Heat-Loss Calculator › Reference › DN125 (5″) @ 150°C
Heat loss from a DN125 (5″) pipe at 150 °C
A bare DN125 (5″) pipe running at 150 °C in 20 °C still air loses about 571 W per metre. Wrap it in 50 mm of removable lamella mat and loss drops to about 77 W/m — a 86% cut — while the outer surface falls to about 30 °C (touch-safe). Figures use the ASTM C680 steady-state method.
These conditions are typical of low-pressure saturated steam, medium-temperature hot water (MTHW) and thermal-oil return lines. A bare surface here is already a burn risk and loses a large, continuous amount of heat that insulation recovers almost entirely.
Per-metre heat loss
| Quantity | Bare | Insulated (50 mm) |
|---|---|---|
| Heat loss | 571 W/m | 77 W/m |
| Heat loss (imperial) | 593.5 BTU/hr·ft | 80.3 BTU/hr·ft |
| Reduction | — | 86% |
| Outer surface temp | ~150°C | 30°C |
| Conductivity k (at 85°C mean) | — | 0.055 W/m·K |
How much insulation thickness?
The same DN125 (5″) line at 150 °C, with different removable-insulation thicknesses (lamella mat, k≈0.055 W/m·K). Heat loss and surface temperature both fall as thickness increases — with diminishing returns past 50–75 mm.
| Thickness | Heat loss | Reduction | Surface temp |
|---|---|---|---|
| 25 mm | 124 W/m | 78% | 41°C |
| 40 mm | 90 W/m | 84% | 33°C |
| 50 mm | 77 W/m | 86% | 30°C |
| 75 mm | 59 W/m | 90% | 26°C |
| 100 mm | 49 W/m | 91% | 25°C |
At this temperature the bare surface burns on contact within a second; 50 mm of removable insulation typically returns it to a touch-safe ≤45 °C.
Example: a 50 m line with 4 valves
| Result | Value |
|---|---|
| Heat loss, bare | 29.9 kW |
| Heat saved by insulation | 25.9 kW |
| Fuel energy saved | 243.3 MWh/yr |
| Money saved | €17,031/yr |
| CO₂ avoided | 48.7 t/yr (≈ 10.6 cars off the road, 2,317 trees, 10.1 homes’ power, 113 barrels of oil, 199,265 km of driving or 49 transatlantic flights) |
| Payback (removable insulation) | 7.0 months |
Assumptions. 50 m pipe + 4 valves (each ≈0.6 m bare pipe), 8000 h/yr, 85% boiler efficiency, €0.07/kWh fuel, 0.20 kg CO₂/kWh, 50 mm insulation at €250/m². CO₂ equivalences use US EPA / DEFRA conversion factors. Change any of these in the live calculator. Estimates for guidance — confirm with a site survey.
Run your own numbers
Adjust size, temperature, thickness, hours and energy price live — or get an exact heat-loss study and fixed insulation price for your equipment list.
Open the calculator →Get an exact quoteIs a bare DN125 (5″) line at 150 °C a burn hazard?
Yes — a bare DN125 (5″) surface at 150 °C exceeds the ~60 °C touch-safe threshold (ASTM C1055, EN ISO 13732-1) and burns on brief contact. The line also loses about 571 W/m, so insulation fixes safety and energy at once.