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Is It Cheaper To Leave Hot Water On All The Time?

If you have a hot water tank, you may have heard that it’s cheaper to leave your hot water on all the time. But is that true? Take a look at why you should only turn on your hot water, just before it’s needed.

should you leave hot water on all the time?

Is It Cheaper To Leave Hot Water On All The Time?

There are several deep, meaningful questions that we often ask. Is there a god? How did the universe start? Should I leave my hot water on constant or timed? I know the answer to all three, but today, I’ll be giving you the answer to the most important… or at least the only one you might listen to me about.

Do you even need to worry?

If you’ve just moved into a new house, or have never looked at your heating system, you might not even know if you need to turn your hot water on or not. Most homes either have a combi boiler or a system boiler.

If you have a combi boiler, switching your hot water on and off isn’t even something you need to consider. These types of boiler work on demand, so the moment you turn a tap, it will start producing hot water.

However, if you have a system boiler, this will come with a hot water cylinder which you need to heat up when you need your hot water. That means either predicting when you’ll need to use hot water or leaving it on constant.

Energy saving… or so I thought

There doesn’t seem to be a year go by without us hearing that energy prices are on the increase. And that means every year we need to be more careful about how we use our gas and electricity. I had always assumed that I was being frugal by only turning on the hot water when needed. However, I was left questioning that assumption after a brief meeting with a professional.

Big Phil

When I last had my boiler serviced, the engineer “Phil” – that’s what we’ll call him, as that was his name, seemed surprised that I left my hot water on a timer. He told me that it wouldn’t cost me any more to leave it on all day.

He didn’t really explain why so I was left a little confused, and I didn’t want to argue with him because he hadn’t given me the bill yet.

So, I went and did some research on the internet and found there were two schools of thought.

1. Leave hot water on permanently. Your boiler doesn’t have to heat the water from cold, so it’s far more efficient.

2. Only heat the tank when needed. Otherwise, you’re burning energy when you don’t need to.

My lovely looking water tank:

is it cheaper to leave the hot water on constant?

The experiment

I decided to put these two views to the test. Over July and August, using my handy new smart meter, I went about discovering which method would save me the most money.

Now, I am a trained scientist. I have a C in double science G.C.S.E. Read A Brief History of Time for five minutes (before it got too heavy) and watched a fair amount of the Big Bang series. As you can see, there aren’t many more qualified people.

Why July and August?

Both have 31 days (I checked with my kids). Also, I don’t use any heating during the summer months, making the experiment far easier to measure. Plus, we weren’t going away for any period during these months. Our holiday is booked for sunny November – thanks for asking.

The variables

These are things that can affect the results.

1. The number of baths. This is where we use the most hot water. We are definitely a bath family and very rarely use the shower. For this experiment, I made sure that we had exactly the same number of baths. This is actually harder than it sounds and ended with me running some unused baths!

2. Washing-up. If we haven’t turned the water on for baths, we use the kettle to boil water and use this to wash-up. So to stop any discrepancies, the washing-up was done using a kettle for both months. And for those interested, on average it costs 2.5 pence to boil your kettle (not the most financially efficient way to wash-up).

3. The Hob. The only other gas device we have is the hob. It isn’t used much and not for long. So I haven’t taken this into account for the results.

So, how much does it cost to leave hot water on constant?

I was a little surprised by what I found.
July (the month where the water was turned to constant), we used 253 kWh.

August (where water was only turned on when needed), we used 199 kWh.

gas usage when leaving hot water on

So leaving the water on constantly meant we used over 25% more gas. Sounds quite a bit doesn’t it? But what does that translate to in cost?

For gas, we pay 3.071p per kWh.

So, in July, that meant we spent £7.78 on heating water and in August £6.11.

So, is it more efficient to leave hot water on all the time?

As expected, if you leave hot water on permanently, it does cost more. However, £1.67 is quite a small difference. I know this site is about money saving, but I also like convenience. If it costs less than £2 per month to have hot water on tap, then it’s a price I’m willing to pay.

Bear in mind that your results may differ slightly. The better insulated your tank, the lower the cost will be to keep your water warm for longer. And the amount you save or spend will depend on how much you pay per unit. Take a look at your latest energy bill to see how much you pay for gas. Obviously, if you’re paying far more than 3p per kWh, then leaving the hot water on could be significantly more than £1.67 per month.

If you’re interested in keeping a closer eye on your energy usage, take a look at smart meters, and why I don’t think they’re that useful… apart from if you’re conducting experiments on whether it’s cost-efficient to leave your hot water on all the time.

Or, take a look at how you could cut your bills and earn £50 by switching to Octopus Energy.

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Friday 11th of May 2018

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 :)

Money Saver Pete

Friday 11th of May 2018

Glad to help Victoria. One day I'll do a post trying to work out how much teenagers increase your water bill by!

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