Pipe Heat Loss Calculator

How heat leaves a pipe

Heat flows from the hot water through the steel wall, through the insulation, and from the outer surface into the air. The steel and the thin film of water inside barely resist it; nearly all of the resistance sits in two places: the insulation layer and the boundary between the outer surface and the air. For a round pipe, the loss per metre is:

Here r₁ is the outside radius of the pipe, r₂ the outside radius of the insulation, λ the thermal conductivity of the insulation and h the surface heat transfer coefficient, which covers both convection and radiation. ΔT is the difference between the water and the surrounding air.

What the numbers usually look like

A bare DN100 steel pipe carrying 90 °C water in a 15 °C room loses roughly 350 W for every metre. That is a small electric heater per metre, running all year. Wrap it in 50 mm of mineral wool and the loss drops to under 30 W/m, a cut of more than 90%. Over 100 m and 8,000 hours, that is the difference between about 290 MWh and about 22 MWh a year.

The first few centimetres do almost all of the work. Going from 0 to 25 mm saves far more than going from 50 to 100 mm, which is why the thickness table in the results is useful: it shows where extra insulation stops paying for itself at your heat price.

Bare valves and flanges add up

Even well-insulated networks leak heat through the parts nobody wraps: valves, flanges, pump casings, strainers. A rule of thumb used in many energy audits is that an uninsulated valve loses about as much heat as one to two metres of bare pipe of the same size, and a flange roughly half a metre. On a plant with dozens of them, removable insulation jackets often pay back within one heating season.

Assumptions and limits

The calculator uses typical surface heat transfer coefficients: about 9 W/(m²·K) for insulated surfaces indoors, 9.4 + 0.052·ΔT for bare steel, plus 6·√v for wind outdoors. Real values depend on the jacket material (shiny aluminium cladding radiates less than painted steel), on air movement and on the exact temperatures. The conductivity of insulation also rises with temperature and with moisture: wet mineral wool can conduct several times more heat than dry. For buried pre-insulated district heating pipes, the soil adds resistance and different methods apply.

Heat lost from a network has to be made up by the boilers and moved by the pumps. The heating flow rate calculator converts that heat into flow, and the pump energy calculator shows what moving it costs.

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Frequently asked questions

Which λ value should I use for my insulation?

Use the value from the product data sheet at the mean insulation temperature, which is roughly halfway between the water and the air. Typical values are 0.035 to 0.045 W/(m·K) for mineral wool, 0.022 to 0.030 for PUR foam and 0.033 to 0.040 for elastomeric foam. Old or damp insulation can be much worse than the data sheet.

Why is the surface temperature important?

It tells you whether the insulation is doing its job and whether the pipe is safe to touch. Many safety guidelines aim for surface temperatures below about 50 to 60 °C where people can reach the pipe. A warm spot on otherwise cool cladding usually means wet or missing insulation underneath.

Can I use this for chilled water pipes?

The heat gain is calculated the same way; enter the chilled water temperature and the warmer air. But for cold pipes the main concern is condensation, not energy. Check the surface temperature against the dew point of the surrounding air with the dew point calculator.

How do I convert the result to Gcal?

The calculator shows annual energy in both MWh and Gcal. One Gcal is about 1.163 MWh. If your heat price is per Gcal, choose “per Gcal” next to the price.