A guide to hot water cylinders

Throughout the UK, many of us rely on a hot water storage tank to heat our homes and supply warm water to our taps.

Choosing the right hot water cylinder for your home, however, can be a tricky task due to the wide range of appliances on offer. To help you, we've put together this guide. Here, we explore the different types of hot water cylinders and their benefits so you can understand what's suitable for your property.

So, if you don't know unvented from vented hot water cylinders, we've got you covered.

What is a hot water cylinder?

A hot water cylinder is a large, insulated tank that stores hot water after a boiler system has heated it via a separated coil within it. The cylinder also has a thermostat, amongst other controls attached to it. This ensures your hot water remains at a temperature that inhibits legionella bacteria growth and at a reasonable temperature so it's ready to use whenever you need it without keeping your central heating system running all the time.

What are the types of hot water cylinders?

Hot water cylinders in the UK generally fall into two categories: vented and unvented. The most suitable cylinder will vary depending on several factors, like your boiler system and demand for hot water.

The main difference between vented and unvented hot water cylinders is how water enters each system. While water is in the systems, the heating process works in the same way - cold water is fed into the base of the cylinder and rises as it is heated before being piped around the house.

What is an unvented hot water cylinder, and how does it work?

Unvented hot water cylinders operate directly from the main water supply, resulting in a powerful flow and negating the need for a cold water storage tank.

Another benefit to unvented hot water cylinders is that they can be retrofitted to existing pipework. And, because the water is taken straight from the mains, the tanks can be located pretty much anywhere in a property.

What is a vented cylinder, and how does it work?

A vented hot water cylinder relies on gravity and static head pressure to produce the flow rate of hot water to your shower and hot water taps. In these systems, water is fed from a cold water tank, that's typically kept in the loft, through a pipe into the hot water cylinder.

Because it relies on gravity, vented hot water cylinders must be located below the cold water tank to generate pressure to flow through the property.

What's the difference between direct and indirect hot water cylinders?

When deciding between vented and unvented hot water cylinders, you'll also have to choose whether you want a cylinder that’s heated directly or indirectly.

A direct hot water cylinder heats water through an immersion heater found inside the cylinder, so it is not connected to a boiler.

An indirect hot water cylinder heats water through a coil heated by an external heat source, i.e. a boiler.

How do you choose the right hot water cylinder?

If you want a hot water cylinder installed in your home, there are things you need to consider. 

Unvented cylinder systems take up less space as they don't require a water tank, giving you greater flexibility over where the cylinder can be installed in your home. If you live in a larger property with multiple bathrooms, an unvented cylinder might be the best solution for you as it will give you mains pressure hot water across multiple outlets. If fitting an unvented cylinder, you will need an unvented qualified professional to come and survey the property to ensure you have the correct cold water mains and flow rate for it to operate.

On the other hand, if you have a vented hot water cylinder and, for whatever reason, the mains water needs to be turned off, you'll still have access to limited hot water for a while. You also need to think about space for the additional header tank as well as how much water pressure you want at your hot water outlets. 

Before making a decision on which system is best for your home, we recommend thinking about the following things.

1. Your home's demand for water
2. The mains water pressure
3. The room required to store both vented and unvented cylinders
4. The changes that may need to be made to your current hot water system
5. Whether you need a direct or indirect cylinder

What size hot water cylinder will you need?

Some people may worry that having a hot water storage cylinder means you'll run out of water while waiting for it to fill up again, but if you select the right size for your property this shouldn't happen. 

Getting the right size hot water cylinder is crucial. If it’s too big, you'll waste energy heating and storing more water than you need. But, if it’s too small, the cylinder won't be able to keep up with your demand for hot water (and no one likes rinsing the shampoo out of their hair in a cold shower).

Unfortunately, there is no foolproof answer to hot water cylinder sizes, as each household will have different water consumption habits. As a general guide, allow around 35-45 litres for every occupant, which would equate to a cylinder with a capacity of between 130 and 170 litres for a family of four or five.

It's good to know that vented and unvented hot water cylinders are available in a wide range of sizes, varying anywhere between 74 litres (16 gallons) and 450 litres (99 gallons), so you’ll be able to choose one that best suits your household’s needs.

How to insulate your hot water cylinder

Insulating your hot water cylinder is one of the easiest ways to save both energy and money. Along with speeding up the heating process, it maintains the desired operating temperature. 

By insulating the tank, you can significantly reduce the heat that escapes - making the cylinder more energy efficient and reducing your bills.

To insulate your hot water cylinder, you can invest in cylinder 'jackets', which fit over the tank and pipes to reduce heat loss and keep your water hotter for longer.

Pros and cons of hot water cylinders

Pros of hot water cylinders

• Suitable for larger homes
• Provide a backup source of water
• Can be powered by renewable energy sources, i.e. solar panels

Cons of hot water cylinders

• Performance varies on water pressure
• Complex and expensive installation process
• Limited hot water supply
• Unvented cylinders require specific qualifications to be installed, maintained and repaired

Home heating help from BOXT

Getting the right central heating system for your home can be tricky, with so many options on the market and so much riding on your decision. 

At BOXT, we’re working to make heating your home stress-free, one new boiler at a time. Tell us a few quick details about your home and current heating system, and we’ll provide you with personalised quotes in minutes. It's that easy.