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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: September 29, 2021. Updated: March 6, 2024

Pros & cons of combi vs conventional boilers

Deciding which new boiler is right for your home can be complicated. There are plenty of factors to consider as well as benefits and limitations for each.

To guide you through this decision we have provided all the essential information on each boiler type. As well as a complete inventory of pros and cons. When making your decision, you should ensure that your boiler will meet your needs.

The first step is to understand what Combi and Conventional boilers are and how they differ from one another. Once you know the difference you can go and look at new boiler costs.

What is a Combi Boiler?

A combi boiler, also known as a combination boiler, is a heating system that provides hot water on demand. This type of boiler is designed to perform two functions: supply hot water and run your central heating through a single compact unit. The water supply comes directly from the mains and is heated within the combi boiler on demand.

There is no need for a cold-water supply tank or hot water cylinder. The water is supplied directly to the boiler and diverted to the tap or central heating. This offers space-saving efficiency but can also hinder its ability to handle multiple requests for hot water throughout a house.

The boiler functions with valves that operate in different directions. As a result, water can either flow into the central heating system or to a tap. It is not equipped for the demands of both. Multiple running taps and showers can also cause a drop in pressure/flow rate, preventing hot water from being delivered.

Depending on the size of the combi boiler, it may only be able to deal with one request for hot water at a time. However, larger, more powerful combi boilers could handle two showers. This means combi boilers aren’t suited to all homes, especially big houses or homes with high hot water demand.

The size of the combi is important and looking at the incoming flow rates into the property must always be checked. Some manufacturers also make more powerful boilers and storage combi boilers with multiple taps.

What is a Conventional Boiler?

Conventional Boilers are typically found in older or larger houses. This is due to the extra space required for separate components of the system. As part of the conventional boiler, a cold-water tank typically needs to be fitted into a loft space, and an airing cupboard is required for hot water storage.

A conventional boiler heats a large quantity of water at once and stores it in a hot water cylinder. When hot water is needed in the house or for central heating, there is a ready supply to meet the demand. In contrast, a combi boiler heats water only on demand.

This method is much more effective for properties with multiple bathrooms, requiring constant heating and hot water supplies. Of course, after a certain amount, the hot water supply will run out, and the system will need to be refilled to heat more water. If water is not heated in advance, you could be facing a thirty-minute or hour wait for a supply of hot water (dependent upon the amount required and the boiler’s specification).

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Which Boiler is Best?

It is important to understand that the two boilers have different methods to supply hot water and central heating. In order to determine which one is best for your household, it is essential to consider the specific needs and requirements of your home.

You may find it helpful to weigh the advantages and disadvantages of each option in relation to your home and hot water usage. By doing so, you can make an informed decision that will ensure maximum comfort and efficiency in your household.

Combi Boiler Pros:

  • Cost Effective – If you’re looking to reduce your energy bills, a combi boiler may be the right choice for you. By upgrading from an older model to a new combi boiler, you could save up to £590 a year on your energy bills. Modern combi boilers have condensing technology that recycles energy to reheat returning water from the central heating system. This means they use less fuel, which saves you money.
  • Less space required – The combi boiler is a self-contained unit that takes up very little space. You can even move it out of sight to get more space. For example, the loft or garage. Modern combi boilers are ergonomic, making utility rooms or storage cupboards suitable locations. Remember to consider the delay between turning on the hot water tap and the distance the water will have to travel when choosing the location of your boiler.
  • Energy efficient – A combi boiler heats water when you need it, which prevents unnecessary heating and energy waste. Condensing boilers typically have an efficiency rating over 90%, making them one of the most energy-efficient options available. This not only saves you money but is also an environmentally friendly choice.
  • Instant Hot Water – Instant hot water is convenient for washing up, taking a bath, or a shower. Traditional water storage systems are disappointing when the hot water runs out. Combi boilers provide hot water instantly, solving this problem.
  • Fewer Pressure Issues – Water is supplied directly from the mains within the combi system. This means the mains pressure will guide the water throughout the house. Conventional boilers typically require a pump to offer this consistent pressure. However, before installing a combi boiler, it is a good idea to have the mains pressure/flow rate tested at your property to ensure you have a reliable amount of cold water

Combi Boiler Cons

  • No storage capability – Combi boilers don’t have water storage tanks, which can be a problem if they break down. Unlike conventional boilers, combi boilers can’t provide hot water or heating when they’re out of action. However, we offer storage combi boilers from Viessmann that combine the benefits of both.
  • Reliant on mains pressure – Good mains pressure will ensure that the system works efficiently, but poor pressure can cause problems. The heating circuit in the combi boiler will shut off automatically if the pressure drops below 0.5 bar, which is a safety feature to prevent damage to the boiler. However, this can be inconvenient when you need hot water. It is common to need to repressurise the boiler occasionally.
  • Struggle to keep up with demand – Combi boilers provide hot water instantly on demand, but using multiple taps or showers will cause water pressure to drop. This can be inconvenient for larger households with high hot water demand.
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Conventional Boiler Pros:

  • Better for larger homes – The number of radiators and the size of the property will impact your choice of boiler. Conventional boilers are good for larger properties with high demand for hot water. They offer consistent hot water while maintaining indoor temperature. However, if you have good water pressure a system boiler may be a better option.
  • Compatibility with older heating systems – Installing a conventional boiler in a home with existing heating components is easy.  Combi boilers require more power and a larger gas supply pipe. Opting for a quick and easy installation process can save you a lot of money.
  • Stored hot water – Conventional boilers heat water and fill a storage cylinder. This water can be used throughout the day. It can maintain the water temperature from hours to days, depending on the size and heat loss rating.This means the reserve of hot water can be accessed by multiple people and will receive the same pressure and a consistent temperature. The water storage facility can be very convenient for houses with multiple bathrooms that are likely to use this functionality.

Conventional Boiler Cons:

  • Supply can run out – Whilst utilising the ability to supply multiple taps with hot water, you might find yourself using up the hot water without the function of instantly creating more. The average shower tends to last around 8 minutes, with average water pressure which can use up to 20 gallons of water. If two showers occur at the same time, 40 gallons of water can be used in a short period of time.

    When deciding upon the size of your hot water cylinder, it is important to consider a number of factors. You should consider how much hot water is needed per person. But also be aware that with a larger tank, heat is lost much faster.

    Finding a middle ground can mean that hot water will run out every so often. You should be prepared to wait for the cylinder to refill with hot water as it is not instantly heated.

  • Additional space required – For smaller properties, a conventional boiler might not be appropriate. Typically, space is required in the attic or loft for cold water supply tanks. These usually have a minimum capacity of 25 litres for the central heating and 100 litres for the domestic hot water These will take up a considerable amount of space.

    Additional space is needed for the hot water cylinder. These are typically kept within airing cupboards or within loft space. Consider whether your property is capable of housing this equipment and if it is completely necessary to sacrifice the space within a smaller house

    Conventional systems often only become essential for larger houses; smaller properties can benefit from the compact nature of combi boilers.

  • Higher costs – You are unlikely to find a service that replaces a combi boiler with a conventional boiler. Installation services typically replace a boiler with a new system in the same location or move the new system to a different location for an increased cost. More commonly, services will include replacing a conventional boiler with a combi system. This may initially cost more for the installation but will certainly save you plenty of money in future energy bills.


When choosing a boiler, it is important to consider the factors that matter the most to you. Take into account your household’s daily routine and think about how frequently hot water is used.

If multiple bathrooms or hot taps are used simultaneously, a new conventional boiler may be the better option. This type of boiler provides improved energy efficiency and a reserve supply of hot water.

However, if you live in a smaller property or only have one or two bathrooms that aren’t constantly in use, a combi boiler may be a better choice. This type of boiler can offer monthly savings and on-demand hot water whenever required.

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Frequently asked questions

It will massively depend on your home’s usage.

A conventional system boiler will heat water on a timer no matter the usage. This is generally more expensive that a combi boiler. But it can produce more hot water in the home in one go, often powering multiple showers and hot taps.

A combi boiler, depending on cold water flow rate and power can produce hot water for a couple of showers but may struggle if a third or a tap or two is used.

Speak to one of our engineers today about which is best for your home. Grab on online quote and we’ll call you back.

Yes if you have 1 or 2 showers and less than 20 radiators. But get a quote first and we’ll call you back to discuss it further.

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