Heat-Loss Calculator › Reference › DN125 (5″) @ 250°C
Heat loss from a DN125 (5″) pipe at 250 °C
A bare DN125 (5″) pipe running at 250 °C in 20 °C still air loses about 1009 W per metre. Wrap it in 50 mm of removable lamella mat and loss drops to about 168 W/m — a 83% cut — while the outer surface falls to about 42 °C (touch-safe). Figures use the ASTM C680 steady-state method.
These conditions are typical of medium-pressure saturated-steam distribution and thermal-fluid (hot-oil) headers. Heat loss climbs steeply with temperature in this band, so removable insulation pays back fastest on exactly these lines.
Per-metre heat loss
| Quantity | Bare | Insulated (50 mm) |
|---|---|---|
| Heat loss | 1009 W/m | 168 W/m |
| Heat loss (imperial) | 1050.1 BTU/hr·ft | 174.8 BTU/hr·ft |
| Reduction | — | 83% |
| Outer surface temp | ~250°C | 42°C |
| Conductivity k (at 135°C mean) | — | 0.069 W/m·K |
How much insulation thickness?
The same DN125 (5″) line at 250 °C, with different removable-insulation thicknesses (lamella mat, k≈0.069 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 | 265 W/m | 74% | 64°C |
| 40 mm | 195 W/m | 81% | 48°C |
| 50 mm | 168 W/m | 83% | 42°C |
| 75 mm | 129 W/m | 87% | 34°C |
| 100 mm | 108 W/m | 89% | 30°C |
This is the band where insulation economics are strongest — the heat saved per metre is high and the payback shortest.
Example: a 50 m line with 4 valves
| Result | Value |
|---|---|
| Heat loss, bare | 52.9 kW |
| Heat saved by insulation | 44.1 kW |
| Fuel energy saved | 415.0 MWh/yr |
| Money saved | €29,048/yr |
| CO₂ avoided | 83.0 t/yr (≈ 18.0 cars off the road, 3,952 trees, 17.3 homes’ power, 192 barrels of oil, 339,866 km of driving or 83 transatlantic flights) |
| Payback (removable insulation) | 4.1 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 quoteWhy does insulating a DN125 (5″) line at 250 °C pay back so fast?
Because heat loss rises steeply with temperature: a bare DN125 (5″) line at 250 °C loses about 1009 W/m, so the money saved per metre is high. Removable insulation on these lines typically pays back in well under two years.