How is cylinder heat-up time calculated?

The formula is t = (V × 4.186 × ΔT) / (kW × 3600), where V is volume in litres, 4.186 is the specific heat capacity of water in kJ/kg·K, ΔT is the temperature rise, and kW is the heat input. The result gives time in hours, which we convert to minutes.

Why is coil efficiency important?

An indirect coil in a cylinder doesn’t transfer 100% of the boiler’s output. Typical coil efficiency is 80-90% depending on coil surface area, flow temperature, and flow rate. A 28 kW boiler with 85% coil efficiency delivers about 23.8 kW to the stored water.

How do standing losses affect heat-up time?

While the cylinder is heating, it also loses heat through its walls. A well-insulated modern cylinder loses about 1-2 kWh per day. During the heat-up period, standing losses add approximately 3-5% to the energy requirement, slightly extending recovery time.

Why do heat pumps take longer to heat cylinders?

Heat pumps typically deliver water at 45-55°C compared to a gas boiler’s 70-80°C. The lower flow temperature means less energy transfer through the coil per unit time. Heat pump cylinders often have larger coils to compensate, but recovery times of 2-4 hours are common.

What is the ideal cylinder storage temperature?

The recommended storage temperature is 60°C to prevent Legionella bacteria growth. Water should be stored at 60°C and distributed at 50°C minimum (or via TMV at point of use). Weekly pasteurisation to 70°C may be recommended for larger systems.

TradeCalculator

Cylinder Heat-Up Calculator — Recovery Time

Calculate how long a hot water cylinder takes to heat from cold to target temperature. Accounts for coil efficiency and standing losses.

Cylinder Volume

Hot water cylinder capacity

Heat Source

Primary heat source type

Start Temperature

Water temperature at start (cold fill ~10\u00B0C)

Target Temperature

Desired storage temperature (60\u00B0C recommended)

How We Calculate This

This calculator determines the recovery time for a hot water cylinder using the fundamental heat energy equation, adjusted for real-world coil efficiency and standing heat losses.

Formula

Energy (kWh): (V × 4.186 × ΔT) ÷ 3600
Effective kW: Heat source kW × coil efficiency
Standing loss: Energy × loss factor (typically 5%)
Time (mins): (Total kWh ÷ Effective kW) × 60

The specific heat capacity of water (4.186 kJ/kg·K) means 1 litre of water requires 4.186 kJ to raise its temperature by 1°C. This is a fundamental constant used throughout plumbing and heating engineering.

Frequently Asked Questions

Related Calculators

Pipe Sizing Calculator

Calculate the correct pipe diameter for your plumbing installation.

Boiler Size Calculator

Determine the right boiler kW output for your UK property.

Hot Water Cylinder Sizing Calculator

Size your hot water cylinder based on household demand.

Radiator Sizing Calculator

Calculate the BTU output needed for each room.

Central Heating System Volume Calculator

Calculate total water volume in your central heating system.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.