What is an unvented hot water cylinder?

An unvented cylinder is a pressurized system that takes water directly from the mains supply. Because it is under pressure, it provides excellent flow rates to showers and taps throughout the house without needing a header tank in the loft.

What size cylinder do I need for a 4-bedroom house?

A standard 4-bedroom house with 2 bathrooms typically requires a 250L to 300L cylinder. If the house has high-flow rainfall showers or a very large bath, you may want to consider a 300L+ unit to ensure you don't run out of hot water during peak times.

What is a 'Direct' vs 'Indirect' cylinder?

An 'indirect' cylinder is heated by a primary coil connected to your boiler (or heat pump). A 'direct' cylinder is heated solely by internal electric immersion heaters. Indirect cylinders are generally faster at reheating and cheaper to run if you have a gas boiler.

Are there regulations for installing cylinders?

Yes. In the UK, unvented cylinders must be installed by a G3-qualified engineer. This is because they are pressurized vessels and require specific safety components, such as expansion vessels and temperature-pressure relief valves, to prevent explosions.

What is an unvented hot water cylinder?

An unvented cylinder is a pressurized system that takes water directly from the mains supply. Because it is under pressure, it provides excellent flow rates to showers and taps throughout the house without needing a header tank in the loft.

What size cylinder do I need for a 4-bedroom house?

A standard 4-bedroom house with 2 bathrooms typically requires a 250L to 300L cylinder. If the house has high-flow rainfall showers or a very large bath, you may want to consider a 300L+ unit to ensure you don't run out of hot water during peak times.

What is a 'Direct' vs 'Indirect' cylinder?

An 'indirect' cylinder is heated by a primary coil connected to your boiler (or heat pump). A 'direct' cylinder is heated solely by internal electric immersion heaters. Indirect cylinders are generally faster at reheating and cheaper to run if you have a gas boiler.

Are there regulations for installing cylinders?

Yes. In the UK, unvented cylinders must be installed by a G3-qualified engineer. This is because they are pressurized vessels and require specific safety components, such as expansion vessels and temperature-pressure relief valves, to prevent explosions.

Water Cylinder Calculator

Plan your hot water capacity for maximum comfort.

Bedrooms

Bathrooms

Showers

Expected Demand

High demand = Rainfall showers or large family occupancy.

Suggested Capacity

Efficiency Tip: Unvented cylinders provide mains-pressure hot water to all outlets simultaneously. This is ideal for modern homes with multiple bathrooms where traditional 'gravity-fed' systems often struggle with poor flow rates.

Hot Water Demand: Choosing the Right Cylinder

Domestic Comfort

Nothing ruins a morning routine like a cold shower. Selecting a cylinder that is too small will lead to constant shortages, while an oversized cylinder wastes energy by heating water you never use.

Our **water cylinder size calculator** uses standard UK occupancy data to recommend a capacity that balances your household's peak demand with energy efficiency.

Standard Capacities

1-2 Bedrooms120 - 150L

3 Bedrooms180 - 210L

4+ Bedrooms250 - 300L+

Reheat Times

The total volume is only half the story. The **reheat time**—how quickly your boiler can heat the water from cold to 60°C—is equally important. A high-recovery cylinder with a large internal coil can reheat in as little as 20 minutes, allowing for multiple baths to be taken in quick succession.

If you have a large family, look for cylinders with 'high-recovery' ratings to ensure the system can keep up with heavy morning demand.

Safety (G3)

Unvented cylinders are governed by Building Regulation G3. They must feature an expansion vessel to handle the increased volume of heated water and a discharge pipe (tundish) to safely vent pressure if the system malfunctions.

Insulation

Modern cylinders are factory-insulated with high-density foam. A good quality 210L cylinder should lose less than 1.5kWh of heat over 24 hours, meaning your hot water stays hot all day even when the boiler is off.

Pro Tip: Placement

Try to place your cylinder as close to your most-used bathrooms as possible. The further the water has to travel through the pipes ('dead legs'), the more heat it loses and the longer you have to wait for hot water to arrive at the tap.

Hot Water Systems FAQ

Plumbing Disclaimer

This calculator provides a sizing estimate based on typical UK household usage. Actual demand can vary significantly based on individual habits. For a precise specification, consult a CIPHE-registered heating engineer who can perform a full hot water load calculation.