Is it cheaper to leave hot water on all the time?

One question that is frequently asked is whether it is cheaper to leave the heating system hot water on all the time, or whether it is better to only heat the water when you need it. In fact, this question sometimes goes without being asked at all because many people assume that it is simply better to leave the hot water on all the time. This is a falsehood that has become a popular myth.

The truth of the matter is that it is very unlikely to be cost effective to leave the water on all the time, unless you are using it constantly, and then you’ll be spending a lot of money on your hot water regardless. In fact, if you are looking to save some money on your energy bills, ensuring that the hot water is only heated when and if you need it can save you money overall.

Some of the main misconceptions on this subject

More energy is used to heat the hot water up from scratch than it does to keep it topped up with heat. It is technically true that the hot water boiler will use more energy when it is heating up water from cold, but it doesn’t hold true that the hot water bill will be cheaper if you keep the water temperature topped up, since the hot water in your system is constantly in a state of losing its core temperature and will effectively require heating most the time. This means that the boiler will constantly be trying to keep the hot water warm. For a household that uses an awful lot of hot water, then this may prove to be a better solution, at least in terms of having ready access to hot water, (and general convenience) but for most homes it is better to have the hot water available just when you use it and not at other times.

There are two reasons for this. The first is that typically a household will only use the hot water at a certain time of day for a couple of hours. For example, having a shower in the morning, or doing the dishes in the evening. The other reason is that when the hot water is heated, if the hot water tank has proper insulation the hot water should remain warm for a fairly long period of time. In fact, it can remain warm enough for most of the day, in many household circumstances. Some individuals choose to put the hot water on for an hour in the morning and an hour in the early evening. It is good idea to experiment to discover what works best in your own particular case.

Another myth is that devices such as washing machines and tumble dryers require the hot water to be on for them to operate correctly. This isn’t necessarily true and in the UK most dishwashers and washing machines are capable of heating the water themselves and only need to be connected to the cold water supply in a house. Another appliance not require any hot water is an electric shower since these tend to work by heating the water as and when it is needed. Again, for those individuals heating water for these devices, then money is being wasted here since it isn’t necessary.

We mentioned that the hot water should remain warm in the boiler as long as the tank is well insulated. This is an important aspect in terms of both keeping the hot water warm for a long period of time and making the most out the energy spent heating up the water in the first place, but also to ensure that money isn’t wasted re-heating the water more frequently than might normally be necessary. Heat loss from a poorly insulated tank can result in the temperature falling quickly. Alternatively when a tank is insulated well, you'd be surprised how long a tank of hot water can retain its temperature. In addition to this the larger tank you have, the longer the water will stay at a higher temperature. The downside to this is that heating up a full tank on a bigger system will be more expensive.

Sometimes it can be easier just to use the immersion heater on your boiler if you need infrequent hot water. Just as long as your remember to turn off this separate water heater once you're done. Water heating using the electric immersion in this way can be efficient if you do not use much hot water, or need it irregularly.

Checking your hot water and boilers

So, this is a good area to check on your own boiler and hot water system especially if it is quite an old one and the insulation has been in place for a while. If you notice that your hot water cools down quickly after it has been heated, then this may be costing you extra money in the long run. In these situations then it is worth getting the advice of a plumber or heating engineer to ascertain whether the hot water cylinder needs replacing or not. We stock both vented and unvented cylinders depending on the type of system you have got. In some cases it can be more efficient in the long run to simply buy a new boiler system. Especially if your boiler is very old because the latest models are more efficient boilers. A heating installer can help you ascertain whether you need a new boiler or not.

It should be noted that this doesn’t apply to modern combi boilers since this type of boiler heats the hot water as and when is necessary and doesn’t keep it topped up in the same way as a traditional boiler would.

So, in summary, if you’re been wondering whether to heat your hot water once or twice a day from scratch, or keep your central heating system and hot water ticking over, and saving some money is your primary consideration then you should consider just heating the water when you need it, as it is the most efficient way.

Find out more advice with our how to guides.

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