Use steel or copper for the 70 to 90 °C supply and return mains, because the corpus says plastic pipe softens with heat, that plastic pressure ratings are quoted at 20 °C and that copper is the pipe for hot water, and keep clay for a protective duct, since fired clay pipe takes no real pressure. Plain polyethylene also lengthens 17 times more than steel for the same temperature rise: 100 m over 70 K grows 1.4 m in HDPE against 84 mm in steel.
Part of District Heating
Which pipe materials suit a village hot-water main, which suit only a duct or a low-temperature loop, and why.
What The Pipe Has To Do
The pressure in a village loop is low: static head of a few metres plus the pump head, about 120 kPa in the reference case, so pressure rating is rarely the limit. The limits are the temperature (70 to 90 °C supply), the joints, the expansion over a 70 K temperature swing, and corrosion in the ground.
Materials Compared
| Material | At 80 °C | Joints | Expansion, 100 m over 70 K | Verdict |
|---|---|---|---|---|
| Steel | Yes | Welded, threaded, flanged | 84 mm (α = 12×10⁻⁶ per K, recalled) | Main trunks; coat the outside against corrosion |
| Copper | Yes; the corpus names it as the hot-water pipe | Soldered, brazed | about 120 mm (α = 17×10⁻⁶ per K, recalled) | Building connections; expensive; salvage |
| HDPE | Rated at 20 °C; weakens as it warms | Barbed fittings, clamps | 1.4 m (0.2 mm per metre per K, the corpus’s recalled value) | Not for the supply; cold or low-temperature loops only |
| PVC | No; “do not run hot water through PVC” | Solvent-cemented | not computed | Never |
| Clay | Survives heat, takes no real pressure | Clay or lime mortar, 167 to 333 joints per 100 m | very small | Duct or casing only |
| Bored log | No hot-water figure in the corpus | Spigot and socket | not computed | Up to 15 psi for cold water; not for the supply |
Sources: Salvaged Modern Pipe (plastic pipe softens with heat; do not use PVC for hot water; copper for hot water), Pipe Materials (ratings are quoted at 20 °C, and plastic weakens as it warms), HDPE Pipe (expansion 0.2 mm per metre per degree), Clay Pipe (167 to 333 joints per 100 m, no real pressure) and Pipe Making (bored log up to 15 psi). The carrier page’s table gives HDPE a maximum of 80 °C and clay 100 °C; the HDPE figure is not supported by the corpus pages above, which only say that the pressure rating falls with temperature, and no number is available to say how much.
Working, expansion: steel 12×10⁻⁶ × 100 m × 70 K = 0.084 m; HDPE 0.2 mm/m/K × 100 m × 70 K = 1,400 mm = 1.4 m. The ratio is 1.4 ÷ 0.084 = 17. The 70 K swing is an assumed installation temperature of 10 °C to 80 °C.
Why Not Plastic
A plastic main that carries 80 °C water has a pressure rating of an unknown fraction of its 20 °C class, grows more than a metre per hundred metres, and cannot be made safe by a simple derating because no derating number exists in the corpus. The one exception is a low-temperature loop (underfloor heating at 35 to 45 °C): use a pipe whose maker rates it for the temperature, not salvage whose rating you cannot read.
Why Clay Only As A Duct
Clay resists corrosion and rot and lasts for centuries buried, but it is brittle and weak against internal pressure; its joints are mortar and it cannot hold a pressurised main. As a duct it can carry the weight of the soil off an insulated steel pipe and keep roots and stones away. Mortar joints are not watertight, so a clay duct does not keep groundwater out of the insulation; for moisture, case the insulation separately (see Insulation Methods).
Method: Choosing
- Is the line at 60 °C or more? Then steel or copper.
- Is there a stock of salvaged steel or copper? Test it at 1.5 times the working pressure (Pressure Testing); reject lead, asbestos-cement and anything that carried fuel or chemicals.
- Coat steel on the outside with pitch (Pitch & Waterproofing) or wrap it before insulating; inside, a closed loop that is kept full corrodes slowly because the dissolved oxygen is quickly used up (recalled).
- Plan the joints and expansion together; see Expansion & Venting.
Common Mistakes
- Using the 20 °C pressure class at 80 °C. The class is for cold water.
- Using clay for the pressurised main because it lasts for centuries.
- Mixing copper and steel without isolation. Dissimilar metals corrode (recalled).
Safety
A failed joint at 80 °C scalds
Plastic or poorly soldered joints that fail under heat release hot water at pressure. Test every joint at 1.5 times working pressure with water, support runs so expansion cannot pull a joint apart, and never use a pipe whose temperature rating you cannot confirm.
What’s Next
- Learn next: Insulation Methods — wrapping the pipe you chose
- Combine with: Metalworking & Soldering — soldering and brazing temperatures, described for a solar dish