The Baxi 24 Litre Buffer Vessel (MPN 7851676) is a factory-insulated thermal storage tank engineered to sit between your Baxi air source heat pump and the central heating circuit. By adding essential system volume, the buffer prevents the compressor from short-cycling on low water-content systems, ensures the heat pump has the energy reserve it needs for reliable cold-weather defrost cycles, and decouples the pump's steady flow from the variable demand of thermostatic radiator valves and zoned underfloor circuits. The result is lower compressor wear, a better seasonal COP, and a quieter, more stable system — all from a compact unit that hangs from the wall and connects with standard G 1¼" fittings. At 24 litres nominal capacity (23 L measured), it meets the Baxi minimum system-volume requirement for the 4 kW, 5 kW, 7 kW and 8 kW HP40 and HP50 monobloc heat pumps and is rated Energy Class A with a standing loss of just 21 W.
⚠ Installer Requirement
This buffer vessel must be installed by a competent and qualified heating engineer in accordance with current Building Regulations, BS 7074:1, and EN 12828. The vessel must be protected from freezing and must not be used for potable or domestic hot water storage.
ℹ Sizing Note
The 24 L buffer is sized for Baxi HP40 and HP50 monobloc heat pumps (4–8 kW outputs) where the Baxi installation manual specifies a minimum system volume of >25 L. For 11 kW and 13 kW heat pumps, or cascade installations, the Baxi 40 L Buffer Vessel is required instead. Always calculate total system volume including pipework and emitters before specifying.
Why the Baxi 24 L Buffer Vessel
Eliminates Compressor Short-Cycling
Low water-content heating systems — particularly those with many thermostatic radiator valves — offer the heat pump little thermal mass to work against. The 24 L buffer adds the system volume the compressor needs to run in sustained, efficient cycles rather than starting and stopping repeatedly. Fewer compressor starts means less wear, lower noise, and a longer service life for the heat pump.
Reliable Cold-Weather Defrost
When outdoor temperatures drop, the heat pump must periodically defrost its outdoor coil. The buffer stores enough thermal energy to sustain flow to the radiators or underfloor circuits during the defrost cycle, preventing occupants from experiencing a cold drop in temperature. This is particularly important in the UK climate where overnight temperatures regularly trigger defrost events.
Hydronic Pressure Break
In some system layouts, the buffer vessel provides a hydraulic separation point between the heat pump's primary circuit and the distribution circuit. This protects the heat pump from pressure spikes caused by multiple zone valves closing simultaneously, and allows the primary and secondary circuits to operate at different flow rates — critical for zoned systems with mixed radiator and underfloor heating circuits.
Energy Class A with 63.5 mm PU Insulation
The vessel's 63.5 mm rigid polyurethane foam shell keeps standing losses to just 21 W, earning it an Energy Class A rating under EU Regulation 812/2013. Heat stored in the buffer is retained efficiently between heat pump cycles, reducing the frequency of top-up heating and contributing to a better seasonal efficiency figure for the whole system.
10 Bar / 95 °C Rated — Future-Proof
With a maximum working pressure of 10 bar and a maximum temperature rating of 95 °C, the vessel comfortably exceeds the operating parameters of current Baxi heat pumps. It complies with the Pressure Equipment Directive 2014/68/EU and EN 12897:2020 Appendix B, giving installers confidence in long-term performance and regulatory compliance.
Wall-Hung with Multiple Connection Points
The buffer ships with wall-mounting brackets and a thermometer as standard. Connections include G 1¼" flow, return, heater, and auxiliary ports plus three G ½" sensor pockets, a G ½" thermometer port, and a G ½" drain — giving installers the flexibility to add immersion backup heating, temperature sensors, and service connections without additional fittings. Unused ports ship with plug and gasket kits included.
Technical Specifications
| MPN |
7851676 |
| Nominal Capacity |
24 L (measured: 23 L) |
| Maximum Working Pressure |
10 bar |
| Maximum Working Temperature |
95 °C |
| Standing Loss |
21 W |
| Energy Class |
A |
| Insulation Material |
Rigid polyurethane foam (PU) |
| Insulation Thickness |
63.5 mm |
| Empty Weight |
9 kg |
| Air Vent Connection |
G 1¼" |
| Heater Inlet / Outlet |
G 1¼" |
| Auxiliary Connection |
G 1¼" |
| Thermometer Port |
G ½" |
| Sensor Pockets |
3 × G ½" |
| Drain Connection |
G ½" |
| Immersion Heater Port |
1½" BSP (plug and gasket supplied) |
| Mounting |
Wall-hung (brackets supplied) |
| Compatible Heat Pumps |
Baxi HP40 & HP50 monobloc ASHP (4–8 kW) |
| Warranty |
12 months parts and labour |
| Directives |
2009/125/EC (ErP), 2014/68/EU (PED), EU 812/2013, EU 814/2013, EN 12897:2020 App B |
What's in the Box
-
1 × Baxi 24 L Buffer Vessel with wall-mounting brackets and thermometer pre-fitted
-
1 × 1½" immersion heater plug & gasket (caps the immersion port if no backup heater is fitted)
-
3 × ½" sensor pocket plugs (caps unused sensor pockets)
- 1 × Energy label
- 1 × Installation and Maintenance manual
Frequently Asked Questions
Do I need a buffer vessel with my Baxi heat pump?
It depends on the total water volume of your heating circuit. Baxi's installation manuals specify a minimum system volume of greater than 25 L for their 4–8 kW monobloc heat pumps. Many modern heating systems — particularly those with mostly TRV-controlled radiators or small pipe runs — contain less water than this. A buffer vessel tops up the volume to the required minimum. Your installer should calculate system volume at the design stage; the buffer is needed if the calculated volume falls short of the manufacturer's minimum.
Is the 24 L vessel the right size for my heat pump?
The Baxi 24 L Buffer Vessel (MPN 7851676) is designed for use with Baxi HP40 and HP50 monobloc heat pumps in the 4 kW to 8 kW output range. For larger Baxi heat pumps — the 11 kW and 13 kW models — the Baxi installation manual specifies a buffer of greater than 40 L, so the Baxi 40 L Buffer Vessel is required. Cascade installations of two outdoor units also require a buffer of more than 40 L per outdoor unit. Always refer to the heat pump installation manual for the applicable minimum system volume figure.
Can I use this buffer vessel with a non-Baxi heat pump?
The vessel itself — with G 1¼" connections, 10 bar rating, and 95 °C maximum temperature — is compatible with the operating parameters of most air source heat pumps. However, Baxi designed and tested this product specifically within their heat pump system. If you are using a different heat pump brand, check the manufacturer's minimum system volume requirement and verify the connection sizes match before purchasing. Most UK-standard buffer vessels use G 1¼" (male) connections on primary flow and return.
Can the buffer vessel be used to store domestic hot water?
No. The Baxi 24 L Buffer Vessel is designed exclusively for use in sealed heating circuits containing heating water. It must not be used for the storage of potable or domestic hot water. For domestic hot water, the correct product is a Baxi ASHP Cylinder or Baxi ASHP Pre-plumbed Cylinder, both of which are fully certified for DHW storage and carry the necessary WRAS and unvented approvals.
Does the buffer vessel include an immersion heater?
No, but the vessel includes a 1½" BSP immersion heater port with a plug and gasket supplied to cap it if not used. Some installers fit an electric immersion element here as a backup heat source for periods of very low ambient temperature when the heat pump output is reduced, or as an emergency fallback. Refer to the Baxi HP40/HP50 installation manual for guidance on backup heater connection options and control requirements.
What is the warranty on the Baxi 24 L Buffer Vessel?
Baxi provides a 12-month parts and labour warranty covering manufacturing defects. If a fault arises within the warranty period, contact your installer in the first instance; they should then liaise with their supplier for a replacement unit. Warranty does not cover damage resulting from improper installation, failure to follow the installation manual instructions, or inadequate maintenance.
