A hot-water network has three things that grow or collect when it heats: the pipe, the water, and the air. A 100 m steel main lengthens by 84 mm between 10 and 80 °C and, if held rigid by backfill, would carry about 168 MPa of compressive stress, so it needs bends, loops or expansion pads; the village’s 17.7 m³ of water swells by 2.87%, 509 L, which needs an open expansion tank at the highest point; and dissolved air collects at crests and must be vented from every high point.
Part of District Heating
Absorbing the thermal growth of pipe and water in a village heating loop, and removing the air that would block it.
Pipe Growth
For a steel pipe of length L heated by ΔT, the change is ΔL = α × L × ΔT with α = 12×10⁻⁶ per K (a handbook value, recalled). Installed at 10 °C and run at 80 °C: ΔL = 12×10⁻⁶ × 100 m × 70 K = 0.084 m, 84 mm per 100 m; for the 250 m main, 210 mm.
If the pipe is fully held, the same strain becomes stress: σ = E × α × ΔT = 200×10⁹ Pa × 12×10⁻⁶ × 70 = 168 MPa (E = 200 GPa, recalled). That is a large fraction of mild steel’s yield, in compression, which buckles a pipe at a bend and pushes against any fixed point. The corpus Tensile Strength gives mild steel tensile strength 345 to 483 MPa, so a stress of 168 MPa from heating alone uses up a third to a half of it before any pressure is added. Backfilled pipe is partly restrained by soil friction, so movement shows up where the pipe turns or ends.
Worked Example: A U-Loop That Takes 84 mm
A U-shaped loop of two legs each of length L absorbs the growth as a bend. Each leg behaves like a cantilever guided at its tip, with the stress in the leg σ = 3 × E × D × δ ÷ L² for a tip deflection δ (from beam theory). Inputs: pipe outside diameter D = 0.0603 m; the loop takes the 84 mm growth of the run, so each leg deflects δ = 0.042 m; allowable stress 100 MPa (an assumed value, well below yield); E = 200 GPa.
L = √(3 × E × D × δ ÷ σ) = √(3 × 200×10⁹ × 0.0603 × 0.042 ÷ 100×10⁶) = √15.2 = 3.9 m.
So a 50 mm pipe needs a loop with legs of about 3.9 m for every 100 m of straight run, in a trench wide enough for them and filled with a compressible pad (a layer of soft material beside the legs) so the loop can move. Reduce the loop by laying the pipe warm, or by stretching it cold, practices this page does not size. Use the elbows of the route as natural expansion bends: a leg that turns 90° for 4 m does the same job.
Water Growth
Water’s density falls from 999.7 kg/m³ at 10 °C to 971.8 at 80 °C (handbook values, recalled), so its volume rises by 999.7 ÷ 971.8 − 1 = 2.87%.
| Part | Volume |
|---|---|
| Mains, 800 m × 2.165 L/m (52.5 mm inside diameter) | 1.73 m³ |
| Heat store (from the boiler article, 480 kWh over 30 K) | 13.8 m³ |
| Boiler circuit (assumed) | 1.0 m³ |
| Radiator circuits, 30 × 40 L (assumed) | 1.2 m³ |
| Total | 17.7 m³ |
Expansion = 17.7 × 0.0287 = 0.51 m³ = 509 L. An open tank must hold that plus a margin, so size it at about 0.75 m³ (an engineering margin, not a sourced figure). Without a heat store (3.9 m³ total) it is 113 L.
Where The Tank Goes
- At the highest point of the system, above the highest radiator, so no part of the system is below atmospheric pressure.
- Open to the air with an overflow to a safe drain. This is the form the corpus recommends for hot-water loops: Hot Water Systems says the loop must stay open at the tank with no valve between the heater and the tank.
- Connected near the pump suction (recalled convention), so the pump pushes into the system and cannot draw air in through joints.
- Insulated and covered, because it holds 0.5 m³ of warm water that evaporates and freezes.
A closed pressure vessel with an air cushion is possible but is a pressure vessel with a fire under it; if you use one, fit a safety valve (Safety Valve) and size it as in Boiler Construction.
Venting Air
Water holds dissolved air, which comes out as the water heats and collects at crests. An air pocket blocks or throttles the flow and can stop circulation altogether in a gravity-fed loop.
- Fit a vent at every high point on the route. Use a manual cock first; automatic float vents are better if available.
- Slope the main continuously toward the plant so air travels to the vent and does not sit in a sag.
- Vent after any repair and after a week of running, since air keeps appearing.
- Fill slowly from the lowest point so the water pushes the air ahead of it.
Common Mistakes
- Burying a straight rigid run. Thermal growth has to go somewhere; it goes into a joint.
- Closing the expansion tank off. A fired boiler cut off from its vent is a bomb.
- Neglecting the vents. Air makes pumps cavitate and radiators cold at the top.
Safety
A blocked expansion path bursts the boiler
Water expands by about 2.9% when heated from 10 to 80 °C and has nowhere to go if the tank is isolated or frozen, and a sealed fired boiler can burst like a steam boiler. Keep the expansion pipe free of valves, protect it from frost, check the overflow is clear, and never plug a vent to stop a drip.
What’s Next
- Learn next: Building Connections — the end of the pipe
- Combine with: Supply & Return Piping Layout — where the vents and loops go