What Buffer Tank Volume Do I Need
What Buffer Tank Volume Do I Need
The question "how big a tank do I need" is one of the most common technical questions we deal with when designing a heating system with a buffer tank. The answer isn't a single number that applies to every house - it depends on the type of heat source, its output, the building's heat loss, and whether the tank serves merely as a balancer or is meant to actually store heat for hours ahead. In this article we walk through a practical calculation procedure, the specific recommended ratios for individual source types (solid fuel boiler, pellet boiler, heat pump, solar collectors), and we also show a real calculation example for a specific family house. The goal is that after reading it, you'll be able to estimate for yourself the volume range worth considering for your house - regardless of the specific brand or model, since this category currently has no stocked products and the article is purely advisory.
Why tank volume is a key decision
A buffer tank (often also called a buffer or balancing tank) has two distinct, though related, roles in the system. The first is hydraulically separating the heat source circuit from the consumption circuit - ensuring the boiler or heat pump can run independently of how much heat the radiators or underfloor heating currently need. The second, and for solid fuel boilers the more important, role is actual energy storage - the tank stores the heat produced during one firing (for example 3 to 5 hours of burning wood at full output) and gradually releases it into the heating system for as much as 12 to 24 hours.
If the tank is too small, the system has to switch on and off far more often than is healthy - this applies especially to heat pumps, where frequent compressor cycling shortens its lifespan and increases electricity consumption, and also to solid fuel boilers, which with insufficient storage either have to burn at a low, inefficient output (more tar, lower efficiency, more chimney fouling), or the surplus heat has to be discharged out of the system via the safety (cooling) circuit. If, on the other hand, the tank is needlessly large, the system needs longer to heat up to a usable temperature, investment costs rise, the tank takes up more boiler-room space, and the heat losses from the large surface of the tank (even with good insulation) are higher than for a smaller, correctly sized tank. The right volume is therefore always a compromise between adequate storage and a sensible investment and space requirement.
Basic calculation procedure
Most recommendations for sizing a buffer tank are based on a simple relationship: the tank volume in litres is calculated as the heat source's output in kilowatts multiplied by a litres-per-kilowatt factor, which differs by source type. In practice the procedure has five steps, shown simplified in the following diagram.
It's important not to confuse the house's heat loss with the heat source's output. A solid fuel boiler is sized for a higher output than the house's actual heat loss, because it doesn't heat continuously - it burns in batches (one firing typically lasts several hours), and the surplus output is stored for later precisely thanks to the buffer tank. That's why the volume calculation is based on the boiler's output, not the house's heat loss - this is a common mistake we see in practice.
Recommended volumes by heat source type
The litres-per-kilowatt factor varies significantly depending on how the heat source operates. Below are approximate ranges commonly used when designing heating systems in Slovakia.
Solid fuel boiler (cordwood, gasification boiler)
Here a buffer tank is practically mandatory - without it, the boiler can't work efficiently and safely. The recommended range is 20 to 55 litres per kilowatt of the boiler's installed output, with the lower bound (about 20 l/kW) being the real minimum that still allows reasonable operation, and the upper bound (50 to 55 l/kW) matching most wood gasification boiler manufacturers' recommendations for storing one full firing entirely. As a practical average when designing, we commonly use a value around 40 l/kW - this value already allows the boiler to burn at rated output (highest efficiency, least tar and soot) and store most of the heat produced instead of it escaping through the safety circuit.
Automatic pellet boiler
A pellet boiler regulates output more smoothly than a cordwood boiler - it can burn at partial output based on current demand, so storage requirements are lower. 10 to 20 litres per kilowatt is commonly used, and many modern pellet boilers can even work with a minimal balancing tank in the 50 to 100 litre range, just for hydraulic separation, especially if they have a wider output modulation range. However, if the house has several heating circuits at different temperatures (for example both radiators and underfloor heating), a buffer tank significantly simplifies control even with a pellet boiler.
Heat pump
With a heat pump, the tank usually doesn't serve long-term heat storage (the temperature difference between inlet and outlet is much lower than with solid fuel combustion), but mainly limits the number of compressor starts and hydraulically separates it from the heating circuits. A common recommended ratio is 10 to 25 litres per kilowatt of heat pump output, with a more conservative value around 15 l/kW often used in practice as a sensible compromise between protecting the compressor and an unnecessarily large investment. For inverter heat pumps with smooth output modulation, the required volume can be even lower, since the unit itself adapts output to current demand and doesn't need to switch on and off as often.
Solar collectors
For solar heating, the tank volume isn't derived from output in kW, but from collector area, since that's what determines how much energy can be gained during a sunny day. A common ratio is 50 to 70 litres of tank volume per 1 square metre of flat collector area (for evacuated tube collectors, a somewhat lower value is sometimes used given the higher thermal gain from the same area). Solar systems are also very often combined with another heat source (boiler, heat pump) into one combined tank with multiple heat exchangers - in that case the volume is calculated as the sum of both sources' requirements, not as a simple addition, since the sources typically complement each other over the course of a day and a year.
The following chart compares the approximate litres-per-kilowatt factor for the three most commonly used heat sources (values match the mid-range recommended value from the ranges above; solar collectors aren't included in the chart, since they're calculated in a different unit - per square metre of area, not per kilowatt of output).
The chart shows a substantial difference between a source that burns in batches (solid fuel) and a source that can smoothly regulate output (a heat pump). This is precisely why there's no single universal recommendation for "how many litres per kilowatt" - it always depends on exactly how the heat source operates.
The effect of the house's heat loss and heated area
The house's heat loss (often stated in kW at the design outdoor temperature, commonly -11 to -15 °C in Slovakia depending on the location) doesn't directly determine the tank volume, but it determines what output of heat source you need at all - and that then feeds into the volume calculation. Simplified: the larger and worse-insulated the house, the higher the heat loss, the higher the required boiler or heat pump output, and therefore the larger the resulting tank volume for the same l/kW factor.
As a rough guide, a typical family house with a floor area of 120 to 160 m² built to current thermal insulation standards has a heat loss of roughly 6 to 10 kW. Older, uninsulated houses of similar size can have a heat loss of as much as 12 to 16 kW. The exact figure is calculated through a thermal engineering calculation (an energy audit or a heating design); a rough estimate based on area can only serve as an initial rough sketch before consulting a designer.
It's important not to forget that a solid fuel boiler's output isn't chosen directly to match the heat loss, but with a certain reserve (for morning start-up, frosty days, DHW preparation), and it's precisely this output reserve that's the reason a buffer tank is needed - without it, the boiler's surplus output would have nowhere to go.
What happens with incorrect sizing - small vs. correct vs. oversized tank
To make the difference between the scenarios concrete, let's look at the model house mentioned above, with a heat loss of 9 kW and a solid fuel boiler with an output of 20 kW, where the calculation gave a recommended volume of 800 litres (rounded to the available size of 750 or 1000 litres). Let's compare what it would mean to choose a significantly smaller or significantly larger tank instead.
With a tank that's too small (500 litres instead of the recommended 800 in our example), one of two problems occurs: either the boiler has to be "choked down" and burn at less than rated output so the heat currently being produced fits into a smaller volume of water - meaning more tar and soot, lower efficiency, and faster fouling of the chimney and heat exchanger - or, conversely, the boiler burns at full output, the tank quickly heats up to maximum, and the surplus heat has to be discharged via the safety (cooling) circuit, which is a pure heat loss with no benefit.
With a significantly oversized tank (for example 1,500 litres instead of 800), the system works safely, but reaching a usable temperature takes longer - especially at the first firing from cold, it can take significantly longer before there's water hot enough in the top of the tank for heating. The investment is higher (a large tank costs substantially more than a smaller one, as does its transport, positioning, and installation), and it takes up more boiler-room space, which might be needed for, say, a hot water cylinder or other equipment.
A practical step-by-step calculation example
To make the procedure fully concrete, let's go through the whole calculation for one real house type.
Given: a family house with a floor area of 150 m², built to a typical level of thermal insulation, with a heat loss per the heating design of 9 kW at the design outdoor temperature. The owner has chosen a solid fuel boiler (a cordwood gasification boiler) with a rated output of 20 kW - the output is deliberately higher than the heat loss, because the boiler won't run continuously but in batches (one firing lasting several hours), and the surplus heat will be stored in the buffer tank.
Step 1 - source output: 20 kW.
Step 2 - source type and factor: solid fuel boiler, average recommended factor 40 l/kW.
Step 3 - calculation: 20 kW × 40 l/kW = 800 litres.
Step 4 - rounding to an available size: the nearest standard tank sizes on the market are usually 200, 300, 500, 750, 800, 1000, 1500, 2000, 3000 and 5000 litres - in this case the choice naturally falls to 750 or 1000 litres, depending on how much space is available in the boiler room and whether the owner prefers a somewhat more conservative (1000 l) or more economical (750 l) option.
If the same house used a heat pump with an output of 8 kW instead of a solid fuel boiler (closer to the house's heat loss, since a heat pump can run practically continuously and modulates its output), the calculation would look different: 8 kW × 15 l/kW (the mid-range recommended factor for a heat pump) = 120 litres. In practice, a standard size around 100 to 150 litres would usually be chosen for a heat pump, or a hydraulic separator or a combined tank with a built-in hot water cylinder would be used instead of a separate buffer tank altogether. This difference (800 litres versus 120 litres for the same house) illustrates well why the question in the article's title can't be answered with one universal number without knowing the type of heat source.
Temperature layering in the tank and its effect on effective volume
A buffer tank doesn't work as one homogeneous vessel at a uniform temperature - a correctly designed system uses temperature layering (stratification), where the hottest water stays at the top of the tank and the coldest at the bottom. This layering significantly affects how much of the tank's total volume is actually usable for heating at any given moment.
In practice this means that even though the tank in our example has a total volume of 800 litres, it's primarily the top, hottest layer that's directly usable for heating - the bottom, cooler layer serves as a reserve that gradually gets reheated at the next firing. That's why the pipe inlets and outlets on the tank have their exact spot (hot water from the boiler enters at the top, cold return water from the circuits comes back at the bottom), and that's why it's not advisable to spend the tank's volume on needlessly large connecting pipe diameters or overly vigorous mixing - that would disrupt the layering and reduce the actually usable volume, even if the total tank volume were sufficient according to the calculation.
Combining multiple heat sources in one tank
A buffer tank is increasingly used not just with a single heat source, but combining, for example, a solid fuel boiler with solar collectors, or a heat pump with an electric heating element as a backup source. In that case, the tank volume isn't calculated as a simple sum of the individual sources' requirements, because the individual sources usually don't deliver maximum output at the same time (for example the sun shines during the day, while a wood boiler is mostly fired in the evening and morning).
In practice, when combining a solid fuel boiler and solar collectors, the calculation is commonly based on the requirements of the dominant source (usually the boiler, since it delivers higher output over a shorter time), and the solar circuit is connected via a separate heat exchanger in the lower, cooler part of the tank, where it can still add heat even at a lower collector temperature. The exact sizing of a combined system is always individual, and we recommend consulting it with a heating designer - the general rules in this article serve as an approximate basis, not a substitute for a design for more complex systems with multiple sources.
Summary of recommended values
For quick reference, here's a summary of the main figures from this article:
- Solid fuel boiler (cordwood): 20-55 l/kW of boiler output, practical average 40 l/kW.
- Automatic pellet boiler: 10-20 l/kW of boiler output, or 50-100 l may be enough just for hydraulic separation.
- Heat pump: 10-25 l/kW of output, practical average 15 l/kW, mainly to limit compressor cycling.
- Solar collectors: 50-70 l per 1 m² of flat collector area.
- A typical family house (120-160 m², current insulation): approximate heat loss 6-10 kW; older, uninsulated houses 12-16 kW.
- Standard available tank sizes: 200, 300, 500, 750, 800, 1000, 1500, 2000, 3000, 5000 litres.
These values are approximate and are based on common practice in heating system design - for a specific house, we always recommend having an exact heating design prepared, which will also take into account the actual heat loss based on the current insulation state, the method of hot water preparation, and any combination of multiple heat sources.
Frequently asked questions
Do I need a buffer tank even with an automatic pellet boiler?
It isn't always a necessity, but in most cases at least a smaller tank (50 to 150 litres) is recommended for hydraulic separation of the circuits, especially if the house has several heating circuits at different temperatures (both radiators and underfloor heating). For simple systems with one circuit and a modern modulating boiler, it's sometimes possible to do without one, but the specific recommendation should be confirmed by a designer or the specific boiler's manufacturer.
Can a buffer tank be too big for a wood boiler?
Yes, practically speaking, yes. A significantly oversized tank lengthens the time needed to reach a usable temperature, especially at the first firing from cold, and needlessly increases investment costs and boiler-room space requirements. The goal is to find a volume that covers one typical firing (typically 20 to 40 kg of wood per load for a mid-size boiler), not the maximum size that will fit in the room.
How does the required volume change if the house also has underfloor heating?
Underfloor heating works with lower heating water temperatures (typically 35 to 45 °C) compared with radiators (often 55 to 70 °C), which, combined with a buffer tank, means even the cooler, bottom layer of water in the tank is still usable for underfloor circuits. In practice this extends the time the tank remains "usable" without needing to increase its total volume - the temperature difference between the layers in the tank is thus used more efficiently.
Can the tank volume be increased later if it turns out to be insufficient?
Enlarging the same tank directly isn't possible, but a common solution is adding a second tank connected in series or in parallel with the first (so-called tandem connection). This solution requires modifying the pipework and controls, so it's worth considering in advance already at the boiler-room design stage and leaving enough space for it, even if the second tank is only installed later.
Does the size of the domestic hot water (DHW) cylinder affect the buffer tank volume?
These are two separate vessels with different functions - the buffer tank is for heating, the hot water cylinder is for heating domestic water. Some combined solutions (so-called combi-tanks) have the DHW cylinder built directly inside the buffer tank, in the form of its own cylinder or an instantaneous heat exchanger - in that case the total volume is chosen somewhat higher, so there's still enough room left for heating water around the internal cylinder. You'll find a more detailed comparison of this topic in a separate article on the difference between a buffer tank and a DHW cylinder.
Is there a difference in volume between a new build and a renovated older house?
There's no direct difference in volume, the calculation principle is the same - the difference is that older, worse-insulated houses usually have a higher heat loss, which for a solid fuel boiler often leads to choosing a higher boiler output, and therefore also a higher calculated tank volume. When planning to insulate an older house later, it's therefore sensible to base the source output and tank volume on the state after insulation, not just the current state before renovation.
Related topics
- How to choose a buffer tank
- Buffer tank for a heat pump
- Buffer tank for a solid fuel boiler
- Insulation and heat losses of a buffer tank
- Buffer tank vs. DHW cylinder
See also the full range in the category Buffer Tanks.
Have a question on this topic?
Can't decide what tank volume would suit your house, or dealing with a specific situation involving a combination of several heat sources? Write to us - we're happy to help.