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Quick answer
Preheat (sometimes called keep warm, comfort or hot start) is a combi boiler setting that periodically fires the boiler to keep a small amount of water inside warm, so hot water reaches the tap faster. It improves convenience but uses extra gas, because the boiler fires even when nobody is using hot water. Many modern combis let you switch it off, set it to eco mode, or have it learn your usage. For most households, turning it off or limiting it saves gas with little real inconvenience.
Key takeaways
- Preheat keeps the combi’s internal heat exchanger warm for faster hot water.
- It costs extra gas because the boiler fires periodically with no demand.
- Eco mode on many combis turns preheat off; comfort mode turns it on.
- Some boilers offer a learning or timed preheat that only runs when you usually need hot water.
- The delay without preheat is usually only a few seconds at the boiler, plus pipe run time.
How preheat works
A combi heats water on demand. With preheat off, when you open a hot tap the boiler detects flow, fires, and heats water from cold, which takes a few seconds. With preheat on, the boiler keeps a small store of water warm so the hot water is ready almost immediately. To keep it warm, it fires briefly at intervals.
What it costs
The extra gas depends on the boiler and how often preheat fires. It is small per firing but adds up over the year, especially in homes where hot water is used only at set times. Many manufacturers recommend eco mode for efficiency.
The delay is often the pipes, not the boiler
Much of the wait for hot water is the time taken to push cold water out of the pipe between boiler and tap. Preheat does not change that. Our article on combi hot water delay explains more.
When preheat makes sense
- Households using hot water frequently and at unpredictable times.
- Homes where short hot water draws, such as hand washing, happen often.
- Boilers with smart or learning preheat that avoids firing at night.
When to turn it off
- Homes where everyone is out during the day.
- Overnight, if your boiler cannot schedule it.
- If you are trying to cut gas use this winter.
How to change the setting
Look for “eco” or “comfort” on the display or a button labelled with a tap and a leaf or similar. Settings differ between brands; check your user manual. Our guide to boiler display icons and symbols can help identify it.
Book with Gasmaster
Want help setting your combi for efficiency? Gasmaster Plumbing & Heating, Gas Safe registered (220932), services combis across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. Book through our boiler service booking page.
Frequently asked questions
Does preheat use a lot of gas?
Each firing is short, but over a year it can add up. For most homes it is a modest but real waste.
Does turning preheat off harm the boiler?
No. It changes when the boiler fires.
What is the difference between eco and comfort mode?
On many combis, comfort mode enables preheat and eco disables it.
Sources
- Manufacturer user guides (Worcester Bosch, Vaillant, Baxi, Ideal)
- Energy Saving Trust, hot water guidance (energysavingtrust.org.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
