Turning your boiler off at night might seem like common sense, but if you’re doing it for economic reasons, read this first – you might not be saving much money at all.

Why do some people turn off their boilers at night?

The thinking goes something like this. When you’re tucked up in bed, even in the depths of winter, there’s a good chance you’ll be warm enough. With decent insulation and all your double-glazed windows closed, the home will probably stay warm enough for a few hours after bedtime, and since you won’t be using any hot water, you may as well switch off your boiler. As for summer, well, that’s a no-brainer, isn’t it?

It’s true that boilers do turn on from time to time during the night. Combi boilers work best when the heat exchanger inside is kept warm, so it will need “topping up” a few times overnight to make sure you have instant hot water when you need it. With a system or heat only boiler, it’s the cylinder that gets heated up when its temperature drops and the thermostat tells the boiler to fire up (assuming it’s not on a timer).

Many people don’t really use much hot water from the tap, either. Their dishwasher, washing machine and shower heat water on demand, and their kettle and coffee machine make their morning drinks from cold tap water, too. So apart from half an hour waiting for the radiators to warm up, is there any need to leave the boiler on overnight?

The downsides of turning it off overnight

All of the arguments above make sense. After all, if you’re not heating water, you’re not burning gas – and if you’re not burning gas, you’re not spending money.

Before you make the switch, however, consider a few things.

First, how well insulated is your home? If your have loft insulation, cavity wall insulation, double- or triple-glazed windows and good sealing around your doors and windows, your home should indeed retain a good proportion of its heat, at least for a few hours.

But if any of these are lacking, your home could drop in temperature quite quickly with no new heat being generated. That could mean damp walls as water vapour condenses on them, which will not only increase heat loss, but could also cause structural or cosmetic damage that will have a cost attached to them. In more extreme circumstances, you could get frozen pipes, which could be much more of a headache – and a costly one.

Second, there’s the cost of bringing your temperature back up to a comfortable 18–20 °C in the morning. If your home’s temperature has sunk to below 10 °C overnight, your boiler is going to have to work extra hard to warm up in the morning, whereas if the boiler was ticking over with the thermostat on, say, 15 °C, it wouldn’t have as much work to do in the morning.

That’s not to say that it would be cheaper to leave the boiler on, or that you’d use less gas – neither claim would be true. But you might not be saving quite as much as you think you are, because of the extra energy required the following day.

Using your thermostat and timer

It is entirely possible to have your boiler effectively switched off overnight without physically isolating the power. This can be done in two ways.

Thermostat

You can set your thermostat to its minimum temperature before you go to bed in the evening. If you want to be sure the heating doesn’t come on, set the temperature as low as it will go, and it’s unlikely the heating will come on unless the temperature in your house falls dramatically.

Alternatively, you can set the thermostat in its mid-range, for example 12–14 °C. Most nights, it probably won’t switch the heating on, but there will be a safeguard for extremely cold nights. That way you can help protect your home from the risks mentioned above, and on average you’ll save money.

Timer

All modern boilers come with a timer on them, usually on a separate panel that’s wired up to the boiler itself. You can set the on and off times for every day of the week. So, you can set it to switch off at, say, 11 p.m. every night and come on at 7 a.m. every weekday (and 8 a.m. at weekends). The timer will override the thermostat, so even if the temperature drops below the setting you have it on, the heating will not switch on.

Pre-heating

The timer and the thermostat should not affect the preheating process or a combi-boiler, however. You might hear your boiler fire up from time to time, even when the heating is off and you’re not drawing any hot water. This is completely normal, and is there to ensure you have hot water when you turn it on. If there’s no pre-heat, you might find you have to run the hot tap for 40–60 seconds before any hot water comes out. If you’re trying to save water, that’s not a good thing.

Have you got an immersion heater?

One final thing to consider is the presence of an immersion heater in your system or heat-only cylinder. Your boiler heats up water and circulates it around your cylinder to warm the water inside, but you can also get immersion heaters, which are essentially kettle elements sticking into the cylinder (it’s a bit more complex than that, but not much). That means you’re using electricity, not gas, to warm up the water. Why would you do that? Well, if you have an electricity tariff that gives you cheap overnight electricity, you can wake up to a full tank of water without using a single cubic inch of gas, and this might work out cheaper given the tariff. Just make sure your cylinder is well insulated, as a poorly insulated cylinder will leak heat and the immersion heaters will come on more often. Also remember, this has nothing to do with your central heating, so if you want hot radiators in the morning, you’ll need to leave your boiler on – just use a timer to wake it up before your alarm clock wakes you up.

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