Boiler DHW Calculator

Domestic hot water (DHW) systems integrated with heating boilers provide efficient water heating through indirect storage tanks, tankless coils, and combination systems. Proper sizing ensures adequate hot water delivery during peak demand while maintaining efficiency and preventing Legionella bacteria growth. ASHRAE 90.1 and EN 12828 establish requirements for system capacity, storage volume, recovery rates, and temperature control. Residential demand typically requires 40-60 liters per person per day with morning peak loads, while commercial applications (hotels, offices, restaurants) exhibit different patterns requiring accurate demand estimation using fixture unit methods.

Storage Tank Sizing and Recovery: Storage tank sizing balances peak demand capacity against boiler recovery rate. Optimal size stores 30-60 minutes of peak flow, calculated as tank volume = peak hourly demand - (boiler recovery rate × peak duration). Example: 1,000 L/hr peak with 800 L/hr boiler recovery needs minimum 200L storage plus safety margin, typically sizing to 300-400L total. Indirect tanks separate potable water from boiler water through heat exchangers—residential units recover at 100-200 L/hr while commercial units achieve 1,000+ L/hr through larger coil surfaces and higher boiler water temperatures.

Temperature Control and Legionella Prevention: Temperature control balances Legionella prevention (storage ≥60°C per ASHRAE 188 and WHO) against scald protection (delivered ≤49°C per IPC 607.1.1). Thermostatic mixing valves blend stored hot water with cold water to safe delivery temperatures. Storage below 55°C risks Legionella proliferation while above 70°C accelerates corrosion and scaling. Weekly pasteurization cycles raising tank to 70°C for 30 minutes provide additional control in high-risk facilities like hospitals and nursing homes, ensuring pathogen elimination throughout distribution systems.

Recirculation Systems and Distribution: Recirculation systems maintain hot water throughout distribution piping eliminating waiting time, critical for large buildings and hotels. Pumps sized for 5-15% of peak DHW demand circulate water through supply mains and return lines. Insulation on all hot water piping (25-50mm per ASHRAE 90.1) minimizes heat loss—uninsulated pipes lose 3-10 W/m. Temperature-controlled or time-clock-operated pumps reduce operating hours during low-demand periods achieving 20-40% energy savings with minimal impact on user experience and comfort.

Combination Boiler Sizing and Priority Controls: Boiler sizing for combination heating and DHW requires the greater of space heating load or DHW recovery load, not the sum (peak demands rarely coincide). Modulating condensing boilers efficiently serve both loads by adjusting firing rates. Priority controls allocate full boiler capacity to DHW during calls, temporarily suspending space heating—acceptable since building thermal mass maintains temperature during brief DHW priorities. Combination systems reduce installation cost and space while achieving higher annual efficiency through condensing operation on DHW loads.

Energy Efficiency Optimization: High-efficiency condensing boilers achieve 95%+ AFUE versus standard atmospheric units at 80-85%. Solar thermal preheat reduces fossil fuel demand by 40-70% in sunny climates. Heat pump water heaters achieve 200-300% effective efficiency through refrigeration cycles. ASHRAE 90.1 mandates minimum insulation R-values, limits standby losses, and requires efficient controls—compliance achieved through indirect tanks, outdoor reset controls reducing boiler temperature during mild weather, and modulation reducing cycling losses and improving seasonal performance.

Standards Reference: Design per ASHRAE 90.1 (energy efficiency), ASHRAE 188 (Legionella prevention), EN 12828 (heating systems), and IPC Chapter 6 (DHW systems). Storage ≥60°C for Legionella control, delivery ≤49°C for scald protection. Thermal expansion tanks accommodate 2-4% volume increase. Recirculation insulation 25-50mm thickness. Annual energy typical: 40-60 kWh per person for residential DHW.