Heat-Loss Calculator › Reference › DN125 (5″) @ 120°C
Heat loss from a DN125 (5″) pipe at 120 °C
A bare DN125 (5″) pipe running at 120 °C in 20 °C still air loses about 439 W per metre. Wrap it in 50 mm of removable lamella mat and loss drops to about 56 W/m — a 87% cut — while the outer surface falls to about 27 °C (touch-safe). Figures use the ASTM C680 steady-state method.
These conditions are typical of low-temperature hot-water (LTHW) heating loops, condensate-return lines and hot process water. The temperature is modest, but the line runs continuously, so a bare run quietly wastes fuel all year — and the surface can still sit above the touch-safe limit.
Per-metre heat loss
| Quantity | Bare | Insulated (50 mm) |
|---|---|---|
| Heat loss | 439 W/m | 56 W/m |
| Heat loss (imperial) | 456.6 BTU/hr·ft | 58.1 BTU/hr·ft |
| Reduction | — | 87% |
| Outer surface temp | ~120°C | 27°C |
| Conductivity k (at 70°C mean) | — | 0.052 W/m·K |
How much insulation thickness?
The same DN125 (5″) line at 120 °C, with different removable-insulation thicknesses (lamella mat, k≈0.052 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 | 90 W/m | 79% | 35°C |
| 40 mm | 65 W/m | 85% | 29°C |
| 50 mm | 56 W/m | 87% | 27°C |
| 75 mm | 43 W/m | 90% | 25°C |
| 100 mm | 35 W/m | 92% | 23°C |
Because the process temperature is moderate, modest insulation already brings the surface well into the touch-safe range while still cutting most of the loss.
Example: a 50 m line with 4 valves
| Result | Value |
|---|---|
| Heat loss, bare | 23.0 kW |
| Heat saved by insulation | 20.1 kW |
| Fuel energy saved | 188.9 MWh/yr |
| Money saved | €13,224/yr |
| CO₂ avoided | 37.8 t/yr (≈ 8.2 cars off the road, 1,799 trees, 7.9 homes’ power, 87 barrels of oil, 154,724 km of driving or 38 transatlantic flights) |
| Payback (removable insulation) | 9.0 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 quoteDoes a DN125 (5″) line at 120 °C really need insulating?
Yes — a bare DN125 (5″) line at 120 °C loses about 439 W per metre continuously, and over 8000 h/yr that is real, recoverable fuel. Removable insulation also keeps the surface touch-safe for personnel.