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Last updated: 26th October, 2022


Can A Combi Boiler Run Hot Water and Heating Simultaneously?

Can A Combi Boiler Run Hot Water and Heating Simultaneously?

You’ve likely heard of a combi boiler before and you may even have one in your home, controlling your heating and hot water, but do you actually know what it does and how it works?

If you have a combi boiler in your home or are thinking about switching to one, it’s wise to familiarise yourself with what they are and how they differ from other boiler types.

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What is a Combi Boiler?

A combi, or combination, boiler is a type of boiler that combines a water heating system and central heating unit into a single body. This acts as a unified way to handle heating and hot water without the need for a separate tank.

How Do Combi Boilers Work?

A combi boiler is connected directly to the water mains and heats hot water by passing it over a heat exchanger inside the boiler body.

This means you can get hot water instantly, rather than heating it and storing it in a cylinder to be used as a regular boiler would and therefore doesn’t require a hot water tank or cylinder, saving on space.

Does a Combi Boiler Run Hot Water and Heating at the Same Time?

When you request hot water from a tap, a sensor within the combi is signalled, telling the boiler to turn on. As there is no hot water storage tank, the boiler can only either send hot water to your taps or radiators.

The control valves within the boiler work in different directions to one another, meaning the system will choose where to send the hot water depending on where the sensor has been triggered from. This means you cannot run the heating and hot water at the same time.

Similarly, if one person in the household is having a shower, you should avoid attempting to use hot water elsewhere in the house e.g. hot taps, washing machines, dishwashers, otherwise this can either cause the shower to become cold or significantly reduce the hot water flow rate, making their shower less enjoyable.

Pros and Cons of Combi Boilers

Not sure whether a combi boiler is suitable for your home? Here are some pros and cons of owning a combi boiler.


On-Demand Hot Water.There’s no need to wait for the water to be heated.

Space Saving.These boilers are typically compact in size and don’t require the additional space for a tank or cylinder.

Efficient.Most combi boilers are incredibly efficient with a rating of over 90%. See our most efficient boilers here.

Wallet Friendly.Thanks to low prices with iHeat plus high efficiency levels, you could reduce your heating bills by up to £380 a year by installing a brand new A-rated efficient boiler.

Fresher Water.As the water is delivered directly from the mains, it tends to be more fresh and safe than water that has been stored for long periods of time.


No Backup.Combi boilers don't have backup immersion heaters to heat water should there ever be a breakdown.

Single Use.There may be a decline in pressure, or temperature, if more than one outlet is using hot water at the same time, so they may not be ideal for larger, more busy households that take showers at the same time.

Get a Brand New Combi with iHeat

With iHeat, you can get a fixed price quote on a brand new A-rated efficient boiler plus install. Get a quote today.

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Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.