Hot Water Cylinders: A Guide

Whether you are replacing or upgrading your boiler, choosing the right hot water cylinder increases your chances of:

• Strong water pressure
• Reliable hot showers
• Lower energy bills. 

This guide explains how hot water cylinders work, the key differences between unvented and vented models, and more.

What is a hot water cylinder? (& how does it work?)

A hot water cylinder is an insulated tank that holds heated water until it’s needed (for taps or showers). Many store a large volume of hot water so that multiple bathrooms can run simultaneously.

The two main kinds are:

Indirect hot water cylinders

These rely on an external heat source (such as a gas or oil boiler, heat pump, etc.). 

Heated water from the boiler travels through a coiled metal pipe inside the cylinder, transferring heat to the surrounding water without mixing with it. 

Most indirect cylinders also house an immersion heater as a backup heat source if your primary boiler breaks down.

Direct hot water cylinders

Direct cylinders don’t connect to an external boiler. Instead, they heat water with an internal electric immersion heater. However, this is their primary heat source, not a backup. 

So unlike indirect cylinders, if the immersion heater fails, the cylinder stops producing hot water altogether.

Direct hot water cylinders are common in homes without a mains gas supply or in all-electric flats.

Indirect and direct describe how your cylinder is heated. Vented and unvented describe how it’s pressurised. Most modern UK homes have an indirect unvented cylinder — boiler-fed, connected directly to the mains.

Smart top-down heating and stratification

Traditional cylinders heat water slowly from the bottom up. This heats the entire tank even if only a fraction of the water is needed (as is often the case). 

Modern smart cylinders from manufacturers like Mixergy use top-down heating and natural thermal stratification. 

By heating water from the top down (which naturally sits above cold water without mixing), a smart cylinder lets you heat precise volumes. It cuts reheating times and reduces standby heat loss.

Unvented vs vented hot water cylinders

Unvented hot water cylinders

An unvented cylinder connects directly to your cold water mains pipe. This means stored hot water is distributed around your property at high mains pressure.

Because cold water enters under mains pressure, you can* get powerful showers without needing extra booster pumps (*depending on your local mains supply).

Unvented tanks also save space and are simpler to fit, since they don’t require a cold water storage cistern. 

That said, the units themselves cost more. Installation must be carried out by a G3-certified engineer, they require an annual safety check of the following:

• An expansion vessel (to absorb expanding hot water) 
• A pressure relief valve
• Twin thermostats (this combines an operating thermostat with a high-limit safety cut-out that automatically shuts off the heat source if temperatures approach boiling point).

Vented hot water cylinders

A vented cylinder is a traditional gravity-fed system that relies on a cold water storage tank located in the loft.

Cold water drains down from the attic tank into the cylinder. As the water heats and expands, excess air and steam escape safely through an open vent pipe that routes back up to the loft cistern.

Vented cylinders are reliable and cheaper to install. They work well in older homes with delicate pipework that cannot handle high mains pressure. 

However, water pressure at your taps depends on the vertical distance between the loft tank and the outlet. The result is often a weaker shower flow unless you fit a booster pump.

What size hot water cylinder do you need?

# of Rooms & Bathrooms	Indirect Hot Water Cylinders	Direct Hot Water Cylinders
1 Bedroom & 1 Bath/Shower	75/120	120/150
2 Bedroom & 1 Bath/Shower	150	180
2 Bedroom & 2 Baths/Showers	180	210
3 Bedroom & 1 Bath/Shower	180	210
3 Bedroom & 2 Baths/Showers	180	210
4 Bedroom & 1 Bath/Shower	180	210
4 Bedroom & 2 Baths/Showers	210/250	250/300
Larger	300+	300+

Picking the right cylinder capacity ensures you don’t run out of hot water and avoid wasting energy heating an oversized tank. 

Cylinder size is measured in litres and depends on the number of bedrooms, bathrooms, and residents in your property.

Here is a general sizing guide for domestic hot water cylinders:

• 1 bedroom with 1 bath or shower: 120 to 150 litres (direct electric) or 90 to 120 litres (indirect gas boiler)
• 2 to 3 bedrooms with 1 bathroom: 180 to 210 litres
• 3 to 4 bedrooms with 2 bathrooms: 210 to 250 litres
• 4 or more bedrooms with multiple bathrooms: 250 to 300+ litres

If you have a high-flow rain shower or a large family bath, select a cylinder at the higher end of these capacity ranges.

Hot water cylinder costs

Cylinder costs vary depending on:

• Type
• Size
• Brand
• Location (for install costs).

For the unit itself;

• A vented cylinder typically costs £200–£500 (plus installation: £300–£600) to fit
• An unvented cylinder costs £400–£900 (plus installation £500–£1,000).

If you’re on an Economy 7 or other off-peak tariff, pairing your cylinder with a timer to heat water overnight can significantly reduce ongoing running costs.

How hot water cylinders act as smart energy batteries

A standard 200-litre hot water cylinder stores roughly 10 kWh of usable thermal energy.

Rather than relying purely on gas, modern smart cylinders can integrate with dynamic green energy tariffs and rooftop solar panels.

Using a solar immersion diverter, excess electricity generated by your solar panels is automatically routed to heat your cylinder instead of being exported back to the grid.

Smart cylinders can also link with dynamic off-peak tariffs. They automatically charge with cheap, green electricity during off-peak night hours, or when wind power generation is high. 

And this gives you low-cost hot water whilst helping balance the national energy network.

Insulating and maintaining your cylinder

Proper insulation is essential for keeping stored water hot and reducing your energy bills.

Modern cylinders come fitted with a thick layer of polyurethane foam insulation, which keeps heat loss low.

If you have an older uninsulated copper cylinder, fitting an 80mm thick lagging jacket can pay for itself in just a few months by preventing standby heat loss. 

You should also check that the hot water pipes connected to the cylinder are lagged with foam pipe insulation.

To keep your cylinder running efficiently:

• Check the thermostat setting: Keep your cylinder thermostat set to 60°C. This is hot enough to kill harmful Legionella bacteria whilst preventing scald risks and reducing limescale build-up.
• Service unvented safety valves annually: Unvented cylinders must be checked yearly by a qualified Gas Safe or G3-certified engineer to test the expansion relief valves and check the internal pressure controls.

In hard water areas, limescale builds up on internal heating coils and immersion elements. Fitting an inline scale reducer helps protect the tank and maintains fast reheating speeds.

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