Should You Turn Off Your Unvented Cylinder at Night to Save Energy?

If you’re trying to cut down on your energy bills or just curious about how your hot water system works, you might have asked yourself this simple question:

Should I turn off my unvented hot water cylinder at night?

The short answer: Not necessarily. In many cases, turning it off won’t save as much energy as you think and it could even end up costing you more.

Let’s explore why that is, how unvented systems work, and the smartest ways to save money on your hot water without compromising comfort.

How an Unvented Cylinder Works

An unvented hot water cylinder stores heated water at mains pressure. It’s sealed, insulated, and heated by either:

• An immersion heater (usually electric)
• A boiler (gas, oil, or system boiler)

Because it’s pressurized and sealed, the hot water doesn’t need to be stored in the loft like a vented system, and you get strong flow rates from every tap and shower.

Most modern cylinders also have very high levels of insulation, which reduces heat loss significantly overnight.

What Happens If You Turn It Off?

Turning off your cylinder at night stops the immersion or boiler from reheating the water. That might seem like a great way to save energy, but the cylinder still loses a small amount of heat over time.

In the morning, you’ll likely find the water isn’t as hot, and your system will have to work harder and longer to reheat it.

So, while you may save a little by avoiding overnight reheating, you might spend the same (or more) reheating it from a lower temperature when you need it again.

When It Might Make Sense to Turn It Off

There are a few specific scenarios where turning off your unvented cylinder overnight can save money:

• If you’re on an Economy 7 or Economy 10 tariff: These off-peak tariffs give you cheaper electricity at night. In this case, it’s actually better to heat your water overnight, not during the day.
• If you’re going away or won’t need hot water for a while: Going on holiday? By all means, turn it off. Just remember to switch it back on a few hours before you need hot water again.
• If your cylinder has a programmable timer: You can set it to run only when you need hot water (early morning and evening, for example) without heating all night long.

What About Leaving It On?

In many cases, leaving your cylinder on continuously doesn’t use much more energy, especially with modern insulation. The system only tops up the temperature when necessary.

Some immersion heaters also include thermostats that shut off the heater once the water reaches a set temperature, reducing waste.

So, unless you’re constantly using hot water, your cylinder likely only needs brief heating cycles to maintain temperature.

Best Practices to Save Energy with an Unvented Cylinder

• Check your cylinder insulation: Most modern units lose less than 1 kWh of heat per 24 hours.
• Use a timer: Heat water only during periods of high use (like 6 to 8 a.m. and 5 to 7 p.m.).
• Lower the thermostat: Most cylinders are set to 60 to 65°C, but you might be able to reduce it to 55°C safely (ask a professional first).
• Use less hot water: Install low-flow taps or showerheads, and fix drips quickly.
• Consider solar or thermodynamic upgrades: Supplementing with renewables can cut costs significantly.

Should You Turn It Off?

If you have a well-insulated, modern unvented cylinder (and you’re not on a special electricity tariff), turning it off at night probably won’t save you much.

In fact, it might lead to longer reheating times and reduced comfort in the morning.

Instead, focus on using a timer, insulation, and smart temperature controls to get the best balance between comfort and efficiency.

Your hot water system works hard to give you a great shower, why not let it work smarter, too?