Buffer Tank for a Heat Pump
Buffer Tank for a Heat Pump
A heat pump is a different type of heat source from a gas or solid fuel boiler, and a buffer tank plays a somewhat different role with it too. Whereas with a wood or pellet boiler a tank is essential so heat from one intense combustion cycle can be stored, with a heat pump the main reason is something else - protecting the compressor from switching on and off too often (so-called cycling) and ensuring stable hydraulic water circulation in the system. In this article we explain in practical terms when a buffer tank is actually needed with a heat pump, when it's unnecessary, how it differs from a classic buffer tank for solid fuel, and what volume proves itself in practice.
One important note right at the start: this category currently has no specific stock on offer (current items are temporarily unavailable), so the article won't refer to any specific model or price. The goal is that after reading it, you'll be able to judge for yourself whether you need a buffer tank with your heat pump, and if so, what volume range to aim for - the rest of the decision (specific manufacturer, design, insulation) can then be worked out with a designer or installation company.
Why a heat pump needs a tank at all
The compressor in a heat pump likes long, steady runs - ideally tens of minutes at a stretch - and doesn't like frequent starts and stops. Every start is the mechanically and energetically most demanding moment of the whole cycle; the compressor first has to build up a pressure difference in the refrigerant circuit before it actually starts delivering output. If the heat pump switches on and off too often (referred to technically as "short cycling"), it leads to:
- increased compressor wear and a shorter lifespan,
- higher electricity consumption per unit of heat delivered (starting up is energetically more expensive than steady running),
- noisier, less smooth operation, which is especially noticeable at night,
- in the worst case, premature failure and loss of warranty, since most manufacturers directly limit cycling in their installation conditions.
Cycling occurs when there's too little water in the system relative to the heat pump's output, and the building's heat loss at that moment is lower than the minimum output the pump can run at (the so-called modulation minimum). Modern inverter heat pumps can smoothly reduce output, often down to 20-30% of rated output, so they're less sensitive to cycling than older single-stage units - but it's never completely eliminated, especially during transitional periods (spring, autumn), when the house's heat loss is small.
The tank's second, equally important function is hydraulic - separating the primary circuit (the heat pump) from the secondary circuit (the pipework to radiators, underfloor heating, possibly also DHW heating), so that flow rates and pressure conditions in one circuit don't affect the other. In practice this is handled either as a hydraulic separator (smaller volume, just balancing flows) or as a full buffer tank (larger volume, also providing a thermal reserve).
How it works in practice - the connection diagram
The most common connection has the heat pump heat the water in a buffer tank (or flow through it), and only from the tank does the heat get distributed further to the individual heating circuits via a manifold. The advantage is that the heat pump "sees" a large volume of water and doesn't have to react to every small fluctuation in the house's heat demand - the tank balances that out.
A smaller and cheaper variant of this connection is the so-called hydraulic separator (also commonly called a "low-loss header") - it's basically a short, vertical piece of larger-diameter pipe with a volume of just a few to a few dozen litres. It handles hydraulic separation of the circuits but provides practically no thermal storage. A real buffer tank has a volume on the order of hundreds of litres and, besides hydraulic separation, also provides real thermal inertia that limits cycling.
When a tank isn't needed at all
With modern installations using underfloor heating and a sufficient water volume in the pipework itself (typically in larger houses with extensive underfloor heating), a solution without a separate buffer tank is increasingly common - the so-called "bufferless" connection. Underfloor heating itself has a large thermal inertia (the screed, the water in the loops themselves), so to some extent it takes over the accumulation function. The heat pump manufacturer's installation manual always states the minimum water volume in the system required for that output - if the actual water volume in the pipework (including radiators or underfloor loops) exceeds this limit, a separate tank can be omitted or replaced with just a small hydraulic separator.
Conversely, with radiators (a smaller water volume in the system), with single-stage (on/off) heat pumps without output modulation, or when combining several circuits at different temperatures at once (e.g. underfloor heating + radiators together), a buffer tank is practically always recommended.
What buffer tank volume to choose
The basic rule most commonly used in practice is an approximate ratio of tank volume to heat pump output. For heat pumps, installation practice commonly recommends roughly 15 to 25 litres of tank volume per 1 kW of output for the heat pump, with a value around 20 l/kW usable as a practical mid-range estimate. This figure is significantly lower than for solid fuel boilers, where 50 to 100 l/kW is used - the reason is simple: a heat pump can smoothly regulate its output and start whenever needed, whereas a wood boiler has to burn through its whole fuel load in one go.
Here's what this looks like with specific examples, using an approximate factor of 20 l/kW:
These figures are only an approximate starting point, not a binding calculation - the actual design is always done by a designer or installation company based on the specific heat pump type (its minimum modulated output), the type of heating system (underfloor vs. radiators), and whether the tank also serves DHW preparation or just heating. For a combined tank (heating + DHW preparation via a built-in cylinder or heat exchanger), the volume is usually increased by a further 80 to 150 litres above the heating portion, depending on the number of people in the household.
Comparison with other heat sources
For a better picture, it's useful to compare how the need for buffer volume differs by heat source type - the differences are genuinely significant and explain why different figures are used for heat pumps than for solid fuel boilers.
| Heat source | Recommended volume | Main reason for a tank |
|---|---|---|
| Heat pump | 15-25 l/kW | Limiting compressor cycling, hydraulic separation of circuits |
| Gas condensing boiler | 0-10 l/kW (often just a separator is enough) | Hydraulic separation of multiple circuits |
| Electric boiler | approx. 10 l/kW | Levelling peaks, possibly using off-peak tariff |
| Solid fuel boiler (wood, coal) | 50-100 l/kW | Storing the whole combustion cycle, which can't be regulated |
The comparison shows why a buffer tank for a heat pump is often (mistakenly) confused with one for a solid fuel boiler - some customers order an oversized tank "for a reserve", but with a heat pump this tends to bring drawbacks: a larger water volume has to be heated to the required temperature first, which lengthens the system's start-up phase and slightly increases heat losses through the tank jacket, without bringing any real benefit for limiting cycling beyond the recommended range.
Temperature stratification in the tank
A well-designed buffer tank works with temperature stratification - the hottest water stays at the top, the coldest at the bottom. For a heat pump system, in practice we're commonly looking at these approximate temperatures (they vary depending on the heating curve settings and circuit types):
- top layer (outlet to the heating circuits): approx. 50 °C,
- middle layer: approx. 40 °C,
- bottom layer (return from the circuits back to the heat pump): approx. 28 °C.
This temperature difference between the top and bottom layer (about 22 °C in our example) matters - the better this layering is preserved (i.e. the less the water in the tank mixes), the more efficiently the heat pump works, because it draws in the cooler return water from the bottom without it being diluted by warmer water from the top. With incorrectly placed inlets and outlets (for example when flow from the manifold heads straight through the middle of the tank), the layers get mixed and the efficiency of the whole system drops.
Insulation and heat losses
The buffer tank must be well insulated, otherwise the benefit of the thermal reserve is lost to losses to the surroundings. Good-quality insulation (e.g. 80-100 mm of rigid polyurethane foam) keeps heat losses low - for a tank around 500 litres held at roughly 45-50 °C, in practice we're looking at roughly 1 to 1.5 kWh of heat loss over 24 hours. Compared with the total daily heat demand of an average family house (typically 15-40 kWh/day depending on the season and how well the house is insulated), this is a negligible figure, but with weak or damaged insulation (a thinner layer, an older tank, leaky pipe penetrations), these losses can grow two- to threefold.
A practical recommendation from installation practice: only place the tank in unheated space (e.g. a utility room) if that room is part of the house's thermal envelope (insulated, without large losses to the outside) - otherwise losses from the tank "escape" outside the heated space entirely needlessly. We cover this topic in detail in a separate article on the insulation and heat losses of a buffer tank.
The most common mistakes we encounter
A handful of typical mistakes keep recurring in practice when designing and installing a buffer tank with a heat pump:
1. An oversized tank "just to be safe"
As mentioned above, a needlessly large tank lengthens the start-up phase (it takes longer for the system to reach operating temperature) and increases heat losses with no real benefit. The recommended range of 15-25 l/kW is proven across dozens of installations and there's no need to deviate significantly upward from it without a specific technical reason.
2. An undersized tank and frequent cycling
The opposite extreme - the tank (or just the water volume in the system) is smaller than the minimum volume required by the heat pump manufacturer. This shows up as the compressor switching on and off frequently, especially during transitional periods (spring/autumn), when heat demand is low. The solution is either adding to or enlarging the tank, or (if the manufacturer allows it) fine-tuning the compressor's minimum run time in software.
3. Incorrectly placed inlets and outlets
As discussed in the stratification section - if the inlet and outlet aren't positioned to support natural layering (warm at the top, cold at the bottom), the tank loses its purpose and functions merely as an ordinary "mixing" reservoir with no stratification effect.
4. Forgetting future expansion
When designing the volume, it's worth also considering any future connection of another source (e.g. a fireplace insert with a heat exchanger, solar collectors) - modifying the hydraulic scheme later is always more complicated and expensive than if this possibility is already accounted for in the initial design.
A more detailed breakdown of these and other mistakes can be found in a separate article on the most common mistakes when designing a buffer tank.
How a tank for a heat pump differs from a tank for a solid fuel boiler
Besides the different volume (15-25 l/kW versus 50-100 l/kW), tanks for these two heat sources also differ structurally. A tank for a solid fuel boiler often works with higher temperatures (even over 80-90 °C at the top layer) and has to withstand greater thermal stress during an intense combustion cycle. A tank for a heat pump, on the other hand, works with lower temperatures (typically up to 55-60 °C, often even lower for low-temperature systems), which reduces material requirements - but the quality of the insulation and the correct hydraulic connection matter all the more, since a heat pump is more sensitive to small inefficiencies in the system (it runs continuously over longer periods, so any efficiency losses accumulate). We cover this topic in detail in the article on the buffer tank for a solid fuel boiler and in the comparison of a buffer tank with a DHW cylinder, where we also explain the difference between a heating water tank and a domestic hot water cylinder.
Summary - what not to forget
- A buffer tank with a heat pump mainly serves to limit compressor cycling and hydraulically separate circuits, not primarily to store heat as with solid fuel.
- Approximate volume: 15-25 l/kW of heat pump output, with a mid-range value of about 20 l/kW.
- With large-area underfloor heating and enough water volume in the pipework, a tank can sometimes be omitted - but the minimum water volume per the specific heat pump's installation manual should always be verified.
- Good insulation significantly limits heat losses - the typical loss for a well-insulated 500 l tank is on the order of 1-1.5 kWh/day.
- Correct placement of inlets/outlets is key to preserving temperature stratification and therefore the efficiency of the whole system.
- Don't oversize the volume "just to be safe" - a needlessly large tank tends to hurt rather than help.
Frequently asked questions
Does every heat pump need a buffer tank?
Not necessarily. If there's enough water volume in the system (pipework, radiators, underfloor heating) per the minimum value stated by the heat pump manufacturer, the tank can be omitted or replaced with just a small hydraulic separator. With radiator systems that have a smaller water volume, however, a tank is almost always recommended.
How big is the difference between a tank for a heat pump and one for a solid fuel boiler?
Significant - a heat pump is calculated at 15-25 litres per kW of output, a solid fuel boiler at 50-100 litres per kW. The reason is that a heat pump can smoothly regulate its output, while a wood boiler has to burn its whole load at once and the surplus heat needs to go somewhere.
Can a tank be too big?
Yes. A needlessly large tank lengthens the time needed for the system to reach operating temperature and slightly increases heat losses through the tank jacket, without bringing further benefit beyond the recommended volume of 15-25 l/kW.
Can heating and DHW preparation be combined in one tank?
Yes, combined tanks exist with both a heating and a DHW section (either a built-in cylinder or an instantaneous heat exchanger). In that case, a further 80-150 litres is usually added to the heating volume for DHW preparation, depending on the number of people in the household.
How much does temperature stratification affect a heat pump's efficiency?
Substantially. If the tank preserves stratification (hot water at the top, cold at the bottom), the heat pump draws in cooler return water and works more efficiently at a lower condensing temperature. If the layers get mixed (for example due to poorly placed inlets), the heat pump has to heat water that's already partly warm, which lowers its actual coefficient of performance (COP).
How much heat does a typical buffer tank lose per day?
With good insulation (80-100 mm) and a tank around 500 litres, losses are on the order of roughly 1 to 1.5 kWh over 24 hours. With weaker or damaged insulation, these losses can grow two- to threefold.
Related topics
- How to choose a buffer tank
- What buffer tank volume do I need
- Installation and connection of a buffer tank
- Insulation and heat losses of a buffer tank
- Most common mistakes when designing a buffer tank
- Buffer tank vs. DHW cylinder
- Buffer tanks
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