Heat-Loss Calculator › Reference › DN65 (2½″) @ 250°C
Heat loss from a DN65 (2½″) pipe at 250 °C
A bare DN65 (2½″) pipe running at 250 °C in 20 °C still air loses about 550 W per metre. Wrap it in 50 mm of removable lamella mat and loss drops to about 109 W/m — a 80% cut — while the outer surface falls to about 40 °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 | 550 W/m | 109 W/m |
| Heat loss (imperial) | 572.0 BTU/hr·ft | 113.8 BTU/hr·ft |
| Reduction | — | 80% |
| Outer surface temp | ~250°C | 40°C |
| Conductivity k (at 135°C mean) | — | 0.069 W/m·K |
How much insulation thickness?
The same DN65 (2½″) 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 | 163 W/m | 70% | 61°C |
| 40 mm | 124 W/m | 77% | 45°C |
| 50 mm | 109 W/m | 80% | 40°C |
| 75 mm | 87 W/m | 84% | 32°C |
| 100 mm | 75 W/m | 86% | 29°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 | 28.8 kW |
| Heat saved by insulation | 23.1 kW |
| Fuel energy saved | 217.2 MWh/yr |
| Money saved | €15,206/yr |
| CO₂ avoided | 43.4 t/yr (≈ 9.4 cars off the road, 2,069 trees, 9.1 homes’ power, 101 barrels of oil, 177,906 km of driving or 43 transatlantic flights) |
| Payback (removable insulation) | 5.7 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 DN65 (2½″) line at 250 °C pay back so fast?
Because heat loss rises steeply with temperature: a bare DN65 (2½″) line at 250 °C loses about 550 W/m, so the money saved per metre is high. Removable insulation on these lines typically pays back in well under two years.