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 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.

Pros

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.

Cons

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.

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Frequently Asked Questions

Why Is My Hot Water and Heating On at the Same Time?

In many boilers, a diverter valve directs hot water to either your radiators or hot water taps. If that valve malfunctions or is stuck, the boiler may send hot water to both the heating circuit and the taps simultaneously. This can also occur if your control settings or wiring are incorrectly configured, causing both systems to activate together.

Is It Cheaper to Have Hot Water on Constant or Timed?

Generally, timed schedules are more cost-effective. Keeping hot water on constantly can waste energy as the boiler cycles to maintain tank temperature, especially when you’re not using hot water. Timers ensure you heat water mainly when you need it, lowering overall fuel consumption.

Can You Run Out of Hot Water with a Combi Boiler?

A combi boiler heats water on demand, so you won’t run out in the same way you could with a tank-based system. However, water flow can weaken if demand exceeds the boiler’s capacity (for instance, multiple taps/showers running simultaneously). Under normal use, the supply is essentially continuous.

Are Hot Water and Heating Systems Separate?

In many older homes, hot water cylinders and central heating are separate circuits. Modern systems, especially combi and system boilers, integrate both functions into a single unit or a smaller set of components, making them more compact. Still, each setup has distinct pipework and controls for heating vs. domestic hot water.

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

Yes, but priority typically goes to hot water. When you turn on a tap, the combi boiler will momentarily divert all heat to hot water production. Once you close the tap, it reverts to heating mode. If you’re showering and running the heating, the radiators may cool slightly until the tap is off.

Why Does My Boiler Heat Radiators When I Run Hot Water?

In a boiler with a faulty diverter valve or incorrect wiring, the hot water circuit can inadvertently send hot water toward the radiators. A properly functioning diverter valve normally prevents this by isolating the central heating loop when hot taps are in use.

Can a System Boiler Heat Water and Radiators at the Same Time?

Yes. System boilers store hot water in a separate cylinder, allowing them to heat water for taps and supply the central heating simultaneously—up to the limits of the cylinder’s heat exchange capacity. Once the cylinder is up to temperature, more heat is available for the radiators.