Is It Cheaper to Leave Hot Water On All the Time? (UK Test)
If you’ve got a hot water tank, you’ve probably heard the old advice that it’s cheaper to leave the hot water on all the time.
Sounds convenient. Sounds logical.
It’s also mostly rubbish.
If you have a hot water cylinder, it is usually cheaper to heat it only when you need it. Leaving hot water on constantly means your boiler keeps topping up heat that slowly leaks out of the tank.
With energy prices where they are in the UK, that wasted heat adds up faster than it used to.
I actually tested this properly in my own house, and the results weren’t even close.
Quick answer: Is it cheaper to leave hot water on constant?
No. In most UK homes with a hot water tank, it is cheaper to heat the water on a timer rather than leaving it on constant.
When hot water is left on all day, the tank slowly loses heat and the boiler repeatedly reheats it. Using a timer means the water is only heated when needed, reducing gas usage.
In my own test over two months, leaving the hot water on constant used around 25% more gas than heating the tank only when required.
Hot Water – Do You Even Need to Worry?
Before you start fiddling with timers, it helps to know what system you’ve got.
Most UK homes fall into one of two camps.
If you have a combi boiler, you can stop reading right now. There’s no hot water tank. Water heats instantly when you turn the tap on, so there’s nothing to leave “on” in the first place.
If you have a system boiler or regular boiler, you’ll have a hot water cylinder. That tank stores hot water and needs heating in advance.
That means you have two options:
• Leave the hot water on constant
• Use a timer to heat it when needed
This is where the debate starts.
Is It Cheaper to Leave Hot Water on All the Time?
There doesn’t seem to be a year that goes by without energy prices rising.
So naturally people look for small ways to reduce gas usage. I’d always assumed I was being sensible by only heating the hot water when we needed it.
Then my boiler engineer planted a seed of doubt.
Also worth reading
If your energy costs continue to rise, find some easy ways to reduce your heating bills.
Big Phil Says Leave Hot Water on Constant
When I last had my boiler serviced, the engineer (we’ll call him Phil… mainly because that was his name) seemed surprised that I used a timer for hot water.
He told me it wouldn’t cost any more to leave the hot water on all day.
He didn’t really explain why, and I didn’t fancy arguing with him while he was still holding the invoice.
So I did what any sensible person would do.
I tested it myself.
The Two Schools of Thought
After doing a bit of digging online, I realised there are two common opinions.
1. Leave hot water on permanently
The argument is that heating water from cold takes more energy, so keeping it hot all the time is more efficient.
2. Only heat the tank when needed
The opposing argument is that heat constantly escapes from the tank, so leaving it on just wastes energy.
My water tank was about to settle the argument.
My Hot Water Experiment
I decided to test both methods using my smart meter.
Before we go further, it’s worth noting that I’m highly qualified for this kind of experiment.
I have a C in double science GCSE, once read five minutes of A Brief History of Time before it got too complicated, and I’ve watched a fair bit of The Big Bang Theory.
So you’re clearly in safe hands.
Why July and August?
Both months have 31 days (my kids confirmed this).
More importantly, we don’t use the heating during summer. That made it much easier to measure how much gas was used purely for hot water.
Also, we weren’t away during either month. Our holiday was booked for sunny November.
The Variables
A few things could affect the results, so I tried to keep them the same.
Number of baths
We’re definitely a bath family and rarely use the shower. Baths use a lot of hot water, so I made sure we had the same number each month.
This was harder than it sounds and resulted in a couple of unnecessary baths being run.
Washing up
If we didn’t turn the hot water on, we used the kettle to wash up instead.
For consistency, the kettle was used in both months.
For anyone wondering, it costs around 2–3p to boil a kettle.
The gas hob
This was the only other gas appliance in the house, but it isn’t used much, so I ignored it.
So How Much Does Leaving Hot Water on Constant Cost?
Here’s what happened.
July (hot water set to constant)
253 kWh of gas used
August (hot water heated on a timer)
199 kWh of gas used
Leaving the hot water on constantly meant we used over 25% more gas.
That surprised me a bit.
What That Cost
The figures below are based on 5.74p per kWh for gas.
That meant:
July: £14.52
August: £11.42
A difference of £3.10 for the month.
That’s roughly £37 per year just from leaving the hot water on constant.
Not exactly life-changing money. But it’s still a takeaway.
Hot Water Setting | Gas Used | Monthly Cost |
|---|---|---|
Constant hot water | 253 kWh | £14.52 |
Timed hot water | 199 kWh | v |
In other words, leaving hot water on constant used 54 kWh more gas per month in my house.
Why Leaving Hot Water on Constant Costs More
The reason is simple: heat loss.
Even well-insulated hot water cylinders slowly lose heat. When the water cools down, the boiler fires up again to reheat it.
If the system is set to constant, that reheating happens all day whether you need hot water or not.
Using a timer means the tank only heats when you actually plan to use it.
Less reheating = less gas used.
Should You Leave Your Hot Water Tank On All the Time?
In most cases, no.
It’s usually cheaper to heat your hot water tank on a timer, especially if you only use hot water at certain times of the day.
However, there are a few situations where constant heating might make sense:
• Very well-insulated cylinders
• Homes using lots of hot water throughout the day
• People who prefer convenience over small savings
But for most households, timed heating wins.
How Long Will a Hot Water Tank Stay Hot?
A typical insulated cylinder will stay reasonably hot for 12 to 24 hours.
Modern cylinders with good insulation lose very little heat.
Older tanks can lose heat much faster, which makes leaving them on constant even more expensive.
If your tank is old, adding a cylinder jacket can significantly reduce heat loss.
How to Reduce Hot Water Heating Costs
Make sure your boiler and cylinder temperatures are set sensibly. Overheating water just wastes energy.
Octopus Energy suggests:
For a combi boiler
Hot water flow temperature around 50°C
Heating flow temperature around 55°C
For a boiler with a hot water cylinder
Boiler flow temperature slightly above 60°C
Cylinder thermostat around 60°C (important for preventing legionella bacteria)
If you want to keep an eye on usage, a smart meter can help. I’m still not convinced they’re life-changing, but they’re excellent if you’re running nerdy experiments to prove boiler engineers wrong.
If you’re trying to cut your bills further, you might also want to read:
• Whether it’s cheaper to leave your heating on constant
• How to stop condensation forming on windows and reduce heating costs
FAQs
Does leaving hot water on cost money?
Yes. If your hot water is set to constant, the boiler will keep reheating the tank as heat escapes. This means it uses gas even when you aren’t using hot water.
Should hot water be on all the time?
Usually not. Most households save money by heating the tank on a timer rather than leaving it on constant.
How long will a hot water tank stay hot?
A modern insulated tank can stay hot for 12 to 24 hours. Older cylinders lose heat much faster, which makes leaving them on constant more expensive.
Is it cheaper to leave hot water on all the time in the UK?
For most homes, no. Heating the tank only when needed usually uses less gas and costs less overall.
Is it cheaper to have hot water on constant or timed?
Timed heating is usually cheaper. Leaving hot water on constant means the boiler keeps reheating the tank as heat escapes, which increases gas usage.
Great experiment and thank you! I’ve wondered this fora couple years. I have moved to a new property that does not have a combi-boiler and have to turn on hot water as and when I need it. This hasn’t been a problem but, when my teenage son turns it on in the morning to take a shower and goes to college leaving it on all day, I absolutely go nuts! I’m thinking the boiler has been bubbling away for 7 hours!! Thanks for reassuring me 🙂
Glad to help Victoria. One day I’ll do a post trying to work out how much teenagers increase your water bill by!
Thak you for your great information
Owning a Townhouse for rent, unnocupied at the present time, the new electric water heater is on, the tank is full, can I shut off the water supply to the house during the winter ?
Hi Fernando. That question is a little out of my comfort zone I’m afraid. The best I could find was from Pete the Plumber (no relation) https://www.petetheplumber.com/turn-off-the-water-when-you-go-away/ He seems to think it will be fine.
Thanks for this useful post! Curious how long does it take for your water tank to heat up? You must need to know somewhat in advance when you plan on taking a bath or shower?
Thanks,
Hi Louisa. It’s a bit of trial and error. But my tank takes around 90 minutes before it’s heated fully, but an hour is enough for one bath.
Does this apply to heating. I’m having the same debate about heating. Would be cheaper to leave on constantly, heat controlled gas boiler central heating or turning on when needed. Is it cheaper to maintain a constant temperature, rather than trying to heat up when it gets too cold
Hi, I haven’t been able to conduct the experiment in person, thanks to the British weather. However, if your home is warmer than the temperature outside, then it will constantly lose heat. So your heating system will continue to work and your house will continue to lose heat, costing you money.
It’s best only to have the heating on during colder periods. Mine comes on in the morning and the evenings, with a boost during the day if it gets really cold.
Hi Pete, its a wonderful and informative post. I moved to a new house and has electric boiler (Tempest stainless).If I switch it off during the night time ,will it save some money ?It does not have timer on it and electric bill has increased enormously. The house has electric storage heaters and electric boiler. If any inputs from your side will be much appreciated. Thanks in advance.
Hi Shamim. Unfortunately, I don’t have any real experience with electric boilers. However, your boiler shouldn’t really be working much at night because the heat should come from the storage heaters.
Ideally, although I’m not a great fan, a smart meter might help. You’ll then get an idea of when exactly you’re using the most electricity.
But the bad news is, electric is a far more expensive way to heat your house compared to gas.
We have found that the cost of replacement timers and their installation outweighs any cost saving, and we like the convenience of having fully heated water available at all times.
Awesome experiment – thanks for the burden of undertaking this for the curious man.
Question, was this only turned on just before say a bath, or via a timer, I.e 30min in the morning and 30min at 6pm for the kids bath
Appreciated
Hi Andy. I used a timer each day, just before I needed the hot water. Otherwise, the water will cool and it can be wasted.
Hi,
I’ve just had a new heating system installed and have always wondered about this question. My suspicion was the same as your result but nice to see it tested.
Thanks .
If you use haft a tank of water at 60 and the hot water is not turned on the the 50% that is in the tank will become Diluted with cold water and the temperature of the hot will drop to around 30 or less so the hot water that was hot is now half what is was.
If you had heated the incoming water by the stat with the boiler on you would not have to reheat the hot water that was already hot.
The boiler should be at least 5% hotter than the hot water tank .
If you set you hot water tank at 65. And your boiler is set at 60 the boiler will keep going as it never gets the temperature.
What a great experiment. Thanks. I like alone every fortnight and I have a fault on the electrical plug connecting my electric water boiler. Found this out after getting a cold water shock in the shower. Brrrrrrr. I have been plugging the tank in just once a week for 4 hours. I can then unplug it for the week and have hot water all week. I wondered if I would save money doing this. I guess the answer is yes. Electrician is coming next week to fix the issue but I will stay unplugged every 2nd week I think. May do the experiment myself.
I write this laid in a hot bath after the tank has been unplugged since Sunday.
Interesting experiment.. thanks! Did you consider hot water used in hand washing in your experiment as I wonder if that impacted the results? Also is any washing machine/ dishwasher cold water feed only?
I’m stuck with a boiler that has my water hardwired permanently on due to a fault with its timer control panel years ago – it was all the plumber could do short of a new boiler – the replacement part was not available. I use the thermostat to control the heating instead. Set to 15C all summer, the heating stays off. So now I’m wondering what to do re the hot water after this experiment
We moved into a larger home and got the plumbing redone. First time living with a tank system. My plumber (old school) is in the first camp. Leave it on and let tank thermostat control the demand for more “boilage”. What a stupid effing SOB! My bill 2 months running used 7x more gas than our previous home!
One of the biggest waste of hot water is long pipe runs. Running that hot tap to fill the kitchen sink up when it’s 10m away from the tank. Same for showers. Mixer showers. When the tank temp is low you draw more heated water from the tank. A point for debate here. I have 4 showers at chez moi 3 teenagers and a builder dad so those showers are flowing. All showers have thermostatic mixers. I have one 300litre tank heated electrical. I found I could maintain a supply of hot water to all showers at peak usage by having the tank temp at 68 degrees. The element is a 3. 5 kw. Which works well and is on all the time my pipe runs are short and insulated. We’ll saying on all the time means when heating to the required temp. Modern well insulated tanks loose very little heat.
Great in-depth experiment as my plumber recently told me the same when i got my new boiler fitted. Thank-you for sharing with a bit of added humour in there too!