Author: C Dunstan
Research: Google Gemini
Date:April 29, 2026
A Modern Take on Hot Water: Non-Pressurised Electric Storage Systems
Traditional hot water systems have served us well for decades, but they are not without their design drawbacks and maintenance requirements. This article introduces an innovative electric hot water design that simplifies installation and maintenance, offering a significant evolution from conventional pressurised systems.
A traditional pressurised hot water system with 160 litre capacity weighs 60 kgs empty and requires a pressure relief valve and an overflow drain. The proposed design uses a sealed 160 litre tank in which water is not under pressure.
The delivery of pressurised hot water is instead achieved by using a stainless steel flat-plate heat exchanger and a circulation pump which circulates stored hot water when water flow occurs through the external pipes connecting to the heat exchanger.
The Problem with Pressure and Water Quality
In a conventional pressurised hot water system, the entire storage tank is designed to withstand the full pressure of the mains water supply. This high-pressure environment necessitates stringent safety features like Pressure Relief Valves (PRVs) and complex overflow drainage.
Beyond the pressure, these systems face a biological and chemical enemy: the constant influx of fresh, mineral-rich water. Conventional systems suffer from two major longevity-limiting issues:
Heating Element Scaling: The submerged heating element is constantly bathed in incoming water, leading to rapid buildup of calcium deposits (limescale). This acts as an insulator, reducing efficiency and eventually causing element failure.
Sacrificial Anode Depletion: To prevent the steel tank from corroding under pressure, a sacrificial magnesium anode is used. This anode slowly dissolves to protect the tank walls. Once it is consumed, the tank itself begins to corrode, inevitably leading to a premature total system replacement.
The Non-Pressurised Redesign: Safety and Longevity
The proposed design takes a fundamentally different approach. It uses a sealed, 160-litre storage tank where the water inside is not under pressure.
1. Enhanced Safety and Simplified Installation
By removing the pressure, the requirement for complex PRVs and dedicated high-volume overflow drains is removed. This reduces installation complexity and eliminates a major point of potential failure.
2. Isolation of Critical Components
The most revolutionary aspect of this design is the separation of the storage water from the incoming water supply. In this system, the water inside the tank is a static volume that remains largely unchanged. The heating element is submerged in this static, mineral-stable environment, meaning it is not subjected to the continuous flow of fresh, scale-forming minerals. Similarly, because the tank does not experience the continuous influx of oxygenated, ion-rich water, the corrosion process is significantly mitigated, potentially removing the need for sacrificial anodes entirely.
3. Targeted Maintenance
The incoming mains water is only exposed to one small, easily accessible component: the stainless steel flat-plate heat exchanger.
If calcium deposits occur, they are contained within the heat exchanger plates.
Instead of replacing an entire 60kg hot water system when the tank corrodes or the element fails, this design allows for the targeted maintenance or replacement of the heat exchanger alone. This turns a major capital expenditure into a minor, simple service task.
Delivering Pressurised Hot Water
This design separates the storage of heat from the delivery of pressurised water using a circulation pump and a heat exchanger. When a hot water tap is opened, the pump pulls stored hot water from the tank, passes it through the heat exchanger, and transfers that heat to the mains water instantaneously. The two water streams never mix, protecting the tank and element from the harsh realities of raw mains water.
Comparison Table: At a Glance
The Future of Hot Water
This new electric hot water design is a significant step forward, offering a safer, simpler, and more durable alternative to traditional pressurised systems. By isolating the most vulnerable components from the damaging effects of water chemistry, this system shifts the paradigm from "replace when failed" to "maintain for longevity." It provides reliable, high-pressure hot water while drastically lowering the long-term cost of ownership for homeowners.
Adopting a "white-goods" form factor (600mm x 600mm x 850mm) transforms the hot water system from a fixed utility into a modular appliance. This design offers significant advantages in spatial efficiency and installation flexibility.
1. Recalculated Storage Volume and Capacity
To calculate the usable volume, we must account for the outer cabinet, insulation (typically 50mm), and the internal space required for the heat exchanger, pump, and controller.
External Volume: (306 Litres).
Insulated Tank Dimensions: Allowing 50mm of insulation on all sides, the internal tank volume is approximately Litres.
Net Usable Storage Volume: After displacing volume for the heat exchanger and circulation pump, the system provides approximately 175 Litres of non-pressurised water storage.
2. Thermal Energy Storage Calculation
We assume the following parameters for a standard Australian environment:
Storage Temperature (): 75°C (The upper safe limit for many non-pressurised setups).
Cold Water Inlet (): 15°C (Average ambient temperature).
Specific Heat of Water (): 4.18 kJ/kg·°C.
Total Thermal Energy Stored ():
3. Continuous Delivery Capacity
The volume of hot water delivered depends on how much energy can be extracted before the stored water becomes too cool to heat the incoming mains water to a comfortable 45°C.
The "Shower Test" Scenario:
Delivery Temperature: 45°C.
Flow Rate: 9 Litres/minute (standard water-efficient showerhead).
Heating Element Input: 2.4 kW (Standard 10A wall plug).
While the water is being consumed, the 2.4 kW element is actively adding heat back into the 175L reservoir.
Energy Required for Flow: Heating 9 L/min from 15°C to 45°C requires 18.8 kW of power.
Net Energy Drain: (The rate at which the storage energy is depleted).
Effective Storage Range: Assuming the heat exchanger can maintain a 45°C output until the storage water drops to 48°C, we have of "usable" energy.
Duration of Hot Water: (~20 minutes).
Total Delivered Volume: 20 minutes at 9 L/min = 180 Litres of 45°C water.
4. Comparative Benefits: The Appliance vs. The Utility
5. Regulatory and Practical Considerations
DIY Installation: Because the unit uses standard flexible hoses (similar to a washing machine) and a standard 10A plug, the installation does not involve permanent modifications to the home’s electrical or pressurized plumbing infrastructure. This "appliance" status potentially allows owners to perform their own installations, saving $300–$600 in typical Australian plumbing and electrical fees.
Recovery Rate: A 2.4 kW element is slower to recover than the 3.6 kW or 4.8 kW elements found in hard-wired systems. After a 20-minute shower, the unit would take approximately 2 hours to return to its full 75°C storage temperature.
Maintenance: Should the heat exchanger suffer from calcium buildup, the owner could simply disconnect the two hoses, slide the unit out like a dishwasher, and replace the $179 heat exchanger themselves, avoiding the need to replace a $1,000+ pressurised tank.
To scale the appliance down while maintaining the standard "white-goods" exterior (600mm x 600mm x 850mm), the internal thermal storage tank is reduced to meet the specific delivery target of 160 litres of 45°C water.
1. Required Energy for 160 Litres
To deliver 160 litres of water at 45°C, starting from a 15°C mains inlet, the total thermal energy required is approximately 20,064 kJ.
During the ~18 minutes it takes to have a 160-litre shower (at 9 L/min), a 2.4 kW heating element adds approximately 2,560 kJ of energy back into the system. Therefore, the "usable" energy that must be stored in the Phase-Change Material (PCM) and its surrounding water is 17,504 kJ.
2. Tank Specifications and PCM Quantity
To provide this energy, the internal storage volume only needs to be approximately 50 litres. Using a mixture of 70% Sodium Acetate Trihydrate (SAT) and 30% water (as the heat transfer fluid), the breakdown is as follows:
Internal Tank Dimensions: ~380mm Wide x 380mm Deep x 350mm High.
SAT Volume: 35 Litres.
SAT Mass: ~47.25 kg.
Water (Heat Transfer Fluid): 15 Litres (15 kg).
Total Thermal Storage (15°C to 75°C): ~17,750 kJ of usable energy (meeting the 160L delivery target).
3. Appliance Weight Comparison
Reducing the storage volume significantly lowers the "wet weight" of the appliance, making it much more manageable for a DIY-style installation or for relocation within an apartment.
4. Design Benefits of the "Air-Gapped" Cabinet
By keeping the external cabinet at washing machine dimensions (600x600x850mm) while using a smaller 50L internal tank, the design gains several functional advantages:
Service Access: There is significant "empty space" inside the cabinet (over 200 litres of void space). This allows the heat exchanger, circulation pump, and electronics to be laid out for easy access. Repairs can be done by simply removing the top or side panel, similar to servicing a dishwasher.
Ultra-Insulation: The extra space allows for much thicker insulation (up to 100mm–150mm in certain areas) than a standard hot water heater. This can reduce standby heat loss to almost negligible levels, keeping the exterior of the appliance cool to the touch.
Floor Loading: At under 100 kg total, the unit exerts significantly less pressure on apartment floors than a full 160L or 250L conventional tank (which can exceed 300 kg). This removes any structural concerns for installation in older buildings or on upper-floor balconies.
5. Summary of the 160L Delivery Model
Form Factor: Standard front-loader size (850mm high).
Installation: Plug into a 10A socket; connect two flexible hoses.
Hot Water Output: 160 litres @ 45°C (Continuous).
Recovery Time: Approximately 2.5 hours to return to 75°C after a full discharge.
Maintenance: No anode to replace; stainless steel heat exchanger can be swapped or descaled by the owner if water quality is poor.
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