A central heating plant saves fuel over individual stoves only if the heat lost in the pipes is smaller than the efficiency gain of the big boiler, and that sets a maximum trench length per house. With an 80% boiler, pipes losing 27.7 W per metre of trench over a 4,800-hour season and 7,680 kWh of heat per house, the break-even is 26 m of trench per house against stoves at 55% efficiency, 58 m against stoves at 40% and only 8 m against stoves at 70%.
Part of District Heating
Deciding between one central boiler and heat from individual or several smaller sources, using fuel as the measure.
The Comparison In One Equation
Per house, over a season:
- Stove: fuel energy = Q ÷ η_s, with Q the heat needed and η_s the stove efficiency.
- District: fuel energy = (Q + L) ÷ η_b, with L the network loss attributed to that house and η_b the central boiler efficiency.
The district system uses less fuel when (Q + L) ÷ η_b < Q ÷ η_s, that is when
L < Q × (η_b ÷ η_s − 1)
The loss per house is L = (trench metres per house) × (loss per metre) × (hours). So the maximum trench per house is ℓ* = Q × (η_b ÷ η_s − 1) ÷ (q′ × t).
Worked Example
Inputs: Q = 7,680 kWh (4 kW design × 40% average load × 4,800 h); boiler η_b = 0.80, the low end of the 80 to 90% on the carrier page; loss q′ = 27.7 W per metre of trench for the supply-and-return pair (from Insulation Methods); t = 4,800 h, so q′ × t = 133 kWh per metre.
| Stove efficiency | η_b ÷ η_s − 1 | L allowed per house | Maximum trench per house |
|---|---|---|---|
| 40% | 1.00 | 7,680 kWh | 7,680 ÷ 133 = 58 m |
| 55% | 0.4545 | 3,491 kWh | 3,491 ÷ 133 = 26 m |
| 70% | 0.143 | 1,097 kWh | 1,097 ÷ 133 = 8.3 m |
The carrier page’s stove efficiency range is 40 to 70%. At the lower end a district network can run 58 m of trench per house and still save fuel; at the upper end it cannot afford more than 8 m. The reference village, with 400 m for 30 houses, uses 13.3 m per house, so it saves fuel against stoves of 55% or worse and costs fuel against 70% ones.
This is only the fuel comparison. The central plant also removes 30 fires, 30 chimneys and 30 fuel stores, and replaces 30 people’s daily tending with one operator’s; none of that is priced here.
Distributed Sources
Several smaller sources can feed one loop: rooftop solar collectors, a workshop’s waste heat, a biogas boiler at the digester. They help when:
- The source is near the loop. A source that needs its own long spur loses heat the way any house does.
- The temperatures match. A solar collector delivering 50 to 80 °C (Flat Plate Collectors) can raise the return water; it cannot lift the supply above its own output.
- The injection is controlled. Each source needs a valve or check so that the loop does not flow backwards through it.
See Solar Thermal Integration and Biogas Boiler Integration. A single plant is simpler to operate, and a small number of sources is simpler to keep balanced.
Resilience
One plant is one fault from 30 cold houses. The existing stoves are a backup, so keep them in working order. Fit the plant with a standby: a second boiler (staging, so one carries the load when the other is out), a store that rides through a few hours, and the stoves.
Common Mistakes
- Comparing boiler and stove efficiency and stopping. The pipes are the cost in between.
- Counting the best case. If your stoves are 70% efficient and the village is spread out, a network may burn more fuel than you now do.
- No fallback. The plant is a single point of failure.
Safety
A shared plant concentrates the hazards
One boiler house holds a fire, a pressurised hot-water vessel and a fuel store that supplies the whole village. Keep it apart from dwellings, vent it, keep a CO check on it, and read Boiler Construction before building the vessel.
What’s Next
- Learn next: Supply & Return Piping Layout — how to keep metres of trench per house low
- Combine with: Fuel Consumption Estimates — the same comparison in tonnes of wood