A Modern Take on Hot Water Systems

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:

1. 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.

2. 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

Feature	Conventional Pressurised System	New Non-Pressurised Design
Tank/Element Exposure	Continuous flow of mineral-rich water	Static, isolated water volume
Scaling Risk	High (affects element and tank)	Minimised (contained to heat exchanger)
Corrosion Protection	Sacrificial anode (needs monitoring)	Greatly reduced internal corrosion
Component Replacement	Entire unit often needs replacing	Targeted, simple heat exchanger service
Installation	Requires PRV and overflow drain	Simplified; no PRV/drain needed
Lifespan	Limited by tank/anode corrosion	Significantly extended

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.

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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: $0.6\text{m}\times 0.6\text{m}\times 0.85\text{m}=0.306{\text{m}}^3$ (306 Litres).

• Insulated Tank Dimensions: Allowing 50mm of insulation on all sides, the internal tank volume is approximately $0.5\text{m}\times 0.5\text{m}\times 0.75\text{m}=187.5$ 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 ($T_s$): 75°C (The upper safe limit for many non-pressurised setups).

• Cold Water Inlet ($T_i$): 15°C (Average ambient temperature).

• Specific Heat of Water ($c$): 4.18 kJ/kg·°C.

Total Thermal Energy Stored ($Q$):

$$Q=\text{Mass}\times c\times (T_s-T_i)$$

$$Q=175\text{kg}\times 4.18\times (75-15)=43,890\text{ kJ}$$

$$43,890\text{ kJ}/3,600=12.19\text{ kWh}$$

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: $18.8\text{ kW}-2.4\text{ kW}=16.4\text{ kW}$ (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 $175\times 4.18\times (75-48)=19,750\text{ kJ}$ of "usable" energy.

• Duration of Hot Water: $19,750\text{ kJ}/16.4\text{ kJ/s}=1,204\text{ seconds}$ (~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

Feature	Conventional Dux 160L	Proposed "Appliance" Design
Footprint	Circular, 530mm diameter	Square, 600mm x 600mm
Total Height	1315 mm	850 mm (Under-bench height)
Installation	Licensed plumber & hard-wired	"Plug & Play" (Socket & Hose)
Location	Laundry/Outdoor cupboard	Kitchen, Laundry, or Under-bench
Energy Stored	~11.1 kWh (at 75°C)	~12.2 kWh (at 75°C)

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.

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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.

Component	Standard 160L Pressurised (Dux)	New 160L (Delivered) PCM Appliance
Main Material	Heavy Steel Pressure Vessel	Light Plastic or Thin Steel Tank
Infill (PCM + Water)	160 kg (Water only)	62.25 kg (SAT + Water)
Empty Weight	59 kg	~34 kg (Cabinet + Tank + Pump)
Total Operational Weight	219 kg	~96.25 kg

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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A challenge to heat and cool buildings efficiently

The efficiency with which we can heat and cool rooms requires a good solution for maintaining low carbon dioxide levels in well-insulated rooms.

Exchanging large volumes of air from inside to outside when the outside air temperature is very cold or hot could be a very costly way to remove small amounts of carbon dioxide.

 

An adult uses about 500 litres of oxygen while breathing about 10,000 litres of air in and out per day.  

See "Oxygen a person uses each day."

A carbon dioxide concentration of 1,400 ppm reduces decision-making ability by 25%. 

See "Rising carbon dioxide ...may directly harm our ability to think."

How much does the CO₂ concentration increase in one day, depending on the total volume of air in a sealed room, with one person occupying it? 

A room that is 4 metres long by 4 metres wide and 2.5 metres high has a volume of 40 cubic metres. 

It contains 40,000 litres of air of which about 32,000 litres (80 percent) are nitrogen and about 8,000 litres (20 percent) are oxygen. 

One person will use about 500 litres of oxygen in a day. So the 8,000 litres of oxygen in this room is ample for one day. However, the small amount of carbon dioxide the person exhales raises the concentration of carbon dioxide so that quite soon it will interfere with the person's ability to think clearly if it is not removed.

One person sleeping or working in this room alone for 8 hours would raise the carbon dioxide concentration to over 4,000 ppm if there was no exchange of air from outside, and no method to remove the carbon dioxide being added. 

The relatively small amount of carbon dioxide - increasing by 21 litres per hour - requires a large volume of air in the room - 40,000 litres - to be either exchanged or otherwise processed each hour to maintain the carbon dioxide concentration at a constant level.

Classrooms with high carbon dioxide levels may be lowering education outcomes for children because their ability to concentrate is being reduced.

Hour	Air Breathed (L)	N2 (L)	O2 in (L)	O2 out (L)	CO2 out (L)	CO2 conc (ppm)
0	-	-	-	-	-	-
1	417	333	83	63	21	521
2	833	667	167	125	42	1,042
3	1,250	1,000	250	188	63	1,563
4	1,667	1,333	333	250	83	2,083
5	2,083	1,667	417	313	104	2,604
6	2,500	2,000	500	375	125	3,125
7	2,917	2,333	583	438	146	3,646
8	3,333	2,667	667	500	167	4,167
9	3,750	3,000	750	563	188	4,688
10	4,167	3,333	833	625	208	5,208
11	4,583	3,667	917	688	229	5,729
12	5,000	4,000	1,000	750	250	6,250
13	5,417	4,333	1,083	813	271	6,771
14	5,833	4,667	1,167	875	292	7,292
15	6,250	5,000	1,250	938	313	7,813
16	6,667	5,333	1,333	1,000	333	8,333
17	7,083	5,667	1,417	1,063	354	8,854
18	7,500	6,000	1,500	1,125	375	9,375
19	7,917	6,333	1,583	1,188	396	9,896
20	8,333	6,667	1,667	1,250	417	10,417
21	8,750	7,000	1,750	1,313	438	10,938
22	9,167	7,333	1,833	1,375	458	11,458
23	9,583	7,667	1,917	1,438	479	11,979
24	10,000	8,000	2,000	1,500	500	12,500

You can add mechanical ventilation to your home with a heat recovery ventilator (HRV) or energy recovery ventilator (ERV). Both exhaust air from the house and bring in outdoor air to replace it. There's a heat exchanger to bring the incoming air to a better temperature.

5/11 pic.twitter.com/geOQQnTecQ

— Joey Fox, P. Eng, M.A.Sc (@joeyfox85) July 20, 2022

Below are extracts from two articles - the first on symptoms in people in rooms with elevated levels of carbon dioxide, the second on symptoms of people in hotel quarantine. 

The similarities may indicate that people in hotel quarantine were being affected by elevated carbon dioxide levels, and not the fact that they were in quarantine. This could be evaluated by comparing the group in hotel quarantine with the experiences of those in quarantine at Howard Springs with self-contained units and access to fresh air.

Rising carbon dioxide ... may directly harm our ability to think

Extract from article published in ScienceDaily:

"It's amazing how high CO2 levels get in enclosed spaces," said Kris Karnauskas, CIRES Fellow, associate professor at CU Boulder and lead author of the new study published today in the AGU journal GeoHealth. "It affects everybody -- from little kids packed into classrooms to scientists, business people and decision makers to regular folks in their houses and apartments."

Shelly Miller, professor in CU Boulder's school of engineering and coauthor adds that "building ventilation typically modulates CO2 levels in buildings, but there are situations when there are too many people and not enough fresh air to dilute the CO2." CO2 can also build up in poorly ventilated spaces over longer periods of time, such as overnight while sleeping in bedrooms, she said.

Put simply, when we breathe air with high CO2 levels, the CO2 levels in our blood rise, reducing the amount of oxygen that reaches our brains. Studies show that this can increase sleepiness and anxiety, and impair cognitive function.

They found that ... indoor concentrations ... of 1400 ppm to be harmful.

"At this level, some studies have demonstrated compelling evidence for significant cognitive impairment," said Anna Schapiro, assistant professor of psychology at the University of Pennsylvania and a coauthor on the study. "Though the literature contains some conflicting findings and much more research is needed, it appears that high level cognitive domains like decision-making and planning are especially susceptible to increasing CO2 concentrations."

In fact, at 1400 ppm, CO2 concentrations may cut our basic decision-making ability by 25 percent, and complex strategic thinking by around 50 percent, the authors found.

Hotel Quarantine And Mental Health

Extract from article by Giulia Fiore, psychologist and founder of Confidence to Achieve:

Hotel quarantine is a costly procedure that necessitates a highly specialised workforce to sustain the system, which includes clinical, welfare, and security services, in order to minimise risk and meet the duty of care obligations. The effects of hotel quarantine on mental health and wellness are arguably one of the most important factors in the hotel quarantine scheme, as even those who have never encountered mental illness may find the experience taxing.

Isolation and quarantine have been shown to have negative mental health effects, including depression, anxiety, stress-related disorders, anxiety and anger. In a rapid study, Brooks et al. found that people who were quarantined have more negative psychological effects, such as post-traumatic stress symptoms, confusion, and frustration. Furthermore, citizens in a state of confinement can experience restraint and express fixation on the disease’s progress, as well as psychosomatic symptoms such as insomnia.