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What is eco mode on my boiler?
On most combi boilers, eco mode switches off hot water preheat. Here's what that means, what it saves, and whether it's worth turning on in your home.
- 1Button or menu setting. You can change it yourself
- 30sRoughly the longest extra wait for hot water at the tap
- 0Effect on your central heating
Ecoone setting on the boiler
Photo 1, wide. Existing hot water preheat image. Swap for a real photo of a combi front panel if you have one.
Eco mode on a boiler turns off hot water preheat. With preheat off, a combi boiler only fires when you open a hot tap. With it on, the boiler fires now and again through the day to keep a small amount of water inside it warm, so hot water reaches the tap a little faster.
That's the whole trade. Eco mode on means less gas burned while nobody is using hot water, and a short wait when you do. Preheat on means hot water a few seconds sooner, paid for in gas used while you're out, asleep or at work.
This guide covers what eco mode actually does, what each manufacturer calls it, who it suits, and the other settings that make a bigger difference to your gas bill.
What eco mode does
It stops the boiler keeping water warm in case you need it.
How does preheat work?
A combi boiler heats water as it passes through, so there's no hot water cylinder. When you open a tap, the boiler has to light, warm its heat exchanger and then push hot water down the pipe to you. Preheat shortens the first part of that by keeping the heat exchanger warm. The boiler lights briefly every so often through the day and night to top the temperature up.
Think of it as a kettle left to keep warm on its base. It's ready sooner, but it's using energy the whole time it waits. Eco mode is the kettle you switch on when you want a cup.
What does it not change?
- Your central heating. Radiators, timers and the thermostat work exactly as before
- The hot water temperature once it arrives
- Frost protection and the boiler's safety devices
- Your manufacturer's guarantee
Does every boiler have it?
No. Preheat is a combi boiler feature. If you have a system or regular boiler with a hot water cylinder, the cylinder is your store of hot water and there's no preheat to switch off. Older combis may not have the setting either.
The eco button
Photo 2, tall. Hand pressing the eco button on a Worcester Bosch combi.
The eco button on a Worcester Bosch combi. On newer models the setting is in the hot water menu.
What each manufacturer calls it
The setting does the same job on every combi that has it. The name changes by brand, and sometimes by model year.
Worcester Bosch
Eco
- Eco button on older Greenstar models
- Hot water menu on newer ones
- Eco lit or showing means preheat is off
Ideal
Preheat
- Set preheat to off for eco
- Found in the hot water or user menu
- Some models offer a timed preheat
Vaillant
Warm start
- Also shown as comfort mode on some models
- Turn it off for eco
- Usually in the hot water settings
Baxi and Potterton
Preheat or eco
- Wording varies by model
- Often a button or a menu option
- Check the user manual for yours
If you can't find the setting, the user manual will show it. Or ask the engineer at your next service and they'll set it for you.
Eco mode on or off: side by side
| Eco mode on (preheat off) | Eco mode off (preheat on) | |
|---|---|---|
| Gas used when no tap is running | None for hot water | Small amounts through the day and night |
| Wait for hot water | Around 10 to 30 seconds longer | Faster from the boiler |
| Boiler firing on its own | Only for heating | Fires briefly now and again to keep warm |
| Central heating | No change | No change |
| Suits | Most homes, especially if you're out during the day | Homes that use hot water in lots of short bursts |
Wait times depend on the boiler and how far the tap is from it. A long pipe run adds time whichever setting you use.
Should I switch eco mode on?
- One or two people, out during the dayTurn eco mode on
Preheat is keeping water warm for nobody. You'll barely notice the wait.
- Busy house, taps on and off all dayTry it for a fortnight
If the wait at the kitchen tap annoys you, switch preheat back on. Nothing is lost.
- Bathroom a long way from the boilerEco mode is still fine
Most of the wait is cold water in the pipe, which preheat doesn't fix.
- System boiler with a cylinderNothing to change
There's no preheat on these boilers. Your cylinder timer does that job.
Try it in summer first
The water coming into your home is warmer from late spring to early autumn, so the extra wait is shorter. Switch eco mode on then, and if you're happy by the time the nights draw in, leave it.
Settings that save more than eco mode
Eco mode is a small saving on hot water. The heating is where most of the gas goes, and these three make a bigger difference.
Turn the heating flow temperature down
A condensing boiler recovers extra heat from its flue gases, but only when the water coming back from the radiators is cool enough. Many boilers are left set near the top of the dial. Turning the radiator temperature down lets the boiler condense more of the time. The house takes a little longer to warm up and costs less to run.
Set the hot water to a sensible temperature
On a combi there's no stored water to keep hot, so there's no reason to run the hot water at its maximum. Somewhere around 50°C is comfortable for most homes and saves topping up with cold. Homes with a cylinder should keep it hotter for hygiene, so ask us before changing that one.
Keep the boiler serviced
A service includes a flue gas analysis, which shows how cleanly the boiler is burning. The engineer cleans the parts that clog, clears the condensate trap and cleans the magnetic filter, so the boiler runs as it was designed to. It also keeps your manufacturer's guarantee valid.
Included in every Home Care plan, from £11 a month.
Common questions
Is it safe to leave eco mode on all the time?
Yes. Eco mode only stops the boiler keeping its own water warm between uses. Frost protection, the heating and every safety device carry on working as normal, and it doesn't affect your guarantee. You can leave it on all year.
Does eco mode affect my central heating?
No. On a combi boiler, eco mode only changes how the boiler handles hot water at the taps. Your radiators, heating timer and thermostat work exactly as they did before.
How much will eco mode save me?
It depends on the boiler, how often you use hot water and how long the house sits empty. The saving comes from the gas preheat burns while no one needs hot water, so it is a modest saving, bigger in homes that are empty all day. Turning the heating flow temperature down usually saves more.
Why is my hot water taking longer since I turned eco mode on?
With preheat off, the boiler lights only when you open the tap, so it needs a few seconds to warm up before hot water starts down the pipe. An extra 10 to 30 seconds is normal. If it takes much longer than that, or the water never gets properly hot, book a repair visit.
I can't find an eco button on my boiler. Does it have eco mode?
If you have a combi, it may be in a menu rather than on a button, and the name varies by brand: eco, preheat, warm start or comfort mode. Check the user manual. If you have a system or regular boiler with a hot water cylinder, there is no preheat setting to change.
Can your engineer set it up for me?
Yes. At your annual service we will check eco mode, the flow temperature and the hot water temperature, and set them up to suit how you use your home. The service is £120 including VAT, or included in any Home Care plan.
Want us to check your settings?
Every service we carry out across Dunfermline, Fife, South Queensferry and Winchburgh includes a look at how your boiler is set up. If eco mode, the flow temperature or the hot water temperature could be set better for how you use your home, the engineer will tell you and change it with you there.
If you'd rather sort it yourself, the user manual for your boiler shows where each setting is. Change one thing at a time so you know what made the difference.
Gas Safe registered
Photo 3, square. Heat Network engineer at a boiler.
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