500 Litre Heat Pump Hot Water Cylinder
Our 500 litre heat pump hot water cylinder is an indirect hot water cylinder designed for use with an external air source or ground source heat pump system. Its large surface area heat exchanger coil is specifically suited to the lower primary flow temperatures produced by heat pumps, helping to transfer heat efficiently into the stored domestic hot water. With a nominal 500 litre stored water capacity (477 Litre actual), it provides a practical solution for homes with moderate hot water demand while still offering good recovery performance, mains pressure hot water when used in an unvented configuration and compatibility with modern low-temperature heating systems to help reduce carbon emissions and energy bills.
This 500 litre heat pump cylinder is a floor-mounted unit that has a class-leading ErP “A” rating in accordance with Commission Delegated Regulation (EU) No 812/2013, an EU regulation that sets out energy labelling requirements and supplementary product information for water heaters and hot water storage tanks.
A 500-litre cylinder is suitable for very large residential properties (country houses, mansions, etc.) or commercial domestic hot water heating requirements.
Heat pump cylinder
A heat pump cylinder is an indirect hot water cylinder with a large, high-efficiency heat exchanger specifically designed to transfer heat effectively from the lower primary water temperatures produced by heat pump systems. These heat pumps can either be an air source heat pump or a ground source heat pump.
These are not integrated heat pump cylinders, which have a heat pump cylinder and an air source heat pump combined in one integral unit, i.e., an integrated heat pump water heater using heat pump technology.
Why Heat Pump Cylinders Need a Larger Coil
A conventional oil boiler or gas boiler may produce primary water at 70°C or higher, whereas a heat pump typically operates at much lower temperatures, usually anywhere from 35°C to 55°C.
Therefore, a heat pump cylinder generally needs a larger heat exchanger surface area to achieve effective heat transfer.
This fact distinguishes a heat-pump-optimised cylinder from a standard indirect cylinder.
Larger 32 mm coil heat exchanger
Our heat pump cylinders use a large 32 mm diameter corrugated stainless-steel heat exchanger coil, designed to accommodate the higher primary water flow rates typically required by air source and ground source heat pumps. The large-bore construction helps to minimise the pressure drop through the cylinder coil, reducing the pump head required to maintain the correct flow rate.
This helps reduce circulation pump energy consumption, allowing the pump to operate at a lower speed and helping ensure that the heat pump receives the water flow it requires to deliver its rated heating output.
| Heat Exchanger Detail | |
| Exchanger type | Heat pump corrugated coil heat exchanger |
| Exchanger design | Corrugated pipe coil |
| Exchanger material | 316L stainless steel |
| Exchanger diameter [mm] | 32 |
| Exchanger length [m] | 45 |
| Exchanger surface area [m²] | 7 |
| Exchanger volume [L] | 29 |
| Maximum pressure [bar] | 10 |
| Maximum temperature [°C] | 95 |
The corrugated surface also promotes turbulence within the primary water, helping maintain effective heat transfer and higher flow rates while benefiting from the lower hydraulic resistance associated with the larger coil diameter.
Duplex 2205 Stainless Steel Hot Water Cylinder
The hot water cylinder is manufactured from Duplex 2205 stainless steel, offering high corrosion resistance, mechanical strength and long service life. This premium stainless steel makes the units suitable for soft and hard water areas. Unlike a glass-lined hot water cylinder, no sacrificial anodes are required.
ErP A Rated Energy Efficiency
Our 500 litre heat pump cylinder has a class-leading ErP energy efficiency rating of A, helping to minimise heat loss from the stored domestic hot water. High levels of cylinder insulation allow the water to remain hotter for longer, reducing the amount of energy the heat pump needs to use to maintain the required temperature. This can help lower overall energy consumption and running costs while allowing the heat pump system to operate as efficiently as possible.
For customers looking to maximise the benefits of either an air source or a ground source heat pump as a heat source, an A-rated hot water cylinder provides an efficient and well-insulated solution for storing domestic hot water.
Specifications of our 500L heat pump water heater
Technical data of our 500 Litre stainless steel DHW cylinder/heat pump water heater
| Dimensions and connection details | ||||
| H | All connections are female BSP | Height | 1828 mm | |
| ØD | Outer diameter | 700 mm | ||
| Ød | Inner diameter | 600 mm | ||
| H | DN 14 | G 1″ | T&P pressure relief valve connection | 1828 mm |
| F | DN 3 | G 1 1/4″ | Heat pump exchanger inlet / outlet | 1460 mm |
| E | DN 4 | G 1 1/4″ | Heat pump exchanger inlet / outlet | 280 mm |
| C | DN 5 | G 3/4″ | Sensor pocket 1 | 1310 mm |
| L | DN 6 | G 1 1/2″ | Electric heating element | 400 mm |
| I | DN 7 | G 3/4″ | Sensor pocket 2 | 620 mm |
| A | DN 8 | G 1 1/4″ | Connection (inlet / outlet) | 280 mm |
| D | DN 9 | G 1 1/4″ | Connection (inlet / outlet) | 1560 mm |
| G | DN 2 | G 1/2″ | Drain port | 33 mm |
| General Product Details | |
| Product type | Stainless steel DHW cylinder / heat pump water heater |
| Rated volume | 500 L |
| Actual volume | 471 L |
| Application | Heat pump / domestic hot water |
| Installation type | Vertical floor-standing |
| Connection layout | 4 connections on two opposite sides |
| Number of exchangers | 1 |
| Height [mm] | 1828 |
| Tilt dimension [mm] | 1958 |
| Outer diameter [mm] | 700 |
| Inner diameter [mm] | 600 |
| Insulation | Highly efficient CFC-free rigid PU foam thermal injected insulation for minimal heat loss |
| Insulation thickness [mm] | 50 |
| Outer casing | Frosted metal, RAL 7030 |
| Weight [kg] (Empty) | 88 |
| Warranty | 5 years |
Please note: installers should allow space on site around the install for future maintenance and inspection of the fitted external expansion vessel if the unit is being installed as a 500 litre unvented hot water cylinder, the drain valve (no drain valve supplied – by others), and other components.
Pressure and temperature ratings
| Tank material | Duplex Stainless Steel 2205 |
| Maximum operating / test pressure | 6/10 bar |
| Maximum tank temperature [C] | 95 |
Heat Pump Compatibility and DHW Heating Performance
Our 500 litre heat pump hot water cylinder is designed for applications where the domestic hot water demand is higher and the heat source may be a larger air source heat pump, ground source heat pump, water source heat pump, high-temperature heat pump or commercial heat pump system. These types of heat pumps are commonly used in hotels, care homes, sports facilities, apartments, offices and other commercial buildings where higher hot water recovery rates may be required.
The large surface area heat exchanger coil allows the cylinder to accept a wide range of heat pump outputs, but the actual DHW heating performance will always depend on the heat pump capacity, primary flow temperature, primary water flow rate and the flow/return temperature difference across the cylinder coil.
Based on a primary flow temperature of 55°C and a return temperature of 45°C, giving a 10°C ΔT, the cylinder heat exchanger can transfer approximately 53 kW when heating domestic hot water from 10°C to 45°C. This is equivalent to approximately 1,295 litres of DHW per hour being raised through a 35°C temperature increase.
This does not mean that a 53 kW heat pump must always be connected to the cylinder. The cylinder can equally be used with smaller heat pumps, with the hot water recovery rate reducing in proportion to the available heat input.
For example, when connected to a 16 kW heat pump, approximately 391 litres of domestic hot water per hour can be heated from 10°C to 45°C, assuming the heat pump can maintain its rated 16 kW output at the required primary flow temperature.
This makes the 500 litre cylinder suitable for a wide range of commercial heat pump systems, from relatively modest single heat pumps through to much larger commercial units or multiple heat pumps operating in cascade.
Matching the Heat Pump to the Cylinder Coil
The heat exchanger coil has a maximum amount of heat that it can transfer under a given set of operating conditions. Installing a larger heat pump therefore does not automatically result in a proportionally faster cylinder recovery time.
For example, if the cylinder coil can transfer approximately 53 kW at the specified operating conditions, connecting a 65 kW heat pump does not necessarily mean that 65 kW can be transferred into the domestic hot water. Once the coil reaches its heat-transfer capability, the heat exchanger limits the system’s performance.
Likewise, when a smaller heat pump is used, such as a 16 kW unit, the heat pump becomes the limiting factor because the coil is capable of transferring substantially more heat than the heat pump can supply.
When selecting a heat pump, it is also important to use the manufacturer’s actual heating capacity at the required leaving water temperature, rather than simply the nominal heat pump rating. A unit described as 50 kW, for example, may produce a different output when required to generate 55°C primary water.
High-Temperature Commercial Heat Pumps
The 500 litre cylinder can also be used with high-temperature heat pumps, including systems using refrigerants such as R290 and CO₂, as well as commercial cascade heat pump arrangements.
These systems can operate at higher primary flow temperatures than conventional low-temperature heat pumps, which can significantly increase the temperature difference between the heat exchanger coil and the stored domestic hot water. This can increase heat transfer and achieve higher cylinder storage temperatures.
Where a high-temperature heat pump is being specified, the cylinder should be sized against the actual heat pump output, primary flow temperature, return temperature, primary flow rate and required DHW storage temperature.
We have a cylinder sizing programme that calculates the expected heat exchanger output, DHW heating rate and cylinder recovery performance for different heat pump operating conditions. Please contact us with the operating temperatures of your heat pump and the delta T of your system, and we can calculate the expected performance of the cylinder.
Unvented cylinders must comply with local building regulations for installation. To legally fit, service, or maintain an unvented hot water cylinder larger than 15 litres in capacity, the installer must hold the current G3 unvented hot water qualification, which is often referred to simply as a G3 certificate or card.
Immersion Heater Backup
Our heat pump water heaters come with an immersion heater port on the hot water cylinder. This allows an immersion heater or immersion element to provide supplementary and backup heating for the hot water tank and thus the hot water system. It also provides a temperature boost for Legionella protection.
Unvented cylinder
We can supply this heat pump cylinder in either an unvented or vented configuration.
Key features of unvented systems
Unvented cylinders can provide higher mains-pressure hot water without requiring a cold-water storage tank. Large unvented cylinders need robust incoming mains water pressure and flow rates for performance.
Unvented systems can deliver balanced water pressure to multiple outlets simultaneously. The compatible unvented kit includes an expansion vessel for installation in the system, which accommodates the expansion volume of the heated water. You will also require a pressure and temperature valve, rated to the power you are feeding into the tank.
Legionella Protection and Risk Management
As with any stored hot water system, the cylinder should be operated with suitable controls to help manage the risk of Legionella bacteria. In heat pump systems, the normal hot water temperature may be lower than that used with a conventional boiler, so the system can be programmed to periodically raise the cylinder temperature to 60°C or more for thermal disinfection.
Depending on the heat pump and its maximum flow temperature, this higher temperature may be achieved by the heat pump itself or with assistance from the cylinder’s immersion heater. The exact temperature settings and operating cycle should follow the heat pump manufacturer’s instructions and the requirements of the overall hot water system.
Commercial Hot Water Applications
In commercial buildings where temperature is used to control the risk of Legionella, stored domestic hot water should be maintained at 60°C or above in accordance with UK HSE guidance for commercial heating.
When a heat pump is used to heat an indirect hot water cylinder, it is therefore important to ensure that the complete system can achieve the required storage temperature. Depending on the heat pump’s maximum flow temperature, the required temperature may be achieved by the heat pump alone or with supplementary heating from the cylinder’s electric immersion heater.
This is particularly important in commercial hot water systems where maintaining appropriate storage and distribution temperatures is part of the building’s strategy for managing Legionella risk.
Contact Flexiheat UK
For more detailed information, stock availability, delivery times, price or to order/purchase our cost-effective and energy-efficient indirect 500 litre heat pump hot water cylinder in premium duplex stainless steel.
Please contact our sales team at 01202 822221 or use our website’s email contact form to discuss your requirements.