How to Choose a Buffer Tank
How to Choose a Buffer Tank
A buffer tank (sometimes also called a balancing tank or a buffer) is one of those pieces of equipment most households don't notice until a problem crops up - the boiler switches on and off too much, the heat pump "cycles", the radiators heat unevenly, or conversely a solid fuel boiler burns down before its heat can be used sensibly. A correctly chosen buffer tank can solve most of these problems; an incorrectly chosen one (most often undersized or poorly placed) can, on the contrary, needlessly drive up costs and complicate the system. In this article we go through everything worth looking at when choosing one - not just the volume in litres, which is the first thing most people associate with a buffer tank, but also the operating pressure, insulation, the number and type of connections, the tank material, how it connects to the heat source, and last but not least the practical side of things - how the tank even gets into the boiler room in the first place.
The article is deliberately advisory and independent of any specific product - the Buffer Tanks category currently has no stocked product available, so the goal of the text is to help you understand the principles and parameters so you can judge any specific tank you end up buying, whether from us in the future or elsewhere.
What a buffer tank is actually for
A buffer tank is basically a large water reservoir (usually 300 to 5,000 litres, even more in industrial applications) connected into the heating circuit between the heat source (boiler, heat pump, solar collectors) and consumption (radiators, underfloor heating, domestic hot water heating). Its job is to separate, in time, the moment heat is produced from the moment it's actually consumed.
A typical example: a solid fuel boiler (wood, wood gas) works most efficiently and cleanly when it burns at full or near-full output from the start to the end of a fuel load. But the house often needs only a fraction of that output at that moment. Without a buffer tank, the boiler would have to throttle its output, which for solid fuel boilers means worse combustion, more tar in the chimney, and lower efficiency. With a buffer tank, the boiler burns the whole load at full output, the surplus heat is stored in the tank, and the house draws heat from it gradually, quite possibly for several hours after the boiler has burned out.
A similar principle applies with heat pumps too, though for a different reason - there it's mainly about limiting the number of compressor starts and stops (so-called cycling), which wears out the compressor and reduces the system's actual efficiency (COP). Here the buffer tank acts as a hydraulic separator and a reservoir that lets the heat pump run in longer, more stable cycles regardless of how demand from the individual heating circuits in the house keeps changing.
First step - find out the heat source's actual output
Before you even start choosing a specific volume, you need to know the heat source's rated (designed) output in kilowatts (kW). You'll find this figure in the boiler's or heat pump's technical data sheet, or in the heating design, if the house has one. Without this figure, buffer tank volume can only be estimated very roughly, and the risk of undersizing or, conversely, needlessly oversizing is high.
A common mistake we see in practice is calculating based on the output the house actually needs (the building's heat loss) instead of the heat source's output. These two figures can differ significantly - especially with solid fuel boilers, the boiler's output is commonly oversized relative to the house's actual heat loss (for example a house with an 8 kW loss might have a 20 or 25 kW boiler, simply because smaller solid fuel boilers aren't available in that range, or because the owner wants a reserve). For calculating buffer tank volume, the source's output is what matters, not the house's heat loss.
Approximate recommendations by heat source type
In practice, a ratio of litres per kilowatt of source output (l/kW) is most commonly used. This ratio differs by heat source type, because each source has a different reason for needing storage and a different output range in which it can work efficiently:
- Solid fuel boiler (wood, wood gas): roughly 25 litres per kilowatt of boiler output, with a common practical range of 20 to 35 l/kW. Reason: the boiler burns at full output even when the house needs much less, so it needs the largest "storage space" of any source type.
- Heat pump: roughly 15 litres per kilowatt of output, common range 10 to 20 l/kW. Here it's more about hydraulically separating circuits and limiting compressor cycling than actually storing a large amount of heat long-term.
- Electric boiler: roughly 12 litres per kilowatt, common range 10 to 15 l/kW. An electric boiler can smoothly regulate output, so the storage need is lower; here the tank often serves mainly to take advantage of a cheaper night-time tariff band (charging the tank at night, drawing heat from it during the day).
- Gas condensing boiler combined with a heat pump (bivalent connection): roughly 8 litres per kilowatt of combined output, common range 5 to 10 l/kW. A condensing boiler can modulate its output over a wide range, so here the buffer tank mainly serves as a hydraulic junction between two sources with different dynamics.
These figures are approximate and are based on commonly stated recommendations from boiler and heat pump manufacturers - a specific heating design or a specific device manufacturer's recommendation may give a different number and always takes precedence over the general rule. The following chart shows these four approximate values side by side, so the difference between source types is visible at a glance.
A practical calculation example
Imagine a house with a wood boiler with a rated output of 20 kW. At the approximate ratio of 25 l/kW, we get a recommended volume of 20 × 25 = 500 litres. If the same house were heated by a heat pump with an output of 10 kW instead, at a ratio of 15 l/kW the recommended volume would be only 10 × 15 = 150 litres - considerably less, because the reason for storage is different (hydraulic separation and limiting cycling, not storing heat from one big fuel burn).
Treat these calculations as a starting point for a conversation with a designer or installation company, not as a final figure that doesn't need further verification. A specific house may have particulars (for example a combined heat source, a strong emphasis on hot water comfort, or conversely limited boiler-room space) that push the recommended volume up or down.
Tank operating pressure
The second parameter that's often underestimated when choosing is the tank's operating pressure. Buffer tanks are commonly manufactured with a maximum operating pressure of 3 bar, which is enough for most typical family homes with a closed heating system and a pressurised expansion vessel. For systems with a heat pump, where a higher static pressure is sometimes used (for example multi-storey buildings or systems with a large height difference between the boiler room and the highest radiator), versions rated to 6 bar are also used.
Manufacturers usually also state the test pressure the tank was verified against during manufacture - commonly 1.5 times the operating pressure, i.e. roughly 4.5 bar for a 3-bar version or 9 bar for a 6-bar version. This figure isn't just a formality - it tells you about the tank's safety margin and should be part of the documentation you get for the tank from the supplier or installer.
Practical tip: before buying, always compare the designed (expected) pressure in your system with the tank's maximum operating pressure, not the test pressure. The test pressure is a manufacturing safety margin, not a value the tank is meant to normally operate at for years.
Insulation and heat losses
A buffer tank only makes sense if it holds heat long enough for the house to actually make use of it. The quality and thickness of the insulation therefore directly affects how much of the stored heat actually turns into a benefit, and how much simply "evaporates" into the boiler room.
In practice, insulation splits into two basic types: soft textile/fleece covers (usually 50 mm) and rigid moulded pieces of polyurethane (PU) foam, commonly made in thicknesses from 80 to 120 mm. PU insulation has a significantly lower thermal conductivity coefficient than soft textile of the same thickness, so the difference between insulation types is bigger than you'd expect from the thickness difference alone.
The following table shows how this shows up in real heat losses. Treat the values as an approximate proportional comparison (100% = the worst commonly available option, 50 mm soft insulation), not as exact physical values for a specific model - those always vary by manufacturer, tank volume, and boiler-room temperature.
| Insulation type and thickness | Relative heat loss | Note |
|---|---|---|
| Soft textile, 50 mm | 100% (baseline) | Cheapest, biggest losses, common on older/cheaper tanks |
| Soft textile, 80 mm | approx. 55% | Common compromise between price and losses |
| Rigid PU foam, 100 mm | approx. 35% | Common standard on higher-quality tanks |
| Rigid PU foam, 120 mm | approx. 25% | Recommended for tanks over 1000 l and for year-round operation |
The same figures in chart form:
Practical consequence: for a tank that stands in the boiler room all year and is meant to serve as an overnight or weekend heat store too (typically with solid fuel boilers or in combination with solar), the investment in better-quality PU insulation pays for itself in genuinely lower fuel or electricity consumption. For a tank that serves only as a short-term hydraulic separator with a heat pump (heat stays in it for minutes to tens of minutes, at most), the difference between insulation types matters less, though it's still not negligible.
Stratification - why the tank's shape and internal layout matter
A good-quality buffer tank isn't just "a barrel full of water" - its interior is designed so warmer water naturally stays at the top and cooler water at the bottom, without mixing. This phenomenon is called stratification (layering) and is key to how efficiently a tank works. The better a tank maintains separated temperature layers, the longer it can deliver genuinely hot water from the top (for example for hot water heating or heating at a higher temperature), even while the bottom of the tank is already considerably cooler.
In practice it looks roughly like this: the top of the tank (outlet toward the radiators or the hot water heat exchanger) can be as much as 80 °C, a slightly lower layer 65 °C, the middle layer around 50 °C, and the bottom part (where cooled water from the system returns, the so-called return) just 35 °C. It's exactly this 35 °C layer at the bottom that goes back to the boiler or heat pump - thanks to the lower return temperature, the heat source works more efficiently (condensing boilers and heat pumps especially have higher efficiency at a lower return temperature).
In practice this means it's worth looking, when choosing a tank, at how many connections (necks) it has and at what heights they're placed - the more connections at different heights, the more flexibly you can connect different circuits (for example a high-temperature circuit for radiators at the top, a low-temperature circuit for underfloor heating from a middle height, and a solar or heat-pump circuit from the bottom). Some tanks also have internal distribution fins or diffusers at the inlet, which prevent the incoming water flow from stirring up and mixing the temperature layers that have already formed.
Heat exchangers and connections - what types exist
Buffer tanks are made in several basic configurations depending on what heat exchangers (coiled heating-water spirals) are built in inside:
- No heat exchanger (bare tank): just with external necks, suitable where all circuits (source and consumption) are hydraulically connected directly, with no need for a separate circuit.
- With one heat exchanger: typically for a solar circuit or a separate boiler circuit, when you need to hydraulically separate one system from another (for example because of a different antifreeze mix in the solar circuit).
- With two heat exchangers: common for a solar + boiler combination, or solar + heat pump, where each source has its own separate circuit.
- With an instantaneous module for domestic hot water: instead of a classic large heat exchanger, a compact plate heat exchanger with a circulation pump is used, which heats hot water instantaneously straight from the buffer tank. The advantage is a lower risk of Legionella growth in stagnant water (since it's not a large volume of DHW being heated, just a small instantaneous flow volume); the drawback is dependency on the module's pump and electronics working correctly.
When choosing, it's important to know in advance how many circuits you'll be connecting to the tank, and to choose the number and type of heat exchangers accordingly - adding a heat exchanger later to an already-purchased tank is generally not possible, since it's a manufacturing configuration.
Material and wall thickness
Most buffer tanks for typical family homes are made from standard structural steel, protected against corrosion on the inside (most often with powder coating or a special coating resistant to high temperatures, since the tank commonly works with temperatures over 80 °C). For systems where the tank also directly stores drinking/domestic water (not just heating water kept separate by a heat exchanger), stainless steel is used, which is more durable but also considerably more expensive.
The wall thickness of the vessel itself is usually a few millimetres and is set by the pressure vessel standard - when choosing, you don't need to compare this value across manufacturers in detail; what matters more is that the tank has a valid declaration of conformity and is rated for the given operating pressure and temperature (see the pressure section above).
Dimensions and the practical side - how the tank gets into the boiler room
This is the point most commonly forgotten in practice, and yet it causes the most complications right before installation. A 500-litre buffer tank is typically about 1.8 to 2 metres tall with a diameter of about 65 to 75 centimetres (exact dimensions vary by manufacturer and insulation thickness). For a 1,000-litre tank, the height is typically around 2 metres and the diameter grows to 790 to 900 mm.
Before ordering, always measure:
- the clear width and height of the doors and corridor the tank will be moved through (including every turn along the route),
- the clear ceiling height in the boiler room (the tank has to fit standing up, with room for future pipe connections from above),
- the floor's load capacity, if the boiler room is on an upper floor or a wooden ceiling - a filled 1,000-litre tank weighs, with the water, roughly a tonne, plus the weight of the tank itself,
- space for servicing and any future maintenance (inspection, replacing seals on the heat exchanger flanges).
Some manufacturers also offer tanks with removable insulation and a narrower body diameter specifically for transport through narrow entrances - if you know in advance that your house has narrow doors or a staircase, tell the supplier before ordering, not only when the tank is delivered.
Step-by-step selection procedure
Let's summarise the whole procedure into clear steps, which we recommend going through in this order:
The most common mistakes when choosing
From experience, we repeatedly encounter a handful of mistake types that are easy to avoid if you know about them in advance:
- An undersized volume "to save money": a smaller tank is cheaper to buy, but for a solid fuel boiler it means more frequent firing and worse combustion efficiency, which ultimately shows up in wood consumption.
- An oversized volume "just to be safe": a needlessly large tank means higher purchase costs, greater space requirements, and paradoxically also somewhat longer for the system to "warm up" to operating temperature after start-up.
- Forgetting the dimensions when moving it in: a tank ordered without checking the door and corridor dimensions sometimes can't be brought into the boiler room in one piece, causing delays and extra costs.
- Underestimating insulation for year-round operation: for a tank meant to store heat overnight or over a weekend, cheaper soft insulation shows up over the years as higher fuel or electricity consumption, as described in the insulation section above.
- The wrong number of heat exchangers for future expansion: if you're planning to add solar collectors in a few years, for example, it's worth considering a tank with an extra heat exchanger now, since adding one later isn't possible.
- Ignoring the return temperature to the source: for condensing boilers and heat pumps, a low return temperature has a direct impact on efficiency - a poor hydraulic connection of the tank (without proper stratification) throws away this advantage.
Summary - what not to forget
Choosing a buffer tank isn't just about "how many litres to buy". It's about the interplay of several parameters - a volume derived from the heat source's output, an operating pressure matching your system, insulation quality matching how it will be used, the number and type of heat exchangers matching the number of circuits being connected, and finally quite a down-to-earth matter - whether the tank can even physically get to where it needs to go. If you work through these points in the order described in this article, you'll significantly reduce the risk of finding out after installation that the tank is too small, too cold, or that it never should have needed to fit through the boiler-room door in the first place.
Frequently asked questions
Does a buffer tank always need a heat exchanger?
No. If all circuits (source and consumption) are hydraulically connected directly with no need for separation (for example due to different heat-transfer fluids), a bare tank with just external necks is enough. A heat exchanger is needed when one circuit has to be hydraulically separated from another - typically a solar circuit with antifreeze from the rest of the heating system.
Can a buffer tank be added to an existing boiler without any work in the boiler room?
Technically yes, but in practice it almost always means modifying the pipework, adding pumps and controls (circulation pumps, valves, and possibly new controls with temperature sensors in the tank), and finding space in the boiler room that wasn't originally planned for it. We recommend discussing this with an installation company already at the planning stage, not only at the point of installation itself.
Is it worth having a buffer tank even with a gas condensing boiler without a heat pump?
For a stand-alone modulating condensing boiler (without a heat pump), the benefit is smaller, since the boiler can smoothly regulate its output over a wide range by itself. Nevertheless, a smaller tank as a hydraulic separator can still make sense if the house has several heating circuits with different dynamics (for example radiators and underfloor heating at the same time).
How often does a buffer tank need checking or servicing?
The tank itself (with no moving parts) generally needs only a regular visual check of the flange seals and possibly a check of the insulation's condition. If the tank has an instantaneous DHW module with a pump and electronics, that module has its own service interval per the manufacturer, similar to ordinary heating circulation pumps.
Is one large tank better than two smaller ones connected in series?
For the same total volume, a single larger tank usually has a somewhat better surface-to-volume ratio (lower relative heat losses) and a simpler hydraulic connection. Two smaller tanks are mainly chosen in practice when space or entry openings don't allow one large tank to be brought to the site, or when the system is expanded gradually over time.
Does the tank's colour or surface finish affect its function?
No, neither the colour nor the appearance of the outer covering (the insulation jacket) affects the tank's function. What matters is what's under the surface - the thickness and type of insulation, the operating pressure, the number and placement of connections, and the quality of the anti-corrosion protection on the inner surface.
Related topics
- What buffer tank volume do I need
- Buffer tank for a heat pump
- Buffer tank for a solid fuel boiler
- Installation and connection of a buffer tank
- Insulation and heat losses of a buffer tank
- Most common mistakes when designing a buffer tank
You'll find the complete range and more information in the main category, Buffer Tanks.
Have a question on this topic?
Can't decide, or dealing with a specific situation in your home? Write to us - we're happy to help.