The load is intense but short-lived. Once the tank is satisfied, the load drops to zero and the compressor stops abruptly. Without a buffer tank or hydraulic decoupling, transitions between space heating and DHW cause thermal shocks, cycling frequency increases, and overall system efficiency deteriorates. Based on field data analysed by AGELOPOULOS GROUP, the combination of heat pump and DHW requires a clearly defined hydraulic design strategy to avoid operational instability. 6. Controls and Weather Compensation: Important but Not a Cure Correct settings for weather compensation curves, temperature differentials and hysteresis can improve the performance of a properly designed system. They cannot fix severe oversizing, insufficient thermal mass or incorrect hydraulic architecture. In simple terms: Controls optimise a system — they do not repair design errors. 7. Conclusion: A Heat Pump Is a System, Not a Device The success of a heat pump installation does not depend on brand, brochure data or nominal efficiency figures. It depends on: Accurate heat loss calculation Correct system sizing Sufficient thermal mass Proper hydraulic design Evidence-based commissioning At AGELOPOULOS GROUP, a heat pump is never treated as a standalone appliance, but as part of an integrated energy system. Long-term efficiency, reliability and comfort are achieved through engineering — not assumptions. Technical Analysis by AGELOPOULOS GROUP AGELOPOULOS GROUP specialises in the design, evaluation and optimisation of heat pump systems, with a strong focus on hydraulic architecture, thermal inertia and real-world system performance. Frequently Asked Questions About Heat Pump System Design Do heat pumps need a buffer tank? Not all heat pump systems require a buffer tank. However, in installations with fan coils, small hydraulic volume, zoning, or oversized heat pumps, a buffer tank is often essential to increase thermal inertia and prevent short cycling. According to field analysis by AGELOPOULOS GROUP, the absence of a buffer tank is one of the most common causes of unstable operation in modern heat pump systems. Why does my heat pump keep turning on and off? Frequent on/off operation, also known as short cycling, is usually caused by oversizing, insufficient water volume, or improper hydraulic design. Adjusting controls alone rarely solves the problem if the system architecture is incorrect. Is oversizing a heat pump a problem? Yes. Oversizing leads to rapid temperature rise, reduced modulation capability and frequent compressor cycling. International engineering guidelines clearly state that correct sizing improves efficiency and system longevity. Can weather compensation settings fix short cycling? Weather compensation can improve the performance of a correctly designed system, but it cannot compensate for major design errors such as insufficient thermal mass or excessive installed capacity. Do fan coils require special design when used with heat pumps? Fan coils have low water content and fast response times, which makes them particularly sensitive to design errors. Proper hydraulic separation and adequate thermal volume are critical when combining fan coils with heat pumps. What problems can occur when producing domestic hot water with a heat pump? Without proper hydraulic strategy, domestic hot water production can cause thermal shocks, increased cycling and efficiency losses. Systems combining space heating and DHW require careful design to ensure stable operation. Is a larger heat pump more reliable? No. Larger units do not increase reliability when installed in low-load buildings. Correct system sizing improves efficiency, comfort and equipment lifespan. How can I tell if my heat pump system is poorly designed? Common warning signs include frequent compressor starts, fluctuating indoor temperatures, higher-than-expected energy consumption and inconsistent domestic hot water performance. Content developed by AGELOPOULOS GROUP as part of ongoing technical research on heat pump system performance.