Accumulation tank in a system with a heat pump or solar collector
Accumulation tank in a system with a heat pump or solar collector
When a heat pump or solar collector appears in the boiler room, the accumulation tank ceases to be a "nice addition" and becomes a key component of the entire system. Without it, the heat pump cycles – turning on and off every few minutes, which drastically shortens the lifespan of the compressor. Without it, solar collectors either overheat the heating circuit or simply waste most of the generated energy. From practice, I know that it is precisely here that the most errors occur in the design of heating systems – the customer invests tens of thousands of euros in a heat pump and saves five hundred euros on the accumulation, which they underestimated. The result? The compressor lasts five years instead of fifteen and maintenance swallows all the savings.
In this article, I will explain why accumulation is so important for renewable energy sources, what types of tanks are considered, how hydraulic separation works, what are the real parameters and dimensions for various scenarios, and how to properly design and connect the entire system. If you are interested in the basic selection of a tank, read the article How to choose an accumulation tank – volume, type and connection to the system, where you will find a more general overview.
Why heat pumps and solar collectors cannot work efficiently without accumulation
A heat pump is a machine designed for long, smooth runs. The compressor consumes significantly more energy at each start than during steady operation – it is said that a start loads the compressor as much as several hours of normal operation. In practice, this means that if the pump operates in so-called "short-cycling" mode (start – run 2 minutes – stop – start), the lifespan of the compressor drops from the usual 15–20 years to 5–8 years. In addition, EER (cooling efficiency) and COP (performance coefficient) are measured in a steady state – with constant cycling, the real values are significantly worse.
The accumulation tank solves this problem simply: the heat pump charges the tank when the need for heat is low, and the tank delivers heat to the heating system when the need is high. The pump can thus run in long, uninterrupted cycles – ideally 30–60 minutes or even longer – and then stops for 20–40 minutes. The result is dramatically lower wear, lower electricity consumption, and a longer lifespan of the entire device.
With solar collectors, the situation is different, but equally critical. Solar energy hits the collectors when the sun wants – not when you need to heat your home or heat water. Without accumulation, the collectors would unnecessarily overheat (which damages the antifreeze mixture and the collectors themselves), or the excess heat would have to be somehow dissipated. The accumulation tank acts as a buffer – it absorbs energy during periods of excess and releases it later when the sun is not shining.
Types of accumulation tanks suitable for heat pumps and solar collectors
Not every accumulation tank is equally suitable for every application. With heat pumps and solar systems, you will encounter several basic types, each with its own place and use.
Simple buffer (pure energy storage tank)
This is the simplest type – a stainless steel or steel tank with thermal insulation, without any internal heat exchanger. Water enters and exits directly. It is used where the heat source and the heating system are capable of working with the same circuit – for example, direct connection of an air-to-water heat pump to underfloor heating via a common circuit.
For simple systems with heat pumps, compact volumes are also suitable. For example, accumulation tank PUFFER PSS 50 (57 liters) is ideal for small apartment units or supplementary heat pumps up to 4–5 kW, where a minimum hydraulic volume is sufficient without a large space requirement. For family homes with heat pumps up to 10 kW, the standard is accumulation tank PUFFER PSS 100 (123 liters), which provides sufficient hydraulic buffer for stable operation.
Buffer with one heat exchanger
This tank has one spiral heat exchanger (pipe or plate type depending on the manufacturer) inside. The heat exchanger allows connecting a solar circuit or boiler as a separate closed circuit with antifreeze, while the heating water in the tank remains clean. This type is common in solar systems with two circuits – solar and heating.
Combined tank (buffer + DHW storage in one)
A very popular configuration in practice, where one tank provides accumulation for the heating system and at the same time an internal storage or heat exchanger for hot water (DHW). It saves space in the boiler room, but at the cost of compromises in sizing – the DHW storage must be large enough for the household's needs, while the buffer part must be large enough for the heat pump. More about the differences between a buffer and a DHW tank can be found in the article Buffer vs. hot water tank – what is the difference and when to use which.
Stratification tanks
A more advanced type, where the internal structure is designed to maximize thermal stratification – that is, to keep cold water at the bottom and hot water at the top without mixing. In solar systems, this increases efficiency, as the collectors return to cold and the warmer water remains at the top for withdrawal. Standard tanks also stratify, but less efficiently.
Hydraulic separation – the basis of correct connection
One of the most important functions of an accumulator tank in a heat pump system is hydraulic separation (or hydraulic coupling) between the primary circuit (heat pump) and the secondary circuit (heating system). This is not just a technical formality – it is a condition for the proper functioning of the system.
Problem without separation: the heat pump has its own pump and the heating system has its own pump (or several in a zoned system). If these circuits are directly connected without hydraulic coupling, the pumps "fight" each other – they create unstable pressure conditions, backflows occur and the whole system behaves unpredictably. The heat pump regulation cannot correctly measure the actual heat demand and the pump cycles again.
The accumulator tank elegantly solves this problem: the primary circuit (HP) charges the tank and the secondary circuit (heating) draws from it. Between them is a large volume of water that acts as a hydraulic separator. Both circuits can work with different flows and pressures without interfering with each other.
Some heat pump manufacturers (e.g. Daikin, Mitsubishi, Bosch) directly specify a minimum buffer volume in their installation manuals as a warranty condition. If this condition is not met and the compressor fails prematurely, the claim may be rejected. Therefore, always insist on documentation proving that this condition is met.
Dimensioning the volume of the accumulator tank for a heat pump
Dimensioning the volume is the most common question and also the most common source of errors. A separate article What volume of accumulator tank do I need for my boiler or heat pump deals with this topic in more detail, here I summarize the key rules for heat pumps.
The basic rule for air-to-water heat pumps speaks of a minimum of 20 liters per kilowatt of heat pump power. This is the real minimum for standard floor heating. For radiator systems, where temperature differences are greater, I recommend 30–50 liters per kW. For geothermal heat pumps (water-to-water), where the heat source is more stable, sometimes even 15 liters per kW is sufficient.
| Heat pump power | Minimum volume (floor heating) | Recommended volume (radiators) | Example tank |
|---|---|---|---|
| 3–5 kW | 60–100 l | 100–150 l | PSS 50 (57 l) as minimum, PSS 100 recommended |
| 6–9 kW | 120–180 l | 200–300 l | 200–300 l tank |
| 10–15 kW | 200–300 l | 400–600 l | 400–500 l tank |
| 15–20 kW | 300–400 l | 600–1000 l | 500–1000 l tank |
Important practical note: these values are minimums for hydraulic separation. If you want real energy storage – the ability to charge the buffer in a cheap tariff (e.g. night rate) and heat during the day without turning on the HP – you need a much larger volume, typically 500–2000 liters. This is a different operating strategy and a different investment.
Dimensioning for solar collector
In solar systems, the philosophy of accumulation is a bit different. Here it is primarily not about hydraulic separation, but about real energy reserve – the more energy the solar collectors "bake" during the day, the larger the tank you need to avoid losing it.
The standard rule for solar DHW heating speaks of 50–80 liters of storage volume per each m² of collector area. For flat collectors with an area of 4–6 m² (typical family house with two adults + two children) this results in 200–480 liters. For vacuum collectors with higher performance, less may be sufficient.
If the solar system is connected to heating (not just DHW heating), the volume increases significantly – we are talking about combined systems, where a tank of 500–1500 liters is common. Here the function of the DHW tank (upper part) and the buffer for heating (lower part) are combined.
The position of the heat exchangers in the tank is critical for solar systems: the solar heat exchanger must be in the lower part of the tank (cold water), because collectors work more efficiently at lower inlet water temperatures. The lower the water temperature entering the collector, the higher its efficiency. On the contrary, the boiler or backup heat exchanger is in the upper part – it heats only if the sun is not enough.
Connection of buffer tank with heat pump – practical schemes
There are several basic connection schemes. The choice depends on whether you have one heat pump, a combined system (HP + boiler), or you are planning solar integration.
Scheme 1: HP + buffer + floor heating
The most straightforward configuration. The heat pump charges the buffer in the primary circuit (e.g., temperature 35–45°C). The secondary circuit (floor heating) draws heat from the buffer through a mixing group with a thermostatic valve, which ensures that water that is too hot does not enter the floor. Control: The HP turns on when the temperature in the buffer drops below a set lower limit (e.g., 38°C) and turns off when it reaches the upper limit (e.g., 48°C).
Scheme 2: HP + buffer + radiators + DHW tank
A more complex configuration, where two secondary circuits lead from the buffer – one for radiators (higher temperature, 55–65°C, depending on the radiator system) and the second for heating the DHW tank. The HP must operate at a higher temperature, which reduces COP, but it is necessary in renovations of older homes with radiators.
Scheme 3: HP + solar + combined tank
Ideal for new builds and renovations with higher environmental ambitions. The solar heat exchanger (bottom) heats the tank content primarily. The HP (middle circuit) reheats if necessary. A gas boiler (top heat exchanger) serves only as an emergency backup during long cloudy weather in winter. This system can achieve coverage of 60–80% of annual heat demand from renewable sources.
Materials, temperatures and operating pressures – what you need to know
Buffer tanks are mostly made of S235 steel with internal anti-corrosion protection (enameling, special coating, or no treatment in the case of a tank used only for heating water). For clean heating water (closed circuit), enameling is not necessary – basic anti-corrosion protection is sufficient. For direct contact with drinking/ DHW water, enameling or stainless steel construction is a necessity.
The maximum operating pressure of standard buffers is 3–6 bar, operating temperatures up to 95°C for standard tanks and up to 110°C for special high-temperature versions. Heat pumps typically operate at temperatures of 35–65°C – this is a comfortable range for standard tanks. Solar systems can reach temperatures above 160°C in stagnation (vacuum collectors), so the solar circuit must be dimensioned for these extremes – including an expansion tank and a safety valve properly set.
Insulation of the tank is another factor. Standard insulation made of rigid PUR foam has a thickness of 50–100 mm. Heat losses of a well-insulated tank are 1–3 °C per 24 hours. This means that if you charge the buffer to 60°C and do not draw any heat, after 24 hours it will still be around 57–59°C. This is acceptable for most applications, but for longer storage (weekend without operation), you need to account for losses.
Protective anode – an important but underestimated component
All steel buffer tanks (including buffers) contain a protective magnesium anode. Its role is to protect the steel tank from corrosion through an electrochemical process – magnesium is sacrificed to protect the tank metal. The anode gradually wears out and must be regularly checked and replaced.
It depends on the water quality, temperature and tank volume, but as a general rule, anode inspection every 1–2 years and replacement when it is worn down to less than 30% of its original weight. Neglecting anode replacement leads to rapid tank corrosion and dramatically shortens the tank's lifespan. More information can be found in the article Protective anode in a buffer tank – what it is, when and how to replace it.
For larger tanks (300–600 liters), a longer anode is suitable, such as protective anode made of magnesium alloy 5/4" x 400 mm for tanks 300–600 l. For smaller tanks, versions with a control device are available, which allow easy checking of the condition without disassembly – for example, protective anode with control device 5/4" x 400 mm or for larger tanks anode 5/4" x 700 mm with control device, where the longer body ensures better protection of the entire tank height.
Control and regulation of the system with accumulation
Proper regulation is an inseparable part of the entire concept. Without it, even a perfectly designed system with accumulation is inefficient or even counterproductive.
For a heat pump, the key parameter is the setting of the buffer's temperature switching hysteresis. Too narrow hysteresis (e.g., 3°C) always leads to cycling. I recommend a minimum of 5–8°C hysteresis for air-to-water heat pumps, ideally 10°C. For example: the heat pump turns on at 40°C and turns off at 50°C. The secondary circuit (floor heating) draws from the buffer continuously and is regulated by its own circulation pump and mixing valve according to the equithermal curve.
In solar systems, a differential controller is responsible for control – it compares the temperature of the collector and the temperature of the bottom part of the tank. When the collector is 5–8°C warmer than the tank, the solar circuit pump is started. When the difference is less than 2–3°C, the pump stops. Proper setting of these values has a direct impact on the efficiency of the solar system.
In modern systems, control via smart home or integrated heat pump control is increasingly used, which takes into account weather forecasts, electricity tariffs, and availability of solar energy (in combination systems with solar panels, not just collectors). These systems can, for example, charge the buffer to a higher temperature before expected cloudiness, thus reducing the need to turn on the backup boiler.
Common installation errors of accumulation tanks in combined systems
From practice, I know several errors that keep repeating over and over again. I won't just list them – I'll explain why they are problematic and what actually happens.
Error 1: Too small a buffer for the heat pump. A customer buys an 8 kW heat pump and the installer installs a 100-liter buffer ("that's enough for a boiler"). In a passive house with floor heating and low heat demand during the transitional period, the heat pump turns on every 5–6 minutes. The compressor breaks down after 3 years and the customer wonders why.
Error 2: Solar heat exchanger in the top part of the tank. The installer mistakenly (or for convenience) connects the solar circuit to the top connections of the tank. The collector heats the hot water, but the cold water at the bottom remains cold. The efficiency of the collector is significantly reduced because it works with a high inlet water temperature. The customer thinks the collectors are faulty.
Error 3: Neglecting the expansion tank in the solar circuit. The solar circuit does not have an adequately dimensioned expansion tank for stagnation temperatures. During summer overheating, the safety valve opens and the antifreeze mixture is lost. After several years, the mixture becomes oxidized and the tank (and also the collectors) corrode from the inside.
Error 4: Unchecked anode. The anode in a 500-liter tank was not replaced for 8 years. The customer notices that the water from the DHW tank smells (hydrogen sulfide from the reaction of magnesium with sulfate bacteria at low temperatures). Sometimes the anode is completely corroded and the tank starts to rust.
Error 5: Poor placement of temperature sensors. The heat pump controller measures the temperature in the middle of the buffer instead of at the bottom. The top part is warm, the bottom is cold – but the controller doesn't see it and the heat pump doesn't turn on. Result: the heating system doesn't receive enough heat, the customer thinks the heat pump is undersized.
More about installation procedures, placement and connection can be found in the article Installation of an accumulation tank – procedure, connection and placement in the boiler room.
Operating costs and economics of accumulation
An accumulation tank itself does not consume energy (except for minimal heat losses through insulation). Its value is purely in what it enables other devices to do more efficiently. Specific numbers will help illustrate the economics of the decision:
Example: a family house with a 10 kW heat pump, annual heat consumption of 20,000 kWh, COP 3.5 with proper operation vs. COP 2.8 with excessive cycling. The difference in electricity consumption: 20000/3.5 = 5714 kWh vs. 20000/2.8 = 7143 kWh. At an electricity price of 0.22 €/kWh, this is a difference of 314 € per year. An investment in a suitable accumulation tank (300–500 €) thus pays for itself in one to two years just from energy savings – without considering the extension of the compressor's lifespan.
Add to that the potential use of a lower (night) tariff: if the heat pump charges the buffer at night at a rate of 0.12 €/kWh and heat is drawn during the day without turning on the heat pump, the savings are even higher. This strategy requires a larger buffer (500–1000 l), but the economics are clear.
Most frequently asked questions (FAQ)
Do I need an accumulation tank for every heat pump?
Strictly speaking, not for every one – some modern inverter heat pumps with continuous power regulation (modulating heat pumps) can operate without a buffer if the heating system (floor heating) is sufficiently large and itself performs the function of a hydraulic buffer. In practice, however, most manufacturers recommend or even require a buffer even for inverter units. For on/off heat pumps, an accumulation tank is mandatory. Always check the installation manual of the specific heat pump model.
What buffer volume do I need if I want to use the night electricity tariff?
For a real "night" strategy – that is, charging the buffer only at night and heating during the day from the buffer without turning on the heat pump – you need a significantly larger volume than for standard hydraulic separation. Roughly: with a 10 kW heat pump and an 8-hour night block, the heat pump would produce 80 kWh of heat. To store this heat at a temperature difference of 20°C (60°C charging, 40°C output to heating) you need a volume: Q = m × c × ΔT, so 80 kWh = 3600 × m × 4186 × 20 → m ≈ 3440 liters. In practice, it is combined with night operation without complete discharge, so real sizes are 800–2000 liters, depending on the size of the house and heat demand.
Can I connect a solar collector to an existing heat pump buffer?
Yes, but only if the buffer has connections for a heat exchanger (or an external plate heat exchanger) and has a sufficient volume for both functions. A standard buffer without a heat exchanger must be replaced with a tank with heat exchangers, or an external plate heat exchanger must be added to the solar circuit. Always use an antifreeze mixture (propylene glycol, not ethylene glycol for DHW tanks) in the solar circuit and check that the solar heat exchanger is located at the bottom of the tank.
How do I find out that the accumulation tank is not working properly?
Main symptoms: the heat pump cycles too often (more than once every 15–20 minutes in normal operating mode), the temperature in the tank is constantly unstable or uneven, the heating system does not react quickly enough to changes in demand. A more technical test: monitor the heat pump controller log (most modern heat pumps have one) and count the number of starts per day. More than 6–8 starts per day during a normal day in the transitional period indicate a problem with the accumulation or hysteresis setting.
Is one large tank better or two smaller ones?
From a hydraulic perspective, one large tank is simpler and better – fewer fittings, lower heat losses, simpler control. Two smaller tanks in series can have a greater effective volume in the case of limited ceiling height
Do you have a question about this topic?
Can't decide or are you dealing with a specific situation in your household? Write to us – we'll be happy to help.