Buffer Vessels
Buffer vessels store heated water for instant demand in heating systems (underfloor heating, radiators).
Buffer vessels, also known as buffer tanks, play a crucial role in modern heating systems. Unlike hot water cylinders that provide domestic hot water for taps and showers, buffer vessels store heated water solely for space heating purposes.
- Temperature stabilisation: Maintain a more constant flow temperature within a heating system
Short cycling prevention: Stop heat sources from turning on and off too frequently
Energy storage: Act as a battery to store excess power for later use (i.e. from solar panels which can produce power even when it isn’t immediately required)
System balancing: Prevent the supply and demand of power from becoming imbalanced if a heat source can’t keep up
Heat source integration: Allows multiple heat sources to work together to satisfy a high demand
Buffer vessels can also be useful in systems with:
- Underfloor heating (which works best with stable flow temperatures)
- Multiple heating zones with different demands
- Heat pumps, which sometimes require minimum system volumes – see Aquinox Heat Pump Buffer Vessels for our range of small buffer vessels
Our range of buffer vessels is designed to store heated non-potable water for instant demand to another system, such as underfloor heating or radiator systems. Buffer vessels are essential for combining heat sources from controlled and non-controlled systems, as integrating a heat pump with a wood burner. Alternatively, a buffer vessel/tank can connect solar thermal panels to a conventional boiler system.
What is a Buffer Vessel?
The most common purpose of a buffer vessel is to accept the heat generated by one or more heat sources, and to store it for on-demand use in a heating system. Unlike hot water cylinders that provide domestic hot water to taps and showers, buffer vessels store heated water solely for space heating purposes.
The heat sources that buffer tanks would typically accept heated water from, include heat pumps, biomass boilers, gas or oil boilers, and solar panels. These are all considered “controllable” heat sources, as they only can be turned off with little to no overrun, ensuring the system is kept within its safe temperature and pressure limits. Basic wood-burning stoves can also be a viable heat source for buffer vessels, but due to often lacking controllability (once it’s lit, it’s impractical to put it out), they are generally only compatible with open-vented buffer vessels and heating systems which are less common.
Buffer Vessel or Thermal Store - What's the Difference?
A buffer vessel stores heated water for space heating circuits only, such as radiators and underfloor heating. It does not produce domestic hot water.
If you need a vessel that also supplies hot water to your taps and showers, you’re looking for a thermal store.
Not sure which is right for your system? Get in touch, and we’ll help you work it out.
Custom Manufacturing for Your System
As with all of our cylinders, we can manufacture buffer units to a wide variety of specifications, such as vented or pressurised systems, multiple heat input and output coils and materials of copper or stainless steel. We can also manufacture our buffer tanks with many different connection sizes and positions to perfectly match your system requirements.
We also manufacture a range of Aquinox Heat Pump Buffer Vessels for use in smaller domestic applications for expanding an existing system to make it suitable for a heat pump system.
The variables that we can offer within the specification of our Aquinox stainless steel buffer vessels include:
- Heights (300-2,050mm)
- Diameters (184-1,300mm)
- Capacities (8 Litres – 2,100 Litres)
- Materials (copper or stainless steel)
- Pressure ratings (between 6-10 bar MWP, depending on diameter)
- Heat source compatibilities (Heat Pumps, Solar Panels, Boilers, Solid Fuel Burners)
- Coil sizes and quantities (0.27-12.0m² – possibly even bigger larger units)
- Connection sizes and quantities (as many ½”- 3” BSP connections and various-sized flanges/hatches as needed)
- Connection positions (specific heights and/or rotational degrees)
- Internal features (dip tubes, baffle plates, sparge pipes, diffusers, innovations, etc)
- Insulation thicknesses (35mm – 150mm)
- Finishes (Metallic silver case, exposed green foam insulation, Armaflex)
Why Heat Pumps Need Buffer Vessels
Buffer tanks can offer certain benefits in heat pump systems, although in many installations, they are not required for optimal operation. In the scenarios in which an installer deems one to be advantageous, there are many options available; however, not all buffer vessels provide an equally optimal solution
A heating engineer may recommend a buffer vessel in situations such as:
- Where the overall system water volume is low, increasing the risk of unstable operation or frequent on/off cycling.
- Where additional thermal mass helps the system ride through defrost cycles without causing noticeable temperature fluctuations.
- Where there is a risk of short cycling due to low heat demand, zoning, or intermittent flow, which can reduce efficiency and increase long-term wear on components.
Mild Steel vs Stainless Steel
- Mild steel is a practical and cost-effective option
- Aquinox Stainless® steel is more robust and adaptable
The material that Buffer Vessels are manufactured from has a significant impact on how well they perform over time.
Standard mild steel buffer vessels remain a practical and cost-effective option in many heat pump installations, particularly where system layouts are simple, water quality is well controlled, and long-term adaptability is not a priority. Their widespread availability and lower upfront cost make them a sensible choice for budget-conscious or time-critical projects. They are designed to meet a basic functional requirement, but have limitations relating to system stability, heat retention, hydraulic flexibility, and adaptability.
In response to these limitations, Aquinox Stainless® steel buffer vessels offer a more robust and adaptable alternative. They represent a higher initial investment, but their value becomes more apparent in systems where system stability, low heat loss, hydraulic flexibility, and system adaptability are important considerations.
Learn about the benefits of copper and stainless steel here.
System Stability
- Mild steel buffers rely on internal coatings and inhibitors to prevent their corrosion
- Inhibitor concentration drifts over time
- Oxygen ingress happens (especially during servicing or partial drain-downs)
- Corrosion particles circulate and end up in:
- heat exchanger coils
- pumps
- valves
- strainers
- plate heat exchangers
Heat retention
- Standard buffers often have thin and inadequate insulation
- They are often installed in colder areas like garages, plant rooms, or outbuildings
- They become constant heat losses
What we hear: “The SCOP figures didn’t quite match my expectations as excess heat was being lost from the buffer”
Aquinox Stainless Steel Buffer Vessels can be insulated anywhere up to an A+ rating on the ErP scale, dependent on the size of the buffer vessel.
| Buffer Vessel Size | ErP Rating |
|---|---|
| 10 – 130 Litres | A+ |
| 140 – 350 Litres | A |
| 360 – 500 Litres | B |
Hydraulic flexibility
“Off the shelf” buffers:
Come with fixed flow/return positions
Assume a generic system layout
Often force awkward pipe runs, unnecessary elbows, and compromised flow paths
Act like big mixing chambers that ignore stratification maintenance
What we hear “I know a better layout exists, but the standard vessels just don’t allow it”
Connection sizes and positions on all Newark Cylinders products are customisable so that you can plan optimal pipe-runs in advance, add features that promote stratification (diffusers, baffles, etc), and ultimately, gain performance.
Adaptability
Standard buffers can’t accommodate:
- Futureproofing
- System/ zone extensions
- Control upgrades as technology progresses
What we hear: “With hindsight, I’d have added some extra connections and pockets”
With an Aquinox Buffer Vessel, you can add as many additional future-proofing features as you can realistically anticipate being beneficial.
An Aquinox Buffer vessel cannot:
- fix a flawed system design
- compensate for incorrect flow rates
- improve the SCOP of an incorrectly designed system
- eliminate the potential need for inhibitors entirely
Sizes
Our range of buffer vessels comes in a comprehensive variety of sizes, manufactured in either copper or stainless steel to suit your system requirements and budget.
| Material | Sizes |
|---|---|
| Copper | 40 – 1,150 litres |
| Stainless Steel | 50 – 2,000 litres |
Example Size Dimensions
| Size | Steel Case Size | Volume |
|---|---|---|
| 760x375mm | 820x450mm | 80 litre |
| 950x375mm | 1010x450mm | 100 litre |
Example System Diagram
This example system diagram demonstrates a large industrial application such as a farm or factory.
The buffer vessel is open-vent and is heated via the wood burner, solar panels and heat pump which can then be circulated around the radiator and underfloor heating circuit.
The unvented cylinder is pressurised, however, by using a coil, the hot water inside the buffer vessel can be used to heat the cylinder. This hot water can then be provided at high pressure to taps and showers.
Frequently Asked Questions
What is a Buffer Vessel?
A buffer vessel (also known as a buffer tank) is used to store heated water for on-demand use in a heating system. Unlike hot water cylinders that provide domestic hot water to taps and showers, buffer vessels store heated water solely for space heating purposes.
What does a buffer vessel do?
A buffer vessel acts as a thermal reservoir between a heat source and a heating system. Its primary purpose is to store heated water so the heating system can draw from it on demand, rather than relying on the heat source to fire every time there is a call for heat. This is particularly important in heat pump installations, where frequent on/off cycling (known as short cycling) can reduce efficiency and shorten the lifespan of the compressor.
By absorbing excess heat and releasing it gradually, a buffer vessel allows the heat source to run for longer, more efficient cycles while ensuring the heating system always has a ready supply of heated water to meet demand.
Buffer vessels are also used to balance systems with multiple heat sources, such as a heat pump combined with solar thermal, by accepting and storing heat from each source in a single vessel.
How does a buffer vessel work?
A buffer vessel is connected between the heat source and the heating distribution system using flow and return connections. Heated water from the heat source (such as a heat pump, boiler, or solar thermal system) flows into the vessel, where it is stored at temperature until the heating system requires it. When there is a call for heat from the building, the heating circuit draws hot water from the buffer vessel rather than directly from the heat source.
As the stored water temperature drops, the heat source is signalled to fire again and replenish the vessel. This cycle ensures a consistent supply of heated water to radiators or underfloor heating, while giving the heat source the freedom to operate at its most efficient output rather than constantly reacting to short-term fluctuations in demand. The vessel itself is typically insulated to minimise standing heat loss during storage.