MPi-325 Heat Pump: Key Attributes

Key Features

A heat pump water heater consumes less energy than an electric water heater - up to 67%^ less. The water heater's evaporator absorbs heat from the surrounding air and transfers this heat into the water.

Even on cloudy or cold days, heat is drawn from the surrounding air. The heat pump will operate when the ambient air temperature is between a minimum of 3°C to 5°C and maximum of 45°C to 55°C. The booster heating unit will operate when the ambient air temperature is outside of this temperature range if heating is required. The efficiency of the water heater increases as the ambient air temperature increases within the operating range of the heat pump.

The 551325 heat pump water heater has a vitreous enamel lined steel cylinder and sacrificial anode for protection against corrosion. It also qualifies for government environmental incentives.

  •  No Solar Panels
  •  Frost protected
  •  Similar efficiency to traditional solar systems
  •  Works day and night, as they don’t require sunlight



* The suggested price is comprised of RRP less the applicable solar incentive and excluding installation. Solar incentive value applies to 551325 system in Zone 3.

^ Energy savings of up to 60% to 67% shown are based on Australian Government approved TRNSYS simulation modelling using a medium load. Savings and incentives will vary depending upon your location, type of Solahart system installed, orientation and inclination of the solar collectors, type of water heater being replaced, hot water consumption and fuel tariff. Maximum financial savings off your hot water bill are achievable when replacing an electric water heater on continuous tariff. 

† Warranty Details: 5 years cylinder replacement, 3 years labour on cylinder, 2 years on sealed system including labour, 1 year on all other parts and labour.

MPi-325 Specifications

Product Code 551325
Suitable for # of People (warm climate) 2 - 5
Height (A) 1631 mm
Width (B) 894 mm
Depth (C) 638 mm
Module Height (D) 1018 mm
Heat Pump Module  42 kg
Tank 88 kg
Storage Capacity  325 lt
Power Input 800 W
Refrigerant  R134a
Element Sizes 1.8, 2.4 kW or 3.6 kW
Boost Capacity 180 lt
Water Connections & Settings  
Inlet  ¾/20 RP
Outlet  ¾/20 RP
Tank Relief Valve Setting 1000 kPa
ECV Setting 850 kPa
Maximum Mains Pressure Settings  
With ECV 680 kPa
Without ECV 800 kPa
Min. Water Supply Pressure

200 kPa

How Heat Pump Water Heaters Work

The MPi325 Heat Pump delivers hot water by efficiently utilising a free and abundant source of energy - the heat that is locked in the air around us. Even on a relatively cold day, the air has some heat which is available to be transferred to a fluid. The secret is to make the fluid even colder than the air. You can see in the diagram at step 1, that liquid refrigerant begins at high pressure before being forced through an expansion valve (2). This sudden release of pressure cools the liquid to allow it to absorb heat from the surrounding air. This valve is designed to carefully regulate how much refrigerant passes through it.

On cold days, less refrigerant passes through; the smaller amount of fluid expands faster into the available space and cools to a lower temperature. On warmer days, more refrigerant is allowed to pass through - it doesn't need to cool down as much because the air has more available heat to transfer. As the cooled refrigerant passes up the coiled tubes, air is blown over the tubes to warm the refrigerant till it becomes a vapour (3). To make the heat pump more efficient, part of the air is actually drawn from around the motor. This helps cool the motor as well as reclaiming potentially wasted heat.

Next, the low pressure vapour needs to be compressed again to return it to its original pressure (4). The temperature has by now reached around 80ºC. From here the refrigerant is pumped to the heat exchanger where it passes down a series of narrow tubes (5). The cold water is pumped up an alternating set of tubes to allow it to absorb the heat. Each time water passes through the heat exchanger it emerges around 10-15ºC hotter. When all the water in the attached storage tank reaches its pre-set temperature, the heat pump switches itself off. 

Electric Boosting

Ice may begin to form on the evaporator reducing the heat pump efficiency, when the ambient air temperature falls below 7°C and the heat pump has been operating for an extended period. At ambient air temperatures below 3°C to 5°C, the water heater deactivates the heat pump operation and switches to the booster heating unit. During this period the evaporator will defrost if necessary.

At ambient air temperatures above 45°C to 55°C, the water heater deactivates the heat pump operation and switches to the booster heating unit. The water inside the tank will be heated to a temperature of 70°C by the electric booster heating unit if heating is required. The temperature setting of the thermostat controlling the booster heating unit is not adjustable and is set at 70°C.

Freeze Protection

The water heater has a freeze protection system. The freeze protection system will protect the water heater from damage, by preventing ice forming in the waterways of the water heater, in the event of freezing conditions occurring.

If the ambient air temperature falls below 1°C and the heat pump is not operating, the system will operate the circulator periodically. During this freeze protection cycle, the circulator will operate for thirty (30) seconds and then rest for fifteen (15) minutes, before the cycle is recommenced. Water is circulated from the storage tank through the heat pump circuit, to prevent freezing in the connecting pipe work and heat pump module. The green LED will flash a series of four (4) flashes whilst the circulator is operating.

Timer Control

A timer can be installed in the electrical circuit to the water heater. The timer must be weatherproof if it is installed outdoors. It may be desirable for the water heater not to operate between certain hours, such as during the peak period when connected to a Time of Use electricity supply due to a more expensive tariff rate applying. A timer will affect the operating times of both the heat pump circuit and the booster heating unit.


Why Solahart Solar Hot Water Systems: Download

MPi325 Heat Pump Hot Water System Brochure: Download

Installation Manuals

MPi325 Heat Pump Hot Water System Owner's Guide and Installation Manual: Download

Solar Incentive Forms

Small-scale Technology Certificate Assignment Form: Download