Unlimited Hot Water for Heat Pump Users: A Hybrid Buffer + Booster Approach

Introduction: How the Idea Began

The idea for this system actually started a few years ago when I was running a combi gas boiler. I had a small buffer tank with a plate heat exchanger acting as a preheater for the combi. One summer, my combi broke down, and I experimented with showers just using the buffer + plate heat exchanger.

To my surprise, it worked really well at first — I could take comfortable showers for about a month without any hot water from the boiler. However, as the weather cooled and mains water temperature dropped, I ran into limits: I couldn’t sustain showers longer than about 15 minutes before the water cooled too much.

That experience planted the seed: the buffer + heat exchanger concept worked, but without a backup heater, it wasn’t robust year-round. Today, that insight has evolved into my hybrid system: a compact buffer, a heat pump supplying most of the hot water energy, and a variable-output electric booster to fill the gaps — giving truly unlimited hot water even in winter.

How the System Works

The system consists of three key components:

1. Buffer Tank (≈100 L) – Stores heat from the heat pump and gives it time to ramp up.
2. Plate Heat Exchanger (PHE) – Transfers heat from the buffer to mains cold water, raising it partway to the desired temperature.
3. Variable-output Electric Booster – Stiebel Eltron DCE-X 6/8 Premium
   • Compact instantaneous heater that tops up water to the target outlet (e.g., 43 °C)
   • 4i Technology: automatically varies the flow rate to ensure the desired temperature is maintained, even if demand suddenly increases, the heat pump fails, or the mains water is unusually cold.
   • This is essential for safety and reliability, guaranteeing showers remain hot under varying conditions.

The heat pump provides the bulk of the thermal energy. The booster only heats the water enough to reach the target, maximizing efficiency while ensuring unlimited hot water. It also acts as a backup, so showers remain fully functional even if someone suddenly opens another tap.

Fixed Heat Pump Output Analysis

Assume:

• Heat pump output: 10 kW thermal
• Target mixer outlet: 43 °C (this feels hot enough for washing up the dishes)
• Mains water: 12 °C (winter) or 18 °C (summer)
• Flow rates: 6 L/min and 8 L/min (my shower is 6 L/min, my bath is 8 L/min)
• Heat pump COP: 3.0
• Booster COP: ≈ 1
• Thermal power formula:

P [kW]=0.06977⋅F[L/min]⋅ΔT[K]P[kW]=0.06977⋅F[L/min]⋅ΔT[K]

Where ΔT is the temperature rise provided by either the heat pump or booster.

Step 1 – PHE Outlet Temperature

ΔTHP=100.06977 F,TPHE,out=Tin+ΔTHPΔTHP​=0.06977F10​,TPHE,out​=Tin​+ΔTHP​

Step 2 – Booster Requirement

ΔTboost=max⁡(0,43−TPHE,out),Pboost=0.06977 F ΔTboostΔTboost​=max(0,43−TPHE,out​),Pboost​=0.06977FΔTboost​

Step 3 – Overall COP

PHP,el=10/3,Pboost,el=Pboost,COPoverall=10+Pboost3.33+PboostPHP,el​=10/3,Pboost,el​=Pboost​,COPoverall​=3.33+Pboost​10+Pboost​​

Results

Winter (Tin = 12 °C)

Summer (Tin = 18 °C)

Insights:

• Lower flow ⇒ higher PHE outlet temperature ⇒ booster does less, raising overall COP.
• In summer at 6 L/min, the booster is nearly idle; the HP alone almost reaches the target 43 °C.
• Flow thresholds exist where the HP alone can achieve 43 °C, making the booster unnecessary.

Buffer Size and Support Time

The buffer provides a key function: buying time for the heat pump to switch to hot water mode and ramp up to full power. How long it can support the shower depends on:

• Buffer storage temperature
• Minimum usable temperature before booster overwork
• Flow rate
• Total load vs. heat pump power

Minimum PHE outlet temperature constraint

In the winter, at 8 L/min, the PHE must supply at least 28.7 °C outlet temperature to keep the booster load ≤ 8 kW. Since there is typically about a 5 °C drop between buffer temperature and PHE outlet, that means the buffer must remain above 33.7 °C.

So, for buffer capacity calculations, we count support time only until the buffer cools from its initial storage temperature down to 33.7 °C.

Updated support times (winter, 8 L/min)

These are the additional minutes the PHE can sustain an outlet ≥28.7 °C before the buffer drops below 33.7 °C, at which point the booster would exceed 8 kW output.

My heat pump takes 3 mins to switch between heating and hot water modes, during which time it stops and restarts the compressor. It then takes a few mins for the flow to get up to temp, so I am looking at about 6 mins before it is delivering it’s full output. In theory the 80 litre buffer would be ok, but the 110 litre gives me some extra headroom. It also means that I have more water stored at a better COP than direct heating, so a bigger buffer means a more efficient and eco system.

You can also see from these numbers why a thermal store or a similar system without a booster would not work. There is no stratification, so the energy that can be delivered from the buffer is quite small, because I cannot let it drop to more than 34 degrees. If I tried to have a shower at that point without the buffer it would be quite cold!

Key Practical Notes

Dynamic flow rate booster: The Stiebel Eltron DCE-X 6/8 will dynamically reduce flow if the buffer runs cold or demand spikes — ensuring hot water temperature is maintained, even if flow is throttled.

Tuning is critical: If your PHE can deliver much more than ~13 kW initially (say up to 25 kW), it will gulp the buffer rapidly, collapsing support time. I use a thermostatic mixing valve immediately after the PHE to cap its outlet at 35 °C. In summer, this limit is often reached; in winter, not always. A further evolution of this design could be to look at smarter ways to control the power delivered by the heat exchanger.

Safety margin: In practice, I recommend designing for PHE_out ≥ 30 °C, i.e. buffer ≥ 35 °C, to allow headroom for sensor error and short-lived demand spikes.

Schematic Description (for illustration)

A simple schematic would show:

[Heat Pump] → [Buffer Tank] → [Plate Heat Exchanger] → [Thermostatic Mixing Valve] → [Booster (DCE-X 6/8)] → [Mixer / Tap]

• Heat Pump: provides bulk of thermal energy
• Buffer: stores heat and smooths supply
• Plate Heat Exchanger + Mixing Valve: preheats mains water, output limited to ~35 °C
• Booster (DCE-X 6/8, 4i tech): finishes heating to 43 °C, automatically varying flow to meet sudden demand or low mains temperatures

The diagram can also annotate thermal splits: HP vs. booster contributions in kW, and typical buffer support times.

Conclusion

This hybrid buffer + heat pump + booster system achieves:

• Truly unlimited hot water for long showers or large baths
• High efficiency, with overall COP often above 2
• Compact footprint, using a small buffer rather than huge storage tanks
• Robustness and safety, thanks to the booster’s 4i variable flow control and thermostatic mixing valve
• No worries about Legionella, as stored water never mixes with mains water

It’s a simple, cost-effective compromise between space, efficiency, and reliability. In practice, it has served me very well over the last year and I have never had a cold shower. There are ways the system could be optimised further, to tune the balance between buffer/HP and booster, but I am very happy with it at the moment. If it could be tuned for a 50L buffer, with a HP that changes modes faster (e.g. without interrupting the compressor), then it could rival a gas combo boiler for convenience and space requirements. Maybe that would convince more people to install a heat pump.