What Is a Smart Hot Water Cylinder, and Is It Worth It?

A smart hot water cylinder is a hot water cylinder, usually an unvented one, with extra temperature sensors and connected controls that let it heat only as much water as you need, track how much hot water is left, and time its heating to cheaper electricity or spare solar power. It still stores hot water like any other cylinder, and a gas boiler can still heat it. Whether it is worth paying extra for depends on how you use hot water and whether you have, or plan to have, solar panels, a time-of-use tariff or a heat pump.

What makes a hot water cylinder "smart"?

The tank is not the clever part; the controls are. A smart cylinder measures the temperature at several heights inside it and uses that information to decide when and how much to heat.

A standard cylinder has one thermostat, usually part-way up the tank. It knows whether the water at that point is above or below the setting, and nothing else. A smart cylinder uses several sensors, so it can estimate how much usable hot water is stored and show it in an app or on a display, often as a number of showers or a percentage. Some designs go further and heat the water from the top down, so a partly heated tank still gives you hot water at the taps without heating the whole volume. Mixergy is the best-known example of this approach in the UK, and other manufacturers now offer cylinders with connected controls of their own.

Most smart cylinders can also be told when to heat. That matters if your electricity is cheaper at certain hours, or if you have solar panels and want surplus power to go into hot water instead of back to the grid.

How is it different from a standard unvented cylinder?

Physically, most smart cylinders are unvented cylinders with the same safety equipment and installation rules. The difference is in the sensors, the controls and the connectivity.

An unvented smart cylinder still runs at mains pressure, still needs an expansion vessel or air gap, a temperature and pressure relief valve and a correctly run discharge pipe, and still has to be fitted by an installer who holds the G3 unvented qualification. The annual safety checks an unvented cylinder needs do not go away because the controls are smarter. If you are choosing between cylinder types in the first place, our comparison of unvented and vented replacement cylinders covers the basics, including what happens if your mains pressure is too low for an unvented system.

Does a smart cylinder work with a gas boiler?

Yes. A smart cylinder can be heated by a system or regular gas boiler through its coil, just like a standard indirect cylinder, with an immersion heater as backup or as the main electric heat source.

What needs care is how the controls talk to each other. The boiler should only fire for hot water when the cylinder actually asks for heat, so the smart cylinder's controls have to be wired into the heating system properly, alongside any zone valves and the room thermostat. Some smart cylinders are designed mainly around electric heating and treat the boiler coil as secondary, so ask how the one you are offered behaves on a gas system before you commit. If you have a combi boiler, there is no cylinder to replace, and adding one purely to get smart features rarely makes sense.

When a smart cylinder is worth it

A smart cylinder earns its extra cost when there is something for it to be smart about: cheap electricity, spare solar power, a future heat pump, or hot water use that changes a lot from day to day.

• You have solar PV. Surplus generation can heat water through the immersion instead of being exported, and a smart cylinder can manage that directly or alongside a solar diverter.
• You are on a time-of-use tariff. Heating water in the cheapest hours is only useful if the cylinder holds enough to see you through, and a smart cylinder shows you whether it does.
• A heat pump is likely later. Some smart cylinders are sold as heat pump ready, with a coil large enough for the lower flow temperatures a heat pump uses. Check this at the survey rather than assuming it.
• Demand varies. In a home where the number of people changes, or where the tank is often heated for nobody, heating only part of the tank can cut wasted heat.

When it is the wrong answer

For many London homes, a well-sized standard cylinder with a good thermostat and a sensible timer does the job just as well for less money.

A smart cylinder is hard to justify if you have no solar panels and no plans for any, pay one flat rate for electricity, and use roughly the same amount of hot water every day. It adds another app, another set of controls and, in most cases, a need for a reliable Wi-Fi signal in the cylinder cupboard, which is often the worst-connected spot in a flat. It also does nothing about the most common hot water complaint, which is a cylinder that is simply too small or too slow to reheat. Getting the size right, covered in our hot water cylinder sizing guide, matters more than the controls.

What to check before choosing one

Treat a smart cylinder like any replacement cylinder first: size, space, water pressure and installer qualifications. Then look at the controls.

• Size and space. Smart cylinders come in standard capacities, and the controls may need a little extra room or a separate display.
• Mains pressure and flow. An unvented smart cylinder has the same supply requirements as any unvented cylinder.
• What happens offline. Ask whether the cylinder still heats on a basic schedule if the internet connection or the app stops working.
• Compatibility with your boiler controls. Make sure the installer explains how the boiler, zone valves and cylinder controls will be wired together.
• The immersion heater. Most smart cylinders rely on one, so check its rating and how it is controlled; our guide to immersion heaters on new cylinders explains the options.

If you want to compare a smart cylinder with a standard one for your home, our hot water cylinder replacement survey can quote for both, so you can see the difference in cost and decide whether the features are worth it for you.