Four steps to the coil length a SIRAC storage vessel needs. Enter your flow and water temperatures; the heat exchange coefficient and pressure drop are read off the SIRAC tubing curve at your actual flow. ← All tools
Choose the coil bore, then enter your total water flow. Each size is rated to a maximum flow per coil, so above that the duty is split across coils in parallel.
Inlet and outlet are the water passing through the coil. Tank temperature is the surrounding stored water, which drives the temperature difference across the coil wall.
Both values are read straight off the SIRAC boiler heat exchange tubing curve at the flow through a single coil. The crosshair below marks exactly where these two numbers come from.
A heat-exchanger coil sits inside a hot water storage vessel and transfers heat from the water circulating through it into the stored water around it. Sizing it means answering one question: how many metres of tubing are needed to move the required duty at the available temperature difference.
| Property | DN25 | DN32 |
|---|---|---|
| Internal bore | 25.4 mm | 32.5 mm |
| Standard coil length | 25 m | 30 m |
| Maximum flow per coil | 2,100 ℓ/h | 3,800 ℓ/h |
| Surface area per metre of coil | 0.154 m² | 0.2351 m² |
| Heat exchange coefficient range | 840 – 1,250 W/m²K | 500 – 1,130 W/m²K |
| Material | Heat-treated 316L stainless steel spiral tubing | |
Heat transfer improves as flow increases, because faster water is more turbulent at the tube wall. Across the DN25 range the coefficient climbs from roughly 840 to 1,250 W/m²K, and pressure drop rises by a factor of ten over the same span. A single fixed value is therefore only correct at one flow rate. This calculator reads both values off the SIRAC boiler heat exchange tubing curve at the actual flow through a single coil, and shows you exactly where on that curve it read them.
At 3,800 litres per hour a single DN32 coil is at its rated flow limit, but it would need 34.9 m of tubing against a 30 m standard coil, so the duty splits across two coils at 1,900 ℓ/h each. At that lower flow the coefficient reads 848 W/m²K, each coil needs 21.2 m, and the pressure drop across the parallel bank is about 16 kPa.
Work out the duty the coil must transfer, then divide it by the product of the heat exchange coefficient, the temperature difference across the coil wall and the coil's surface area per metre. This calculator uses duty ÷ (U × ΔT × area), taking U off the SIRAC boiler heat exchange tubing curve at the actual flow through one coil.
A DN25 coil is rated to 2,100 litres per hour and a DN32 coil to 3,800 litres per hour. Above that the duty is split across multiple coils running in parallel, and the calculator does that automatically.
Faster flow means more turbulence at the tube wall and better heat transfer, so U rises with flow. Across the DN25 range it moves from roughly 840 to 1,250 W/m²K. A single fixed value is only correct at one flow rate, which is why the calculator reads it off the curve at your actual per-coil flow.
No. Coils are piped in parallel, so every coil sees the same pressure drop and the figure quoted is the drop across the bank, not the sum. The value shown is straight-tube friction from the published curve and excludes bend, entry and header losses.
DN25 has a 25.4 mm bore and comes in 25 metre standard coils; DN32 has a 32.5 mm bore in 30 metre coils. DN32 carries almost twice the flow per coil and offers more surface area per metre, so it usually needs fewer coils for the same duty.
Sizing the plant as well? Start with the heat pump sizing calculator, or talk to the SIRAC engineering team about a full design.