Why do heat pump cylinders need larger coils?

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. Heat pump cylinders typically need 2-3 times the coil surface area of a boiler cylinder.

What is LMTD and why does it matter?

LMTD (Log Mean Temperature Difference) is the effective average temperature difference between the coil water and the stored water, accounting for the temperature changing along the coil length. It gives a more accurate heat transfer calculation than a simple arithmetic mean.

What coil surface area do I need for a heat pump?

Heat pump cylinders typically need 1.5-3.0 m² of coil surface area, compared to 0.5-1.0 m² for a gas boiler cylinder. The exact area depends on the heat pump flow temperature, target recovery time, and cylinder volume. MCS standards specify minimum coil areas for certified installations.

Can I use a standard cylinder with a heat pump?

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.

What recovery time should I target?

For a gas boiler, 20-30 minutes is typical. For a heat pump, 60-120 minutes is acceptable due to the lower flow temperature. MCS guidelines suggest sizing the coil for a maximum 4-hour recovery from cold. The actual recovery time depends on coil area, flow temperature, and cylinder volume.

TradeCalculator

Cylinder Coil Calculator — Surface Area & Recovery

Calculate the required coil surface area for heat pump (45°C) vs gas boiler (70°C) flow temperatures. Determine recovery time at low ΔT.

Heat Source

Determines flow temperature to coil

Cylinder Volume

Hot water cylinder capacity

Target Temperature

Desired stored water temperature

Cold Inlet Temperature

Incoming mains water temperature

Target Recovery Time

Desired full recovery time

How We Calculate This

This calculator uses the LMTD (Log Mean Temperature Difference) method to determine the required coil surface area for a hot water cylinder, accounting for different heat source flow temperatures.

Formulas

Energy (kWh): (V × 4.186 × ΔT) ÷ 3600
kW required: kWh ÷ recovery time (hours)
LMTD: (ΔT1 − ΔT2) ÷ ln(ΔT1 / ΔT2)
Coil area (m²): (kW × 1000) ÷ (U × LMTD × efficiency)
Coil length: Area ÷ (π × pipe OD)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.