Hot Water Cylinders: Are You Wasting Money Heating Water You Don’t Use?
If your home has an airing cupboard with a large insulated tank in it, you have a hot water cylinder — and there is a good chance it is quietly costing you more than it should. With the Ofgem price cap still keeping unit rates high through summer 2026, the hot water cylinder cost uk homeowners actually pay comes down to two things almost nobody checks: when the tank heats, and how hot it gets.
The single biggest saving with a hot water cylinder is almost never a new tank — it is setting the timer and the thermostat correctly. A cylinder that heats on a sensible schedule to 60°C costs far less to run than one left heating around the clock or set hotter than it needs to be, and fixing both takes about ten minutes and zero money. Everything below is about getting those two settings right before you spend anything on hardware.
A quick note on scope: this covers vented and unvented hot water cylinders in owner-occupied UK homes heated by gas, oil or electric immersion. If you have a combi boiler with no tank, your water is heated on demand and most of this does not apply — though the timer logic for a system boiler still does.
How your cylinder is actually being heated
Most cylinders are heated one of two ways, and your bill depends heavily on which.
The cheaper route is your boiler. A gas or oil boiler heats the water in the tank through a coil, usually on a timer you set at the programmer. Gas costs roughly a quarter of what electricity does per unit, so heating a tankful by gas is far cheaper than the alternative.
The expensive route is the immersion heater — the electric element that sits inside the cylinder, switched on at the wall, often with its own timer. It is there as backup, and it works, but it heats water using full-price electricity. Running an immersion heater as your main hot water source, rather than as a backup, is one of the most expensive habits in a UK home — and thousands of people do it without realising, simply because a switch got left on years ago.
What an immersion really costs
A typical immersion element draws around 3kW. At the current price-cap electricity rate of around 26p/kWh, an hour of immersion heating costs about 78p. Leave it on for three hours a day and you are spending in the region of £2.35 a day, or close to £860 a year, for hot water you could heat far more cheaply on gas. Even used as a top-up, an immersion left switched on permanently is the most common reason a cylinder bill looks wrong.
The two settings that matter most
The timer: heat it when you use it
A cylinder holds heat for hours, so you do not need to heat it continuously. For most households, two short heating windows work — one before the morning routine and one before the evening — rather than leaving it on all day. If the water is still hot when you come to use it, your windows are too long; shorten them until you occasionally run out, then add ten minutes back.
The thermostat: 60°C, no hotter
Your cylinder thermostat should be set to 60°C, and no higher than 65°C. That is hot enough to kill legionella bacteria, which is the safety reason it should never be set lower than 60°C. But there is no benefit to setting it to 70°C — you simply lose more heat through the tank wall and pay for water so hot you have to add cold to use it. If yours is cranked up, turning it down to 60°C costs nothing and trims standing losses.
Is your cylinder worth keeping?
A well-insulated, correctly set cylinder is not a problem to be eliminated. Modern cylinders come with thick factory-applied foam lagging and lose very little heat. If yours is an older copper tank with a thin red jacket — or no jacket at all — fitting an 80mm British Standard cylinder jacket, which costs around £15 to £20, pays for itself within months.
Replacement is a bigger decision. An unvented cylinder (mains-pressure, no loft tank needed) typically costs in the region of £1,000 to £2,500 supplied and fitted in 2026, and must be installed by a G3-qualified engineer. That is worth it if your tank is leaking, badly undersized, or you are renovating anyway — but rarely worth it purely to save energy if the existing tank is sound.
When a cylinder beats a combi
It is fashionable to rip out cylinders for combi boilers, but a cylinder still wins in two cases: homes with more than one bathroom in regular simultaneous use, and homes planning a heat pump, which need a cylinder to work at all. If a heat pump is on your horizon, keep the cupboard space. See our guide on heat pumps versus boilers for how the cylinder fits in.
What to do in the next 30 minutes
• Find your cylinder and check whether the immersion switch at the wall is on. If it is and you have a working boiler, switch it off and see if you still get hot water.
• Set the cylinder thermostat (a dial strapped to the tank, often behind the jacket) to 60°C.
• Set your hot water timer to two short windows a day instead of “always on” or a long single block.
• Feel the tank: if it is warm to the touch through the insulation, it is losing heat — note whether it needs a jacket.
• Note today’s settings somewhere you will find them again.
Track it so the saving sticks
The reason cylinder savings slip away is that nobody remembers what the settings were, so the next person nudges the immersion back on. Log your immersion settings in Home+ — your timer windows, your thermostat temperature and whether the immersion should stay off — so the whole household knows the cheap setup and you can spot if your bill creeps back up. You can do it free when you sign up to Home+, alongside the rest of your home’s energy notes and our guide to energy and EPC basics.
Frequently asked questions
Is it cheaper to leave the hot water on all day or heat it twice? For almost everyone, two short timed windows is cheaper. A cylinder holds heat for hours, so heating it continuously just pays for standing heat losses you never use.
What temperature should a hot water cylinder be set to? 60°C, and up to 65°C. Below 60°C risks legionella bacteria; above 65°C wastes energy through extra heat loss with no benefit.
Should I use the immersion heater or the boiler? The boiler, in nearly all cases. Gas costs roughly a quarter of electricity per unit, so an immersion is best kept as a backup for when the boiler is down.
How much does it cost to run an immersion heater? A 3kW immersion costs roughly 78p an hour at current electricity rates — which is why leaving it on permanently is so expensive.
Do I still need a cylinder if I want a heat pump? Yes. Heat pumps run at lower flow temperatures and need a cylinder to store hot water, so keep the space if a heat pump is on your plans.
