Cylinder Coil Calculator — Surface Area & Recovery

Heat pumps deliver water at 45-50°C compared to a gas boiler’s 70-80°C. The lower temperature difference between the coil and stored water means less heat transfer per square metre of coil surface, and that difference shrinks to almost nothing near the end of recovery when the store is nearly as hot as the flow. A manufacturer sizes the coil so it can still absorb the heat pump’s full output at that worst-case moment, which is why purpose-made heat pump cylinders typically carry 2-3 times the coil surface area of a boiler cylinder (for example the 300L Telford Tempest heat-pump cylinder uses a 3.3 m² coil). This calculator instead estimates the coil needed for the AVERAGE heating rate over your chosen recovery time, so its figure is smaller than the deliberately oversized coil the manufacturer fits — always size to the cylinder’s published coil kW rating at your flow and return temperatures.

LMTD (Log Mean Temperature Difference) is the effective average temperature difference driving heat from the coil water into the stored water, accounting for the primary water cooling as it flows along the coil. Both coil-end temperature differences are measured against the same stored-water temperature (the cylinder is well-mixed, so it sits at one temperature at any instant) — the difference between the two ends is simply the primary flow/return drop. A low flow temperature shrinks the LMTD dramatically, which is why heat pumps need a much larger coil for the same kW.

Manufacturers fit roughly 2.0-3.3 m² coils to purpose-made heat pump cylinders (e.g. the Telford Tempest heat-pump range uses 2-3.3 m² coils), versus around 0.5-1.5 m² for a boiler cylinder. The exact area depends on the flow temperature, the primary flow/return drop, the target recovery time and the cylinder volume — and manufacturers deliberately oversize the coil so it can absorb the heat pump’s full output even when the store is nearly hot. Use this calculator for a working estimate, then confirm against the cylinder’s published coil kW rating at your flow and return temperatures (the figure MCS MIS 3005-D requires you to design to).

Generally no. Standard boiler cylinders have small coils designed for 70 °C+ flow temperatures. Using one with a heat pump would result in very long recovery times (4–8+ hours). Purpose-designed heat pump cylinders have larger coils and are optimised for 45–55 °C flow temperatures. Use our Hot Water Cylinder Calculator for cylinder sizing, and our Cylinder Heat-Up Calculator to check recovery times.

For a gas boiler, 20-30 minutes is typical. For a heat pump, 60-120 minutes is normal because of the lower flow temperature. There is no fixed regulatory limit — MCS MIS 3005-D states the desired cylinder reheat time should be agreed with the customer and the heat exchanger sized to the manufacturer’s recommendation. The actual recovery time depends on coil area, flow temperature, the primary flow/return drop, and cylinder volume.