One of the most common household money questions has very little to do with heating and everything to do with hot water. Should you leave the hot water on all the time, or switch it on for an hour or two with a timer? It depends entirely on how your home makes its hot water, and a surprising number of people are trying to save money on a system where the timer question does not even apply. This guide works out which type you have, where the cost actually goes, and the handful of settings that trim real money off the bill.
The short answer. If you have a combi boiler with no tank, there is nothing to leave on, so a hot water timer saves you nothing and just gives you cold taps. If you store hot water in a cylinder, a timer is usually a little cheaper than leaving it on constantly, because a tank that is allowed to cool loses less heat overall than one held hot around the clock. For a well-insulated modern cylinder the saving is small, often around £10 to £15 a year. For a poorly insulated tank it is larger, but the better fix there is a cylinder jacket, not fiddling with the timer.
The first thing to settle: tank or no tank
Homes make hot water in one of two ways, and they behave completely differently. A combi boiler heats water on demand, straight from the mains, the moment you open a tap. There is no cylinder anywhere in the house. A stored system, which means a regular or system boiler feeding an insulated hot water cylinder, or an electric immersion heater in that cylinder, keeps a tankful of hot water ready for when you want it.
That difference decides the whole question. With a combi there is no standing tank of water quietly cooling, so the timer debate does not apply to hot water at all. With a cylinder there is, and keeping that tank hot has an ongoing cost whether you draw any water off or not. Before you touch a single setting, look in the airing cupboard. If there is a large insulated tank, you store hot water. If there is not, you have a combi and you can stop worrying about hot water timers entirely.
Combi boilers: the timer question does not apply
On a combi, leaving the hot water on costs essentially nothing when no tap is running. The boiler sits in standby drawing a few watts to keep its electronics and controls alive, and that is the same whether you have set it to summer mode or not. There is no reservoir of hot water being kept warm, so there is nothing to switch off to save money.
Switching hot water off at a combi does not cut your bill; it just means you get cold water until you turn it back on, and then a short wait while the boiler fires. The savings that combi owners can chase live elsewhere: the central heating timer, a sensible flow temperature and shorter or cooler showers. If you have a combi, leave the hot water on and put your attention into the ideas in our hot water savings guide instead.
Stored hot water: where the money actually goes
With a cylinder, the cost splits into two parts. One is the energy to heat the water you actually use, for showers, baths and washing up. That part is fixed by how much hot water you draw, and no timer changes it. The other part is standing heat loss: the warmth that leaks out of the tank into the surrounding cupboard, hour after hour, whether you use any water or not. That is the part the timer question is really about.
How much a tank loses depends almost entirely on its insulation. A modern cylinder with thick factory-fitted foam might lose roughly 1 to 2 kWh a day. An older copper cylinder with a thin quilted jacket, or none at all, can lose 4 kWh a day or more. Every one of those lost units has to be put back in by the boiler or immersion, and that is what you are paying for when the tank is simply sitting there staying warm.
So is a timer actually cheaper?
Usually yes, but by less than most people expect if the tank is well insulated. The reason comes down to a quirk of how things cool. A hot tank loses heat fastest when it is at its hottest, because the temperature gap between the water and the cool cupboard is largest. As the water cools, the gap shrinks and the losses slow down. Hold the cylinder hot 24 hours a day and you keep that gap at its maximum the whole time, so the total standing loss over a day is as high as it can be.
A timer that heats the tank to coincide with your morning and evening use, then lets it coast for the rest of the day, spends fewer hours at full temperature. Less time hot means a smaller average temperature gap, which means less heat leaks out in total. The catch is that the size of this saving tracks the quality of the insulation. On a well-lagged cylinder the two approaches are within a kWh or so of each other over a day. On a badly insulated one the timer saves more, but that same tank is haemorrhaging heat all day regardless, and the real answer is to fix the insulation first.
A worked example
Take a reasonably insulated cylinder losing about 2 kWh a day when held hot constantly. Put it on a timer that heats the water for roughly an hour in the morning and an hour in the evening, and it spends much of the day cooler, cutting the standing loss to something like 1.5 kWh a day. The difference is around 0.5 kWh of gas a day. At an example gas price of 7p per kWh, that is about 3.5p a day, or roughly £13 a year.
Now take an old, barely insulated tank losing 5 kWh a day. A timer might trim that to around 3.5 kWh, saving about 1.5 kWh a day, closer to £38 a year at the same example rate. That sounds like a strong case for the timer, until you notice that a cylinder jacket costing £15 to £20 cuts the loss on that same tank by a similar amount every single day, timer or no timer, and keeps doing so for years. You can put your own tank size, temperatures and prices into the running-cost calculator to see where you land.
Insulation changes the whole answer
Because standing loss is the entire game, insulation matters far more than the timer setting. A modern foam cylinder is usually well covered already, but an older tank often is not. A British Standard cylinder jacket at least 80mm thick, wrapped snugly and taped at the joins, is one of the fastest-paying jobs in the house. Lagging the first metre or so of pipe coming off the top of the cylinder helps too, because that is where a lot of heat sneaks away.
Once a tank is properly wrapped, the gap between running it on a timer and leaving it on shrinks to loose change, so the sensible order is clear: insulate first, then set a timer, rather than agonising over the timer while the tank sits there uninsulated. We cover the job and its payback in our guide to cylinder jackets and pipe lagging.
Immersion heaters and electric-only homes
If your cylinder is heated by an electric immersion rather than a boiler, the timer stops being a nicety and becomes the main event, especially on an Economy 7 or similar off-peak tariff. Heat the tank overnight on the cheap rate, let good insulation carry that heat through the day, and you avoid ever topping it up at the expensive daytime price. Leaving an immersion on constantly at the peak rate is one of the pricier ways to make hot water there is.
One safety point cuts across all of this. Stored water that sits lukewarm for long periods can grow legionella bacteria, so do not set the cylinder thermostat too low in the hope of saving a few pence. Aim for the stored water to reach around 60C, at least on a regular cycle, which is hot enough to keep the tank safe. Our guides to the cost of running an immersion heater and Economy 7 and night rates go through the timing in more detail.
Settings that beat both extremes
The cheapest hot water rarely comes from picking a side in the timer-versus-constant argument. It comes from a few settings working together:
- Insulate the cylinder and pipes first. This is the biggest single lever and it works no matter which control strategy you use.
- Set the cylinder thermostat to about 60C. Hot enough to be safe and to give a decent volume of usable water, without wasting energy keeping it scalding.
- Use a timer that matches your routine. One or two short heat-ups a day, lined up with when you actually shower and wash, beats both a constant setting and a tank you keep reheating from stone cold.
- Do not chase pennies into the cold. Letting a big tank go completely cold and then reheating the whole thing can cost more than a timed top-up, and it introduces the legionella risk above.
- If it is a combi, leave hot water on and look elsewhere. Your savings are in the heating controls and shower habits, not a hot water timer.
The bottom line
For a combi, the whole question dissolves: leave hot water on, because there is nothing to switch off. For a stored cylinder, a timer is generally a little cheaper than running it constantly, but the size of that saving is set by the insulation, not the schedule. Wrap the tank, set the thermostat to a safe 60C, run a timer that fits your day, and you will have squeezed out nearly all the saving there is to get. Everything past that is small change, and probably not worth the cold showers.