Domestic Hot Water Preparation (DHW)
System that heats drinking water for household use to 55-60°C; in modern well-insulated houses, DHW energy demand now rivals space heating.
What is domestic hot water preparation and why does it matter now?
Domestic hot water (DHW) is drinking and sanitary water heated to 55-60°C for household use: showers, laundry, dishwashing, and hand washing. In Slovakia's older housing stock, DHW energy was a minor budget item; the large heating demand dominated all other loads. This has changed profoundly in well-insulated new builds and deep renovations. Passive-house standards reduce space heating to 10-15 kWh/m² annually, while DHW demand remains roughly constant at 30-40 kWh/m² per year because people still shower and wash regardless of building insulation. In many modern Slovak homes, hot water preparation now rivals or exceeds the energy used for space heating. This is why TZB engineers now treat DHW as a distinct system design problem and why energy certificates report it separately.
How is DHW different from the heating system?
Heating systems can operate at low flow temperatures (30-40°C) because the water circulates continuously and never stagnates. Domestic hot water must be maintained at 55-60°C because this temperature prevents legionella pneumophila, a bacterium that thrives in stagnant lukewarm water and can cause legionnaires disease. Storage vessels and pipes carry potable drinking water, not system water, which means they must meet drinking water safety standards. This fundamental difference means DHW cannot simply borrow the heating system's low-temperature philosophy. Instead, DHW preparation has become a separate design choice with its own efficiency penalties and trade-offs. A building may have an excellent heat pump for heating, but the DHW subsystem will always consume more electricity per unit of energy delivered because of the higher temperature requirement.
What are the main DHW preparation methods and how do they compare?
Modern Slovak residential projects choose from five preparation paths, each with different efficiency, recovery speed, space demand, and suitability to a heat pump house:
| Method | Efficiency | Recovery Time (10–20 litre shower) | Standby Loss | Space Required | Typical Use |
|---|---|---|---|---|---|
| Indirect cylinder heated by main heat source | Good; uses heat pump waste or boiler return water | 5–15 minutes (depends on coil size) | High; large tank loses heat to room | Requires mechanical room | Central European standard; suits all heat sources |
| Dedicated DHW heat pump | Excellent; one unit optimized for high-temperature lift | Minutes to hours (depends on flow) | Low; small storage possible | Minimal | Primary choice for heat pump houses; highest COP |
| Instantaneous electric resistance | 100% efficient in conversion but draws heavy current | Instant (no storage delay) | Zero; no standing tank | Tiny; wall-mounted | Supplement to main system; rarely sole heat source due to power demand |
| Instantaneous gas (condensing) | 85–92% efficient; recovers flue gases | Instant | Zero | Requires flue venting | Backup system or hybrid heat-pump-plus-gas approach |
| Solar thermal collectors with backup | Seasonal; summer near-zero grid energy, winter relies on backup | Depends on backup | Tank size drives standing loss | Roof space for collectors; storage tank | Excellent for reducing annual grid energy; needs backup for winter |
In Slovak practice, dedicated DHW heat pumps are increasingly favoured for new passive houses because they isolate DHW from heating, eliminating the temperature-lift conflict. Solar thermal systems are popular where roof area permits, paired with gas boiler or electric backup for winter.
Why is heat pump DHW a design tension in well-insulated houses?
A heat pump optimized for heating works at a small temperature lift: outdoor air at minus 5°C to heating water at 35°C, a difference of 40°C. This small lift gives excellent COP (3.5-4). Domestic hot water requires a much larger lift: minus 5°C to 55-60°C, a difference of 60-65°C. The same pump's COP for DHW drops to 2.0-2.5, consuming more than twice the electricity per unit of hot water. This is the core tension: one pump cannot efficiently do both tasks. The solution is a dedicated DHW unit or accepting lower efficiency. A poorly designed system forces one pump to do both and pays the cost in winter electricity bills.
| Component | DHW-Optimized Heat Pump | Heating-Optimized Heat Pump (heating DHW) |
|---|---|---|
| Compressor type and oil | High-temperature refrigerant; handles 55-60°C discharge safely | Standard air-source refrigerant; efficiency degrades above 50°C |
| Coil design (evaporator) | Larger surface area for low outdoor temperatures | Sized for heating; undersized for DHW volume |
| Flow temperature capacity | 60°C+ continuously or peak | Peaks at 50-55°C with reduced COP |
| Sizing for cylinder recovery | Generously sized to reach temperature in 2-4 hours | Undersized for DHW; recovery slow (8+ hours) or relies on immersion backup |
| Annual COP for DHW | 2.8–3.2 (good for high-lift work) | 1.8–2.2 (poor; heating pump stressed) |
Key insight: if using a heat pump for heating, size the cylinder coil generously rather than the cylinder itself. A small cylinder with a large coil recovers quickly to 55°C, reducing standby loss and legionella risk. A large cylinder with a tiny coil stays lukewarm and stagnant; poor for both efficiency and safety.
What is legionella and why does it conflict with heat pump design?
Legionella pneumophila is a bacterium thriving in warm stagnant water (20-45°C) and becomes a health hazard if inhaled in shower aerosols. The control is storing DHW at 55°C (kills active bacteria) and periodic heating to 70-80°C (annual disinfection kills spores). This creates a genuine design dilemma: heat pumps achieve best efficiency at low temperatures, but DHW heated to 55-60°C continuously conflicts with low-temperature heating philosophy. Large storage vessels or long dead legs where water sits cool are particularly risky. Oversized cylinders with small coils become legionella incubators. The solution is small cylinders with large coils for rapid heating to 55°C, and circulation loops that prevent stagnation. If large storage is necessary (solar systems, off-peak charging), annual thermal disinfection is mandatory.
What are circulation loops and what is their cost?
A circulation loop is a pipe running from the cylinder to distant taps and back, kept warm by a pump and timer. It eliminates cold-water wait time but creates standby loss: the loop loses heat all day and night, consuming energy to maintain temperature. In well-insulated houses, this loss can rival the water heating itself. Circulation loops are best controlled by a timer (night-off, occupancy-responsive) rather than running continuously. The heat loss from even an insulated loop often exceeds its convenience benefit in Slovak residential projects. A simpler solution is placing the cylinder close to the primary bathroom and accepting a brief wait at distant taps.
Why does bathroom location matter for water and comfort?
If a cylinder sits 15 meters from the main bathroom, every shower begins with 10-15 litres of cold water running down the drain while hot water travels through pipe. Over a year, this dead leg wastes hundreds of litres of heated water. It also reduces comfort: waiting in a cold shower wastes time and water. Best practice is placing the cylinder close to high-demand fixtures (main bathroom, kitchen) and accepting longer runs to auxiliary spaces. If layout forces a long run, either install a small circulation loop with a time clock or accept the dead leg as a necessary trade-off.
Frequently asked questions
- Why is DHW now significant in well-insulated houses when it used to be minor?
- Passive-house insulation standards have reduced heating demand to 10-15 kWh/m² per year. Domestic hot water demand (for showers, laundry, dishes) remains relatively constant at roughly 30-40 kWh/m² per year. In old leaky houses, heating dominated; in modern insulated houses, they are now comparable or DHW exceeds heating. This is why energy performance certificates report DHW separately.
- Why must DHW be maintained at 55-60°C while heating can use 30-35°C?
- Drinking water above 55°C prevents legionella pneumophila growth, the bacterium that causes legionnaires disease. Below 20°C, legionella cannot survive. The 55-60°C band kills active bacteria. Heating systems can use much lower temperatures because the system water is not potable and is not stored; it circulates continuously and does not stagnate where legionella could grow.
- Can I use the same heat pump for space heating and hot water?
- Technically yes, but with a major efficiency cost. A heat pump optimized for 35°C heating water must work much harder to reach 55-60°C for DHW, drastically reducing its COP in DHW mode. This is why most modern systems use either a dedicated DHW heat pump, a high-temperature immersion coil in a separate cylinder, or solar thermal with gas backup; not a single pump doing both.
- What does 'instantaneous' hot water mean?
- An instantaneous heater (electric or gas) produces hot water on demand without storage. A shower draws cold water through a heating element or burner, and hot water emerges immediately. No cylinder, no standing losses, but recovery time is zero and flow rate is limited by the heater's power. Electric instant heaters require very high current; gas units need flue venting.
- What is a dead leg and why does it waste water and time?
- A dead leg is pipe run from the main cylinder to a distant bathroom where the water sits motionless, cooling off. Each time you turn on the tap, you run cold water down the drain until the hot water from the cylinder reaches you. Long dead legs waste water daily and reduce perceived comfort. Circulation loops can recover some heat but add standby losses elsewhere.
- Is thermal disinfection the only way to prevent legionella in a large cylinder?
- It is the primary control. Once per year, the cylinder is heated to 70-80°C for 1-2 hours to kill any dormant legionella. This can conflict with heat pump design goals (which favor low temperatures and high efficiency) but is unavoidable for large storage vessels or slow-turnover systems. Smaller cylinders heated rapidly to 60°C and kept there continuously avoid the need for periodic shocks.