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 always to leave your hot water on. But is that true? Take a look at why you should only turn on your hot water just before it’s needed.

Hot water – 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.

Now, you may be thinking why would anyone ever opt for a system boiler? Who wouldn’t want hot water on demand? The fact is, combi boilers are only suitable in homes where the water pressure is right. Plus, they don’t always suit larger families where they may need to run hot water from several taps at once.

Constant or timed?

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, after a brief meeting with a professional, I was left questioning that assumption.

Big Phil says to leave the hot water on constant

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 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:

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.

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 cheaper to always leave the hot water on?

It is a myth that always leaving your hot water on will save you money. Unless you need an endless supply of warm water, your boiler will constantly be heating when there is no real need. It is cheaper and more energy-efficient to switch your boiler on, only when you need hot water.

But, as you can see from my experiment above, the increased cost isn’t huge. £1.67 isn’t really a figure that most people will be concerned with.

Bear in mind that your results may differ slightly. The better insulated your tank, the lower the cost 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. If you’re paying far more than 3p per kWh, leaving the hot water on could be significantly more than £1.67 per month.

Please note that this post was written before 2022. Since then, we have seen a massive increase in prices. Although that doesn’t change the amount of gas used, it does impact the cost. I would strongly recommend only heating your water when you need it.

Final tip for keeping costs down

To ensure that you’re not paying more than you should for your hot water, check that your boiler has the right settings and that water is being heated more than it needs to be.

Octopus Energy suggests

• A combi boiler to have a flow temperature of 50°C for hot water and 55°C for heating
• A boiler and a hot water cylinder – set the flow temperature on your boiler to a few degrees over 60°C for heating and hot water, and set your hot water cylinder to 60°C 

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.

Why not look for the best ways to stop condensation from forming on your windows?

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