Split Arctic with integrated DHW tank

Tank volume, household size and reheat time

Hot water demand is driven by habits far more than by floor area. A shower typically draws 40 to 60 litres of stored water at cylinder temperature, a bath considerably more, and a heat pump cylinder is normally held around 48 to 55 °C rather than boiler temperatures, so blended usable volume is closer to the nominal figure than with a very hot tank.

• 190 l suits roughly two to four people with showers, one bathroom and a normal morning pattern.
• 240 l suits four or more people, two bathrooms, a bath in regular use, or households where several draw-offs land in the same half hour.
• Count simultaneity, not just people. Four people spread across two hours is a lighter load than three people in twenty minutes.
• Do not oversize blindly. A larger cylinder means more standing volume to keep warm and a longer legionella cycle, so pick the smallest tank that covers the real peak.

The internal heat exchanger is what limits how quickly the heat pump can put heat back into the water. Because the machine delivers water at perhaps 50 °C, the temperature difference across the coil is small, and a small difference needs a large surface area to move the same kW. This is precisely why cylinders designed for gas boilers underperform on a heat pump even when their volume is ample.

• Compare the reheat time, not just the litres. The datasheet figure for heating the tank from cold to setpoint is the number that maps to real comfort.
• Check the continuous draw-off rate if the household runs long showers, because that is where the coil either keeps up or does not.
• Stratification matters. A tall tank with a low coil keeps hot water at the top and cool water at the bottom, which raises usable volume and keeps the condensing temperature down.
• Sensor position sets the behaviour. A sensor high in the tank restarts reheating early and often, a sensor lower down allows deeper discharge and fewer, longer cycles.

Footprint compared with a separate cylinder

The whole point of the integrated unit is plant space. A footprint of about 0.36 m² means the appliance occupies roughly the area of a washing machine, against a wall-hung module plus a floor-standing tank plus the interconnecting pipework, valves and insulation that a split installation would otherwise need.

• Fewer joints, fewer leaks. The primary circuit between condenser and coil is factory made and factory insulated instead of site built.
• Faster commissioning. The diverter valve, tank sensor and auxiliary heater are pre-wired to the controller, so there is no field wiring of the DHW logic.
• Plan the height and the door. A tall slim unit needs headroom for the top connections and a route into the plant room. Measure the doorway before delivery.
• Keep service access. Front clearance for the hydraulic components and the element flange is not optional, even though the footprint is small.

Legionella cycle, backup element and priority switching

A heat pump alone rarely reaches disinfection temperature efficiently, so the integrated auxiliary electric heater lifts the cylinder above 60 °C on a schedule, usually weekly, to control legionella. That is normal, short and unavoidable; what is not normal is the element running to make everyday hot water.

• Run the legionella cycle weekly at a quiet hour, and check the log occasionally to confirm it completed rather than timing out.
• DHW priority pauses space heating while the cylinder reheats. In a well-insulated house the room temperature barely moves in the 30 to 60 minutes involved.
• Cap the priority time. Most controllers allow a maximum DHW run time and a minimum heating interval, which prevents a stubborn cylinder from starving the house on a cold morning.
• Watch the element hours. Frequent auxiliary heating outside the disinfection cycle points to a setpoint that is too high, a failing sensor or a scaled coil, not to a faulty compressor.
• Fit a thermostatic mixing valve. Storing at 60 °C for disinfection and delivering at 60 °C to a tap are not the same thing.

Standby losses, insulation and when a separate cylinder is better

A stored cylinder loses heat continuously. Manufacturers declare that as a standing loss in W or kWh per 24 hours and as an energy efficiency class for the storage function, and over a year those watts add up to a meaningful share of the hot water bill. Insulation thickness, the quality of the connection sleeves and whether the pipe tails are insulated all matter.

• Compare the declared standing loss between the 190 l and 240 l variants before defaulting to the larger tank.
• Insulate the tails. Uninsulated horizontal connections drive convective loss out of the top of any cylinder.
• A separate cylinder wins when demand exceeds what a 240 l tank covers, when a solar thermal or solid fuel coil has to be accommodated, when the tank must sit in a different room from the hydraulics, or when a serviceable cylinder is already installed.
• It also wins on replacement. A standalone tank can be changed without touching the heat pump. In that case choose Split Arctic and specify the cylinder separately.

Pairing with photovoltaic surplus

A hot water cylinder is the cheapest thermal battery in the house, and an integrated tank makes the control loop short. With a photovoltaic array and export detection, the system raises the DHW setpoint while surplus is available, so several kWh of midday generation land in the tank instead of going to the grid at export rates. Lifting 240 l from 45 to 55 °C absorbs roughly 2.8 kWh of heat for well under 1 kWh of electricity, at a fraction of the cost of equivalent battery capacity. Prefer raising the setpoint with the compressor rather than the auxiliary element, because the element is a blunt resistive load with no COP multiplier.

Key parameters compared

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