Supplying domestic hot water from a village heating loop works in winter, when the network is hot anyway, but costs more heat than it delivers in summer: 30 houses using 100 L a day heated from 10 to 55 °C draw 157 kWh a day, while holding 400 m of trench at 65 °C supply and 45 °C return loses 9.14 kW, 219 kWh a day, 1.4 times as much. Store water at 60 °C or more against bacteria, temper it at the tap against scalds, and heat summer water locally or with solar collectors while the network is off.
Part of District Heating
Taking washing and bathing hot water from the district loop: demand, the tank and coil, the winter peak problem, and the summer loss problem.
How Much Heat Hot Water Needs
Inputs: 100 L per house per day (assumed), heated from 10 to 55 °C (assumed), c = 4.18 kJ/kg·K.
- Per house: 100 × 4.18 × 45 = 18,810 kJ = 5.2 kWh per day.
- For 30 houses: 157 kWh per day, an average of 6.5 kW.
The figure scales directly with the litres you assume. Per Capita Use gives 20 L a day per person as the minimum for drinking, cooking and basic hygiene and 50 to 80 L with laundry and a garden; only part of that is heated.
The Tank And Coil At Each House
Hot Water Tanks gives a 200 L tank warmed from 20 to 60 °C as holding 33.4 MJ, or 9.3 kWh, about seven baths of 40 L. A well-insulated 200 L tank is more than the 5.2 kWh a day above. A coil in the tank takes heat from the district water, as in Heat Exchanger Design.
Rules from the corpus tank page:
- Store at 60 °C or more; lukewarm water between about 25 and 45 °C lets Legionella grow.
- Temper at the tap with cold water; tank water at 60 °C scalds within seconds.
The Winter Peak Problem
If every house reheats its tank at the same time, the demand lands on top of the space heating. Reheating 200 L from 20 to 60 °C takes 33.4 MJ; at 4 kW it takes 33,400 ÷ 4 = 8,350 s, 2.3 h, and 30 houses doing it together would take 30 × 4 = 120 kW, the whole boiler. Restrict each coil’s primary flow so that a house takes at most about 1 kW (a throttle or a small orifice): the same tank then recovers in 33.4 MJ ÷ 1 kW = 9.3 h, overnight, and the peak adds 30 kW and not 120. The 1 kW figure is a design choice, not a sourced limit.
The Summer Problem
In summer there is no space heating, but the hot water needs a supply of at least 65 °C. Keeping the network warm costs heat even when nothing flows to the houses.
Inputs: pair loss for 50 mm wool-class insulation at 65 °C supply and 45 °C return, from the derivation in Insulation Methods: supply (65 − 8) ÷ 4.113 = 13.86 W/m, return (45 − 8) ÷ 4.113 = 8.99 W/m, a total of 22.85 W per metre of trench.
- 400 m × 22.85 W/m = 9.14 kW.
- Per day: 9.14 × 24 = 219 kWh.
- Against the 157 kWh a day that the water needs: 219 ÷ 157 = 1.4.
The network wastes 1.4 times what the houses use, and the boiler has to burn wood to make up the loss, in the hottest months. The solutions:
- Shut the network from late spring to early autumn and heat water locally: batch heaters or thermosiphon collectors on each roof (Batch Water Heaters), or the solar field of Solar Thermal Integration.
- Run a short summer loop that serves only a few houses near the plant; the loss scales with metres of trench.
- Heat the store once or twice a week and distribute by hand, which only suits a very small group.
Common Mistakes
- Keeping the network hot for hot water in summer. The loss exceeds the delivery.
- Storing at 40 °C to avoid scalds. That invites Legionella; store hot and temper at the tap.
- No limit on coil flow. All tanks reheating together take the whole boiler.
Safety
Hot water scalds and tepid water breeds bacteria
Water at 60 °C scalds within seconds, but water between about 25 and 45 °C grows Legionella, so the answer is storage at 60 °C and tempering at the tap with a mixing valve or tap, not a lower tank setting. Put a relief on every tank coil circuit and keep the district side at a lower pressure than the potable side or double-walled; see Hot Water Tanks.
What’s Next
- Learn next: Fuel Consumption Estimates — the winter fuel, in tonnes
- Combine with: Operation & Scaling — the yearly cycle of running the network