400 Litre Air source heat pump water heater
Our 400 litre air source heat pump water heater is for domestic hot water production only. They are a high-performance R134A refrigerant air source heat pump with a built-in 400-litre storage cylinder manufactured from premium Duplex Steel 2205 (thus suitable for soft and hard water areas), with an efficiency rating of “A+” at 20°C external air temperature or an “A” efficiency rating at 7°C for the hot water system.
Heat pump technology is used by air source heat pump water heaters to efficiently capture heat from the external air and transform it into residential domestic hot water through a heat exchanger process. They need electricity to power the water heater’s heat pump and the backup or top-up immersion heater (electric heating element).
What applications can a 400 litre ASHP water heater cover?
A 400 litre air source heat pump water heater is best suited to heavy domestic and small commercial and light-commercial applications, particularly where hot water demand is spread over the day rather than concentrated into a very short peak. Suitable applications can include small boutique hotels (around 6 to 10 guest rooms) and B&Bs and guest houses (approximately 4 to 8 bedrooms), holiday accommodation, sports facilities, offices, workshops, hair/beauty salons, schools and nurseries, small pubs, cafés and fast food locations, and staff shower facilities.
ideal for B&Bs, guest houses, small hotels, offices, salons, workshops, sports changing facilities and other light-commercial hot water applications
The exact capacity depends heavily on the heat pump’s recovery output and how quickly the stored hot water is used.
How much usable hot water does 400 litres actually provide?
If the cylinder stores 400 litres at 60°C, it contains considerably more usable shower-temperature water when mixed with the incoming cold supply (10°C is the temperature used for the majority of the UK in wintertime conditions; however, Scotland and northern Ireland should use 5°C as the lowest cold water mains temperature).
Assuming the relevant incoming mains water and a mixed water temperature of 40°C:
| Incoming cold water | Cylinder storage temperature | Mixed water temperature | Usable mixed water |
|---|---|---|---|
|
5°C |
60°C | 40°C | 629 litres |
|
10°C |
60°C | 40°C |
667 litres |
|
15°C |
60°C | 40°C |
720 litres |
| 20°C | 60°C | 40°C |
800 litres |
So, in practical terms, a fully heated 400 litre cylinder can provide roughly 600 to 670 litres of usable mixed hot water in winertime conditions before allowing for pipework losses, stratification and the temperature at which the heat pump restarts to heat.
We also show the data for typical summertime main cold water temperatures; please use 15°C for Scotland and Northern Ireland and 20°C for the rest of the UK.
For commercial systems, the 60°C figure is also significant because HSE guidance states that, where temperature is being used to control Legionella risk, hot water should normally be stored at 60°C or above and distributed to achieve at least 50°C within one minute at outlets.
How Many Showers Can a 400 Litre Heat Pump Water Heater Supply?
All the data shows that the actual average showtime is 5 minutes. A standard showerhead uses either 7 or 8 litres, unlike drench showerheads, which can have very high flow rates; 25 litres/minute is not uncommon for these shower types. We’ll use 6 minutes in the following data just to allow a “bit of headroom” on the calculations.
Number of showers at 10°C mains cold water temperature in wintertime
400 litre cylinder storing water at 60°C, with 10°C incoming mains water mixed to a 40°C shower temperature, provides approximately 667 litres of usable mixed hot water. At an 8 litre/minute shower flow rate, this equates to approximately 83 minutes of total showering, or around 13 to 14 six-minute showers, 11 seven-and-a-half-minute showers, or 8 ten-minute showers, before taking any heat pump recovery into account.
Number of showers at 5°C mains cold water temperature in wintertime
A 400 litre cylinder storing water at 60°C, with 5°C incoming mains water mixed to a 40°C shower temperature, provides approximately 629 litres of usable mixed hot water.
At an 8 litre/minute shower flow rate, this volume equates to approximately 79 minutes of total showering, or around 13 six-minute showers, 10 seven-and-a-half-minute showers, or 7 ten-minute showers, before taking any heat pump recovery into account.
In reality, the number of showers could be higher if the heat pump reheats the cylinder while the showers are in use, especially if they are staggered rather than simultaneous.
Baths
Firstly, for a standard UK bath, it is worth distinguishing the fill volume figure from the physical capacity of the bath. A standard bath may have a capacity to the overflow of roughly 180 to 225 litres, but in normal use it is not filled anywhere near that level, as the person getting into the bath displaces a significant amount of water. Waterwise research provides an actual-use figure of about 88 litres for a standard bath, despite a nominal capacity of around 225 litres.
Here is the data for the number of baths (we’ll use 90 litres for this table, just to allow a bit of “headroom” in the calculations).
| Incoming mains temperature | Usable water at 40°C | Approx. 90L baths |
|---|---|---|
| 5°C | 629 litres | 7.0 baths |
| 10°C | 667 litres | 9.4 baths |
| 15°C | 720 litres | 8.0 baths |
| 20°C | 800 litres | 8.9 baths |
Again, these figures are before accounting for heat losses or heat pump recovery while the hot water is in use.
Recovery time
At an ambient air temperature of 20°C, the 400 litre air source heat pump water heater has an estimated full recovery time of approximately 8 hours from 10°C to 60°C. At a lower ambient temperature of 7°C, the estimated recovery time increases to approximately 10 hours.
For 10℃ to 53℃, which is a very usable temperature for DHW bathing, the recovery time is 5.3 hours with inlet air at 20℃ and 6.5 hours with inlet air at 7℃.
The main consideration with an air source heat pump water heater is its longer recovery time compared with conventional high-powered water heating systems. Heat pumps work and achieve their high efficiency by heating the water gradually, rather than using large amounts of electrical power to reheat the cylinder quickly. For this reason, the cylinder should be correctly sized and designed to meet the property’s expected hot water demand, particularly when several showers, baths, or other hot water outlets may be used in quick succession, allowing the water heater sufficient recovery time before the next large hot water demand.
Larger commercial water heating demands
For applications with larger domestic hot water requirements, multiple 400 litre air source heat pump water heaters can be installed in parallel. This modular arrangement increases both the available stored hot water volume and the total heat pump recovery capacity.
For example, three 400-litre units provide 1,200 litres of stored hot water at 60°C (2,000 litres of mixed water at 40°C based on 10°C incoming water temperature) and approximately 9.3 kW of combined heat pump heating capacity at 20°C ambient conditions.
Parallel installation can also provide valuable system redundancy, as individual units can potentially be isolated for maintenance while the remaining water heaters continue to operate.
| Number of 400L units | Total storage | Approx. mixed water at 40°C | Combined heat pump output at 20°C ambient air |
|---|---|---|---|
| 1 | 400 litres | 667 litres | 3.1 kW |
| 2 | 800 litres | 1,333 litres | 6.2 kW |
| 3 | 1,200 litres | 2,000 litres | 9.3 kW |
| 4 | 1,600 litres | 2,667 litres | 12.4 kW |
| 5 | 2,000 litres | 3,333 litres | 15.5 kW |
An important point is that the recovery time of the whole bank does not necessarily become longer as units are added. If all cylinders are depleted by approximately the same proportion and all heat pumps operate simultaneously, each 400 litre cylinder reheats independently. Therefore, two 400 litre units give you twice the storage and twice the heating output, but their full recovery period can remain around the same 8-hour figure at 20°C ambient for 10°C to 60°C, rather than becoming 16 hours.
Domestic hot water heat pump energy efficiency
They have a COP (the coefficient of performance, or COP for short), which is a measure of immediate energy efficiency in heat pumps that is calculated by dividing heat output by electrical input. Improved efficiency and reduced operating expenses are associated with a higher COP. Because heat pumps move existing heat rather than create it from scratch, they routinely achieve a COP above 1.0 (or 100% efficiency).
A COP of 4.0, for instance, indicates that the system generates 4 units of heat using 1 unit of electrical power, which means it operates at 400% efficiency when heating water.
COP = Heat Output (in kW) / Electrical Input (in kW)
Our 400 Litre domestic hot water air source heat pump, depending on the outside air temperature, has COP figures of 2.81 on average (the range is 2.52 to 3.1 depending on air temperature). This means they are 2.81 times more efficient than electric direct water heaters.
By using renewable energy sources to power the air-source heat pump for hot water only, you can lower energy consumption and mitigate climate change with a low-carbon hot water system for your home or business. This line of air-to-water heat pumps is used to provide hot water for homes or social housing or even generate hot water for a light commercial water heating system (recovery/heat-up times dependent) in an environmentally friendly manner.
Our air-source heat pump water heaters, with their high energy efficiency ratings, can significantly lower carbon emissions and are ideal for eco-conscious homeowners and businesses looking to reduce carbon emissions while producing DHW.
Due to current heat pump technology, an on-demand heat pump water heater range with high flow rates is not currently feasible for your hot water needs.
However, heat pump domestic water heaters are highly efficient and perfect for eco-conscious homeowners and businesses to reduce carbon emissions when they produce hot water with our energy-efficient water heaters.
Air source heat pump hybrid hot water heater
Our 400 litre hot water heat pump storage tank is fitted with an additional heating coil as standard, which can be used as an additional heat source for the hot water system, such as a conventional boiler, or with other renewable energy sources such as solar thermal systems.
As a result, they may be referred to as an “air source heat pump hybrid hot water heater” or a “hybrid hot water system” that is heated by two energy sources in a space-saving solution while producing reliable hot water for the home or property.
By special request, we can produce a twin-coil version of this water heater (e.g., one to be supplied by a gas or oil boiler and the other supplied by solar thermal), but these are manufactured to order and may require a minimum order quantity.
Product detail of our 400 Litre heat pump hot water heater
Dimensions of the 400-litre floor-mounted DHW air source heat pump water heater, which is designated the Aquaflex2 – 400DC
Height (E): 1,975 mm
Diametre (A): 726 mm
Empty weight: 115 kg
Electrical supply: 240 volt single phase / Amps required – 6.2 amps for the heat pump + 6.8 amps for the immersion heater, giving a total maximum draw of 13 amps.
Please note that the above data is based on the Europe Average Climate Zone, as defined by the Europe EN16147 regulation for testing domestic hot water heat pumps. This regulation involves an ambient air temperature of 7℃ and heating the water from 10℃ to 53℃. Stage C from EN16147 refers to silent heating mode.
The full details are in the following brochure – Aquaflex 2 400 and 500 litre air source heap pump water heaters brochure
ASHP DHW heater product features
- “A+” energy efficiency rating at 20°C ambient air temperature and “A” at 7°C;
- Factory-sealed with R134A refrigerant;
- A high-performance rotary compressor mounted on anti-vibration pads, a DC variable-speed fan, and a soundproof hood all contribute to the extremely silent operation.
- High-efficiency DC fan motor for reduced electrical consumption and quieter operation;
- Premium 2205 duplex stainless steel hot water storage tank with insulation, making it suitable for both soft and hard water areas.
- Provides useful cooling and dehumidification of the air while recovering waste heat for domestic hot water production;
- Built-in Wi-Fi with app control via the Apple App Store and Google Play;
- Magnesium anode for additional corrosion protection;
- External aluminium micro-channel condenser wrapped around the storage tank, helping to prevent scaling within the refrigerant heat exchanger and eliminating direct refrigerant-to-water contact;
- Integrated 1.6 kW electric immersion heater to provide supplementary hot water heating when required, including during low ambient temperature conditions;
- Integrated control panel with LCD touchscreen display on the 100-litre capacity water cylinder model;
- Programmable weekly anti-Legionella sanitisation cycle;
- Fixed or dynamic domestic hot water (DHW) setpoint control for improved energy management;
- External ON/OFF contact switch for solar PV integration, allowing photovoltaic electricity to be used for domestic hot water production for the property or location
- Modbus communication compatible
The Modbus communication capability allows the Aquaflex air source heat pump water heater to communicate with compatible external control and monitoring systems. This is particularly useful in commercial buildings, plant rooms and building management system (BMS) applications, where the hot water system may need to form part of a wider building controls strategy.
If you require a larger unit, then the next size-up unit is our 500 Litre Air source heat pump water heater.
Solar PV electricity compatible
Our units are SG Ready (Smart Grid Ready) as standard at no extra cost, meaning our heat pump water heater has an interface that lets an external energy management system, smart electricity tariff, or solar PV system control its operation, so electricity consumption can be shifted to favourable periods and surplus energy can be stored as domestic hot water.
Air duct installation options
The most common setup is to duct in ambient outside air and duct out exhaust air from and to the heat pump domestic hot water heater.
The intake air and exhaust air outlets are 160 mm in diameter.
The maximum air intake duct and exhaust air duct distance, including a 90-degree bend off the heat pump using rigid ducting, is a total of 15 metres for X and Y in the below diagram (i.e., 7.5 metres for the intake air and 7.5 metres for the exhaust air ducts); however, if using flexible ducting, this distance is reduced to 8 metres in total (flexible ducting uses internal support rings or wire helixes that disrupt airflow and create more pressure drop than smooth rigid ducting).
Air exhaust options
The exhaust air from a heat pump water heater (HPWH) is cool and dry, which provides a built-in dehumidification and cooling effect ideal for a laundry room or a wine cellar.
or a two-room setup
Heat pump technology
Condenser technology for our hot water heat pump cylinders
Our full range of the “Aquaflex 2” series of DHW heat pumps is equipped with a highly efficient type of condenser that encases the inner stainless steel hot water tank of the models. The condenser is made of aluminium and features flat micro-channels, which increase the contact area between the refrigerant and the tank as the gas flows.
With more heating surface area, more heat is transferred into the hot water tank. This results in faster heating of the domestic hot water, reduces the unit’s running time, and ensures freedom from scaling and gas/water contamination.
Aluminium micro-channel alloy is a very mature technology that has been used in heat pumps and air conditioners for many years.
Hot water cylinder with integrated air source heat pump
Heat pumps can lower electricity consumption by 75% compared to electric heaters. Our range also benefits from a precisely balanced refrigerant cycle due to an electronically commutated motor and an electronic expansion valve, resulting in 75% lower electricity consumption.
Very quiet operation: Noise levels for heat pumps can be similar to a refrigerator; the sound power and sound pressure at 1 metre are 58/47 dB(A).
A 400-litre hot water cylinder can yield approximately 833 litres of hot water at 40°C from a 10°C incoming water temperature and suit new or retrofit hot water systems from houses to commercial water heating applications.
Heat pump water heaters normally need to connect to the outside air via vents or exhaust ducting to produce hot water, as they extract heat from the external air supplied.
Product detail
Contact Flexiheat UK to find out further information
Should you require further assistance, information, check on delivery times, have questions on the product, or wish to purchase or order one of our highly efficient 400 litre air source heat pump domestic hot water heaters, which are an environmentally friendly product and, when powered by renewable energy sources, result in lower CO₂ emissions, with a premium duplex stainless steel tank, then please contact our UK customer sales team on 01202 822221 or click here for our email contact form to send an email.