Domestic Hot Water (DHW) for Heat Pump Systems
Domestic hot water (DHW) is the part of a heating system that provides hot tap water for showers, baths, handwashing, and kitchens. In a heat pump system, DHW is a separate demand category because it combines temperature requirements, hygiene risk control, and user safety.
This page explains DHW as a heat distribution function within a heat pump system.
Learn about other components of heat distribution
Learn about heat pump technology
Facts and figures
- DHW is a major household energy use. A REHVA journal article notes that hot water production and legionella prevention in DHW systems account for around 17% of total energy demand of German households (based on Eurostat figures discussed in that article).
- Legionella growth is temperature-sensitive. World Health Organization guidance highlights keeping water temperatures outside typical growth ranges and notes, for hot water systems, water leaving heaters above 60°C and maintaining above 50°C through associated pipework as part of control strategies.
- Many systems store hotter than the water you actually use. An International Energy Agency Heat Pumping Technologies report notes DHW is often produced/stored/distributed at 55–60°C for legionella control, while desired use temperature at high-risk taps (e.g., showers) is often 30–40°C—which is why mixing/tempering is common.
- Heat pumps may need a DHW “boost” mode. Sustainable Energy Authority of Ireland guidance notes some heat pumps cannot heat a cylinder to 60°C, and in those cases a booster heater/immersion is often used for disinfection cycles.
What “domestic hot water” means
Domestic hot water is sanitary hot water delivered at outlets in a building for everyday use. In a heat pump system, DHW is usually produced by heating water in a storage cylinder/tank or via instantaneous heating (depending on system concept).
What this is
- A heat distribution demand (separate from space heating)
- A system function shaped by hygiene (microbiological risk) and safety (scald prevention)
- Often a load that requires higher temperatures than low-temperature space heating
What this is not
- A different heat pump technology
- A single universal temperature rule (requirements vary by building type, risk level, and national regulations)
Why DHW matters for heat pumps
DHW often drives different operating conditions than space heating:
- Higher temperature targets than many space-heating emitters
- Short, concentrated demand peaks (showers/baths)
- Hygiene control needs (temperature/time strategies or other measures)
This is why DHW is commonly treated as its own operating mode in heat pump system design and control logic.
Key terms and system boundaries
A simple boundary helps avoid confusion:
- Heat generation: the heat pump produces heat (and may have a booster heater for DHW cycles)
- Heat distribution (DHW): cylinder/tank, pipes, circulation (if used), mixing/tempering devices, valves, safety components
- Control: priority switching (space heat vs DHW), timers/sensors, disinfection cycle scheduling, temperature limiting logic
DHW hygiene and temperature control
A core reason DHW is treated differently is legionella risk management.
- WHO guidance describes temperature as a key control lever and references hot water leaving heaters at or above ~60°C and maintaining above ~50°C in pipework as part of control strategies.
- European Centre for Disease Prevention and Control technical guidance similarly discusses reduced colonisation at higher hot-water temperatures and highlights approaches such as circulating at ~60°C with ≥50–55°C at outlets (or before thermostatic mixing).
User safety: scalding risk and temperature limiting
Hygiene control often pushes storage/distribution temperatures upward, while user safety pushes outlet temperatures downward.
- UK safety guidance notes the scalding risk can become significant for vulnerable users and mentions thermostatic mixing valves (TMVs) as a control method in relevant settings.
Practical meaning: many systems store water at higher temperatures for hygiene control, then temper it at the point of use to a safer delivery temperature—while keeping the “hot side” hot enough to manage microbial risk.
Main DHW system configurations
DHW concepts can be grouped by how hot water is prepared and delivered.
Storage DHW (cylinder / tank)
- Water is heated and stored in a cylinder.
- Supports peak demand by drawing from stored volume.
- Often paired with scheduled high-temperature cycles (depending on risk strategy and system capability).
Instantaneous DHW (on-demand)
- Hot water is produced when a tap opens (using a heat exchanger).
- Can reduce stored hot water volume, but still needs hygiene and safety management consistent with local rules and the overall system concept.
DHW circulation (recirculation loop) vs non-circulated
- A circulation loop reduces waiting time and can help maintain temperature in larger buildings, but it also increases heat loss from pipework and adds a temperature-control requirement.
- Some building guidance discusses maintaining circulation temperatures in a 50–55°C band as part of legionella control strategy (context-dependent).
Integration with other distribution components
Radiators often appear in systems that include additional distribution components:
- Radiators + underfloor heating (mixed emitter systems): usually needs clear temperature management (often with a mixing concept)
- Radiators + domestic hot water (DHW): DHW has different temperature and control priorities than space heating
- Multiple zones: increases the importance of stable hydraulics and good control logic
- Hydraulic separation or buffering: may be used to stabilize flows and operating behavior in some system layouts (depends on topology, zoning, and control strategy)
Core components in a DHW system
A typical DHW setup includes:
- Heat source interface: coil/heat exchanger connected to the heat pump circuit
- Storage cylinder / tank (if storage-based)
- Distribution pipework (hot, cold, and sometimes a circulation return)
- Circulation pump (if a recirculation loop is used)
- Mixing/tempering valve (to limit outlet temperature and improve safety)
- Safety components: temperature/pressure relief, expansion provisions, backflow protection (exact requirements depend on national codes)
- Sensors and controls: cylinder temperature sensor(s), timers, disinfection cycle control, priority logic
What influences DHW performance and efficiency
These factors show up across most DHW discussions:
- Temperature targets: Higher DHW temperatures generally increase the heat pump’s temperature lift requirement and can reduce efficiency relative to low-temperature space heating.
- Storage and distribution losses: Tanks and long pipe runs lose heat continuously; circulation loops can increase losses if not well controlled.
- System capability at high temperature: Some heat pump systems rely on a booster heater/immersion for periodic high-temperature cycles.
- Demand profile: Short peaks (showers) and usage timing affect how often the system needs to reheat the tank.
- Hygiene strategy and monitoring: Temperature control is a common approach; cleaning, flushing, and stagnation reduction are also discussed in public health guidance.
Integration with other distribution components
DHW interacts with the rest of the heating system:
- DHW + space heating: many controls give DHW priority during reheating, then return to space heating.
- DHW + buffer/storage concepts: DHW cylinders are different from space-heating buffer tanks, but both are storage elements that affect cycling and stability.
- DHW + mixed-temperature systems: mixing/tempering is common to balance hygiene temperatures with safe outlet delivery.
The other components of heat distribution in heat pump system include:
