Compact thermal storage for domestic hot water

Author: C Dunstan
Research: Google Gemini
Date: February 28, 2026

I said

I would like help to design a compact thermal energy storage cabinet... Initially the "fully heated" heat store has a uniform temperature of 60 degrees centigrade. Assume the maximum flow rate of water to be heated by the heat store is the expected amount for a shower. For calculation, assume at first a shower head has a flow rate of 15 litres per minute, and 10 litres of this is hot water. The other 5 litres are cold water that mixes with the hot water to moderate the temperature.

An external Brazed Plate Heat Exchanger (BPHE) with a circulation pump is a highly effective "active" option. In the world of thermal storage, this is often called an External Heat Interface Unit (HIU).

This configuration is generally superior to the submerged copper tube bundle because BPHEs are designed for extremely high turbulence and surface area in a tiny footprint.

---

1. Viability of the $800\text{ L/h}$ Pump

To determine if an $800\text{ L/h}$ pump is sufficient, we look at the Energy Balance. We need to transfer $26\text{ kW}$ of heat from the store water loop to the shower water loop.

$$Q={\dot{m}}_{store}\cdot C_p\cdot (T_{hot}-T_{return})$$

Using your parameters:

• Shower Demand: $600\text{ L/h}$ ($10\text{ L/min}$) needing to rise from $15^{\circ }\text{C}$ to $52.5^{\circ }\text{C}$.

• Store Flow (${\dot{m}}_{store}$): $800\text{ L/h}$ ($13.3\text{ L/min}$) starting at $60^{\circ }\text{C}$.

Calculating the Return Temperature ($T_{return}$): If the shower takes $26\text{ kW}$, the $800\text{ L/h}$ of store water will drop by:

$$\Delta T=\frac{26,000\text{ W}}{(800/3600\text{ kg/s})\cdot 4180\text{ J/kgK}}\approx 28^{\circ }\text{C}$$

• Return Temperature: $60^{\circ }\text{C}-28^{\circ }\text{C}=32^{\circ }\text{C}$.

Verdict: This is very viable. A return temperature of $32^{\circ }\text{C}$ is well above the cold inlet of $15^{\circ }\text{C}$, meaning the heat exchanger has a healthy "temperature buffer" to work with.

---

2. Key Considerations for this Setup

BPHE Sizing (The "Camping" Warning)

The Scintex units you mentioned are often designed for vehicle engine cooling loops (which have very high flow rates and temperatures). For a domestic-style shower at $10\text{ L/min}$, you need to ensure the BPHE is rated for at least $30\text{ kW}$.

• A 30-plate or 50-plate unit (approx. $190\text{mm}\times 75\text{mm}$) is usually the sweet spot.

• If the BPHE is too small, you won't reach $52.5^{\circ }\text{C}$ even if the pump is fast enough, because the "approach temperature" will be too wide.

Pump Head Pressure

Plate heat exchangers have narrow channels that create significant fluid resistance.

• Cheap $12\text{V}$ "solar" pumps often advertise $800\text{ L/h}$ at zero head.

• Once you connect it to a BPHE and $1.5\text{m}$ of pipework, that flow might drop to $300\text{ L/h}$.

• Ensure your pump has a "max head" of at least $3\text{ to }5\text{ meters}$ to maintain that $800\text{ L/h}$ flow through the plates.

Stratification & The "Later Stage"

Returning $32^{\circ }\text{C}$ water to the bottom of the tank is a double-edged sword:

1. Initial Stage: If the bottom is $15^{\circ }\text{C}$, the $32^{\circ }\text{C}$ return water is "hotter" than the surroundings and will want to rise, potentially mixing the tank and breaking your stratification.

2. Later Stage: If the bottom is already cold, the $32^{\circ }\text{C}$ return water effectively "pre-heats" the bottom half. This is actually helpful, but it means your "cold" half won't stay $15^{\circ }\text{C}$ for long.

---

3. Comparison: Submerged Tubes vs. BPHE + Pump

Feature	Submerged Copper Tubes	BPHE + Pump (Active)
Complexity	Low (No moving parts)	Medium (Pump, sensors, power)
Efficiency	Medium (Laminar flow)	High (Turbulent plate flow)
Space	Takes up tank volume	External (Saves tank volume)
Maintenance	None	Pump may fail; BPHE can clog
Performance	Drops as tank cools	Consistent until tank hits $\approx 55^{\circ }\text{C}$

Summary Recommendation

The BPHE + Pump is the modern way to build a thermal store. It allows you to use a much smaller tank because you can extract almost all the heat effectively.

One final tip: Ensure the pump is installed on the return (cooler) side of the BPHE if possible, or ensure it is rated for $60^{\circ }\text{C}+$ continuous operation.

Would you like me to help you check the specifications of a specific pump or BPHE model you are considering to see if they match these head-pressure requirements?

I said

Before that, I would like to know the quantity of a Phase Change Material (PCM) suitable for the compact heat store. The PCM would be to supply the same amount of thermal energy that would be stored in a 250 litre electric hot water system

To replace a standard 250-litre electric hot water system, you need to store roughly $13\text{ kWh}$ ($47,000\text{ kJ}$) of thermal energy. This assumes the water is heated from a cold inlet of $15^{\circ }\text{C}$ to a standard storage temperature of $60^{\circ }\text{C}$.

For a compact heat store, the most suitable PCM is Sodium Acetate Trihydrate (SAT), often called "hot ice." It is preferred over paraffin wax for domestic hot water because it has a higher energy density and a melting point of $58^{\circ }\text{C}$, which is almost perfect for hot water delivery without requiring extreme charging temperatures.

1. Quantity Required to Match 250L of Water

To achieve the same $13\text{ kWh}$ of storage using a PCM-based system (operating between $40^{\circ }\text{C}$ and $70^{\circ }\text{C}$ to ensure a complete phase change), the requirements are:

Material	Required Mass	Required Volume	Size Reduction
Water	$250\text{ kg}$	$250\text{ L}$	0% (Baseline)
Sodium Acetate (SAT)	$\approx 145\text{ kg}$	$\approx 110\text{ L}$	\approx 55\text{%} smaller
Paraffin Wax	$\approx 185\text{ kg}$	$\approx 225\text{ L}$	\approx 10\text{%} smaller

Note: While SAT is significantly heavier than water, it is nearly twice as energy-dense by volume. This allows you to build a heat store that is roughly half the size of a traditional water tank while delivering the same amount of hot water.

---

2. Why SAT is the Standard Choice

• High Latent Heat: It stores about $250\text{ kJ/kg}$ just during the phase change at $58^{\circ }\text{C}$.

• Thermal Conductivity: It has better thermal conductivity than paraffin, meaning it can charge and discharge faster (crucial for your $10\text{ L/min}$ flow requirement).

• Safety: It is non-flammable and non-toxic (it's actually used as a food additive in salt-and-vinegar chips).

---

3. Practical Design Implications

If you move forward with a PCM store using the Brazed Plate Heat Exchanger (BPHE) and pump configuration we discussed, there are two critical factors for the PCM tank:

1. Encapsulation: PCMs like SAT are usually sealed in thin tubes or flat plates (macro-encapsulation). The "heat store" water (the $800\text{ L/h}$ loop) flows around these containers to pick up the heat.

2. Expansion Room: PCMs expand and contract during phase changes (SAT expands by about 10\text{%} when melting). Your tank must have an internal expansion volume or a flexible membrane to prevent the tank from splitting.

Summary Recommendation

To match a 250L tank, aim for a $120\text{ to }130\text{ litre}$ internal tank volume filled with approximately $150\text{ kg}$ of SAT modules. This leaves enough room for the circulating "heat transfer fluid" (water) and the necessary expansion gaps.

Would you like me to calculate the specific dimensions for a tank to house $150\text{ kg}$ of SAT, including the necessary spacing for water flow?

I said

Yes. The dimensions for the storage tank are a central requirement.

In particular, the problem I am trying to solve is to fit the storage tank in an apartment that has limited space available. To place a Hot Water Tank outside on a balcony is very expensive - needing large parts of the ceiling to be removed to install pipes, and then to repair and repaint the ceiling.

On the compact heat store, one option is to make it tall and thin, and the same width as kitchen cupboards. There is space near the front door, next to the hot and cold water supply pipes where it could be installed without the need for long lengths of pipes to be installed in the ceiling.

Designing a thermal store for an apartment requires balancing energy density with aesthetic integration. To fit within a standard kitchen cupboard footprint while providing the energy of a 250L water tank, a "Slimline PCM Tower" is the most efficient solution.

Here are the calculated dimensions and design specifications for your compact unit.

1. The "Slimline Tower" Dimensions

A standard kitchen cabinet is 600mm wide and 600mm deep. However, for a hallway or entry-way placement, a "Half-Depth" or "Slim" profile (300mm deep) is often preferred to keep the walkway clear.

Target Capacity: 150 kg PCM + ~35 L circulating water + insulation.

Dimension	Measurement	Notes
Outer Width	600 mm	Matches standard cupboard width.
Outer Depth	300 mm	"Slimline" profile for hallways/tight spaces.
Outer Height	1,650 mm	Approx. head height; fits under standard 2.1m ceilings.
Insulation Thickness	40 mm	High-efficiency PIR foam or Vacuum Panels (VIP).
Internal Volume	160 Litres	Total usable space for PCM and water.

2. Internal Volume Breakdown

To match a 250L electric tank (~13 kWh), the internal 160L volume is utilized as follows:

• PCM (Sodium Acetate): 115 Litres (~150 kg).

• Heat Transfer Fluid (Water): 35 Litres. This is the water the pump moves through the BPHE.

• Expansion/Air Gap: 10 Litres. Crucial for the $10\%$ expansion of PCM during melting.

3. Internal Design for High Flow ($800\text{ L/h}$)

Because you are using an external pump and BPHE, the internal structure must allow the $800\text{ L/h}$ flow to touch as much PCM surface area as possible.

• Macro-encapsulation: Instead of one giant block of PCM, the 150kg should be stored in thin HDPE plastic containers (about 20mm thick) stacked vertically with 5mm gaps between them.

• The "Radiator" Effect: The pump pulls water from the very top (hottest) and returns it to the very bottom. As the water rises through the 5mm gaps between the PCM plates, it "wipes" the heat off the PCM very efficiently.

4. Integration of BPHE and Pump

Since you want this to look like a cupboard, you have two options for the "Plumbing Rig" (the Scintex BPHE and the pump):

• Option A (Top-Mounted): The height of the tank is reduced to 1,400mm, and the BPHE, pump, and control valves are mounted on top, hidden by a decorative cabinet header. This makes maintenance very easy.

• Option B (Side-Integrated): The tank is made slightly narrower (500mm wide), leaving a 100mm "plumbing lane" inside the 600mm cupboard shell for the pump and pipes.

5. Apartment-Specific Engineering Tips

1. Weight Loading: 150kg of PCM + 35kg water + 40kg tank/cabinet = ~225 kg. This is roughly the weight of three adults standing in a small circle. Most modern apartment floors can handle this easily (it's less than a full 250L water tank, which would weigh ~300kg).

2. Vacuum Insulation Panels (VIP): If space is extremely tight, use 15mm VIPs instead of 40mm PIR foam. This could reduce the outer depth from 300mm to 250mm, making it almost flush with a door frame.

3. Silent Mounting: Since it's near the front door, use rubber vibration isolators for the $800\text{ L/h}$ pump. In a quiet apartment, a pump bolted directly to a wooden cabinet can act like a speaker and hum throughout the home.

Summary Specification for a Fabricator:

• Tank Material: Polypropylene or Stainless Steel.

• External Shell: White powder-coated steel or furniture-grade MDF.

• Footprint: $600\text{mm}\times 300\text{mm}$.

• Total Energy Density: $\approx 110\text{ Wh/L}$ (compared to $\approx 50\text{ Wh/L}$ for a water tank).

Would you like me to create a simple "plumbing schematic" to show how the Scintex BPHE and pump should be piped to the hot/cold supply lines near your door?

I said

Yes please.

To keep your apartment quiet and your system reliable, the "electrical brain" needs to be simple but robust. Most flow switches are delicate—if you run the full power of a pump directly through them, the internal magnets can weld shut over time.

Here is how to wire it properly and which pump to choose.

---

1. The Wiring: Using a Relay for Longevity

Instead of the flow switch "powering" the pump, it should "tell" a Relay to power the pump. This protects the switch from the electrical surge (inductive load) of the pump motor.

The Component List:

• Flow Switch: $1/2"$ BSP Magnetic Reed Switch.

• Relay: A "Solid State Relay" (SSR) or a standard 12V DC Relay (if using a DC pump). For an AC pump, use a 240V DIN-rail relay.

• Power: A small 12V transformer (like a LED driver) if you are using a DC control circuit.

Wiring Logic:

1. Trigger Circuit: Connect the 12V (+) from your power supply to one wire of the Flow Switch. Connect the other wire of the switch to the Relay Input (+). Connect the Relay Input (-) back to the power supply (-).

2. Power Circuit: Run your Main Power (240V or 12V depending on the pump) through the Switching side of the Relay.

3. Result: When you turn on the shower, the flow switch closes, the relay "clicks" (or silently activates if it's an SSR), and the pump gets full power.

---

2. The Pump Recommendation: "Library Quiet"

Since the unit is inside your apartment near a hallway, a standard industrial pump will hum through the floorboards. You need a Wet Rotor circulation pump.

Top Pick: Grundfos COMFORT (UP 15-14 B PM)

This is the gold standard for domestic hot water.

• Noise Level: Virtually silent. You usually have to touch it to feel if it’s running.

• Heat Rating: Designed for continuous $95^{\circ }\text{C}$ water.

• Low Power: It uses high-efficiency permanent magnet technology (only about $5\text{ to }8\text{ Watts}$), so it won't spike your power bill.

• Vibration: Extremely low internal mass movement.

Budget/DIY Pick: 12V Brushless DC "Solar" Pump (e.g., Topsflo TS5)

• Noise: Very quiet "whirring" sound.

• Safety: Running 12V near water is inherently safer for a DIY build.

• Note: Ensure you get the "High Temp" version (often with a stainless steel or Ryton head) to handle the $60^{\circ }\text{C}$ store water.

---

3. How to Kill the Vibration

Even the quietest pump can vibrate the cabinet walls like a drum.

1. Flexible Hoses: Do not connect the pump to the tank with rigid copper pipe. Use braided stainless steel flexible hoses. These act as "shock absorbers" for the fluid pulses.

2. Rubber Mounts: Mount the pump (or the board it's attached to) on anti-vibration rubber bobbins.

3. Acoustic Foam: Line the inside of your 300mm-deep cabinet with egg-carton foam. It’s cheap and will soak up the high-frequency "whine" of the pump.

---

One final "Pro Tip" for Apartments:

Since you are installing this near the front door, add a Leak Detector. You can buy a simple $20 sensor that sits on the floor inside the cabinet. If a fitting ever drips, it will sound an alarm (or you can wire it to a solenoid to shut off the main water supply automatically).

Would you like me to draft a "Shopping List" of these specific components with their estimated costs so you can check your budget?