What size hot water cylinder do I need for a family of 4?

A family of 4 with a mix of baths and showers typically needs a 150-180 litre cylinder. If baths are used frequently, go for 180L. For shower-only households, 120-150L may be sufficient. Always factor in the number of bathrooms as well.

What is the difference between vented and unvented cylinders?

A vented cylinder is fed by a cold water tank in the loft and works on gravity pressure. An unvented cylinder connects directly to the mains, giving much better water pressure at taps and showers. Unvented cylinders must be installed by a G3-qualified engineer under Building Regulations (Part G3).

How long does a hot water cylinder take to reheat?

Reheat time depends on the cylinder size and boiler output. A 150L cylinder heated by a 24kW boiler typically takes 40-55 minutes. An immersion heater (3kW) takes considerably longer — around 3-4 hours for the same cylinder. System boilers with higher outputs reheat faster — use our Boiler Size Calculator to find the right kW for your property.

Can I replace a vented cylinder with an unvented one?

Yes, this is a common upgrade in UK homes. However, you need adequate mains water pressure (minimum 1.5 bar, ideally 2+ bar), a G3-qualified installer, and the work must comply with Building Regulations. Your plumber will also need to fit pressure-relief and expansion vessels.

What are standard UK cylinder sizes?

Standard UK hot water cylinder sizes are 90L, 120L, 150L, 180L, 210L, 250L, and 300L. The most common domestic sizes are 150L and 180L. Larger homes with multiple bathrooms may need 210-300L cylinders.

Hot Water Cylinder Size Calculator — What Size Cylinder Do I Need?

How We Calculate This

Our hot water cylinder calculator estimates the correct capacity based on the number of occupants and their hot water usage patterns. The calculation follows UK industry guidelines for domestic hot water demand.

The calculation method

Daily demand = Number of occupants x Usage per person per day

We estimate daily hot water usage at 60°C as:

Shower only: ~35 litres per person per day
Bath & shower: ~45 litres per person per day
Mostly baths: ~55 litres per person per day

Peak demand sizing

The cylinder does not need to store an entire day's hot water — it reheats throughout the day. We size based on peak demand (65-75% of daily usage), which represents the busiest period (typically morning). Multi-bathroom homes need a higher peak factor because multiple outlets may be used simultaneously.

Standard UK sizes

We recommend the nearest standard UK cylinder size that meets or exceeds your peak demand: 90L, 120L, 150L, 180L, 210L, 250L, or 300L. These align with commonly available products from manufacturers like Megaflo, Heatrae Sadia, and Telford.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Hot Water Cylinder Sizing Calculator — What Size Cylinder Do I Need?