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Witten by Bernard Morgan (Engineer).
First Published: September 29, 2021. Updated: November 28, 2023

Pros & cons of combi vs conventional boilers

Deciding which boiler is right for your home can be a complex task. There are plenty of factors to consider as well as benefits and limitations for each. BoilerHut is set to offer you both Combi Boilers and Conventional Boilers. However, you must first decide which product is best for your home.

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

The pivotal first step in your decision is to understand what Combi and Conventional boilers are and how they differ from one another.

What is a Combi Boiler?

A combi boiler, or combination boiler, is a compact heating system that heats water on demand. The boiler provides two functions; it can supply hot water and central heating via one compact unit. Water supply comes direct from the mains and is heated within the unit 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 demands from both. Multiple running taps and showers can also cause a drop in pressure/flow rate preventing hot water from being efficiently delivered. Generally, the combi boiler can only deal with one request for hot water at a time. Therefore, the best applications for combination boilers tend to be within smaller houses with less consistent demand for hot water.

The size of the combi is very important and looking at the incoming flow rates into the property must always be checked

Some manufacturers also make more powerful boilers and storage combis for use 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 that is required for separate components of the system. As part of the conventional boiler, two cold-water tanks typically need to be fitted into a loft space and an airing cupboard is required for hot water storage.

Unlike the combi boiler which heats water on demand, a conventional boiler will heat many litres at once and store it within a hot water cylinder. As hot water is requested throughout the house or within the central heating, there is a supply to meet all demand.

This method is much more effective for properties that contain multiple bathrooms, require constant heating and supplies of hot water. Of course, after a certain amount, the hot water supply will run out and the system will need to refill 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) Interested in a conventional or combination boiler?

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Which Boiler is Best?

As discussed, the two boilers operate differently in the methods used to supply hot water and central heating. There is no direct answer for which is best, as it entirely depends on your needs and the requirements  of your household. Consider the pros and cons below in relation to your home and hot water usage.

Combi Boiler Pros:

  • Cost Effective – A key advantage of the combi boiler is its capability to cut energy bills in half. When switching from an older model to a new combi boiler, you could be able to save as much as £300+ per year in energy bills. Most newer versions of combi boilers will include condensing technology. This prevents the escape of waste energy and instead recycles the energy to reheat returning water from the central heating system. This uses far less fuel which consequently saves you money
  • Less space required – Instead of the bulky system that accompanies a conventional boiler, the combi boiler is its own self-contained unit that hardly takes up any space at all. This means you have access to more space within your home and the choice to relocate your boiler out of sight.

    For quieter boilers, the attic can be a great spot when there’s limited space in the rest of the house. Garages and basements are also perfect locations as it means the boiler is completely out of the way, allowing you to enjoy all of your available space. Utility rooms or storage cupboards will also suffice due to the ergonomic nature of many modern combi boilers. Consider the delay between turning on the hot water tap and the distance the water will have to travel when choosing your location.

  • Energy efficient – A combi boiler, as previously mentioned, heats water on demand. This means water is not heated unnecessarily and energy wasted when unused. Condensing boilers are also typically manufactured with an efficiency rating around 90% +.Offering you one of the highest levels of energy efficiency available for any type of boiler. This provides an eco-friendly and money saving option.
  • Instant Hot Water – For the moments when you want to fill a washing-up bowl, take a bath or a quick shower, instant hot water can be extremely convenient. This is the alternative to water storage systems that offer a disappointing experience once the hot water has run out. In most situations, you may not have the time or patience to wait 30 minutes to be able to complete that task. Combi boilers offer the perfect solution for hot water in an instant.
  • Fewer Pressure Issues – Within the combi system, water is supplied directly from the mains. 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 – Water storage tanks aren’t included with most combi boilers which prevents storage of hot water for later use. This typically becomes an issue when the boiler breaks down. Houses with conventional boilers may be lucky enough to have several gallons of pre-heated water to use before the boiler is repaired and even an electric immersion heater to see them through a day (although this is a very expensive way to heat the water).

    With a combi boiler, once it is out of action, hot water and central heating capabilities aren’t available. Water is heated on demand so whilst the boiler is not operational, there is no functionality to heat water. We do offer storage combi boilers from Viessmann. These boilers are a mix of conventional and combi; read more about what a storage combi is and does >

  • Reliant on mains pressure – We previously mentioned that mains water pressure can be a benefit for combi boiler users. However, the same factor can also be a disadvantage.

    The system will work best when the mains pressure is good. However, if there is poor pressure in your area, problems may arise. Once the pressure within a combi boiler drops below 0.5 bar of pressure on the heating circuit, the system will typically shut off. This is a safety mechanism to prevent overheating and damage to the boiler but can be an issue when you require hot water. It is normal to need to repressurise the boiler every now and again.

  • Struggle to keep up with demand – Combi Boilers are designed to heat water to a set temperature on demand. This can cause issues when multiple sources request hot water. Directing hot water to two locations can cause a drop in water pressure (flow) at the tap. This means that using two showers simultaneously will create lower water pressure (flow) or cooler water. For larger households that require these kinds of capabilities, it can become inconvenient to not have access to hot water when it is needed through multiple demands.
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Conventional Boiler Pros:

  • Better for larger homes – Depending on the number of radiators within a property, as well as the size of the property, certain boilers are able to heat the property better than others. When large amounts of hot water are needed at the same time, the conventional boiler can offer a far more consistent supply. Heat is maintained within the property whilst being able to offer hot water in several taps throughout.
  • Compatibility with older heating systems – If your house already has the components of a traditional heating system, a conventional boiler can be incredibly easy to install. Replacing an older conventional boiler with a newer one also offers a simpler install experience as many of the same components are utilised. Because combination boilers heat water on demand they have to generally be more powerful (have a higher kW) than a conventional boiler this means that where a conventional boiler can normally run with a 15mm gas pipe supply a combination boiler will need a 22mm gas supply pipe.

    Overall, a quick and easy install process will likely cost you far less than if you had one system removed and an entirely different one installed. Fewer working hours and minimal extra supplies and equipment equate to far lower costs.

  • Stored hot water – Unlike a combi boiler which heats water on demand, a conventional boiler heats enough water to fill a storage cylinder. This water can be used throughout the day and maintains its temperature for a number of hours or even days depending on the size and heat loss rating of the cylinder.

    This means the reserve of hot water can be accessed by multiple users and will receive the same pressure and a consistent temperature. This is a feature that combi boilers are unable to offer. For houses with multiple bathrooms that are likely to use this functionality, the water storage facility can be very convenient.

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.


To make your decision, you should consider each of these factors individually. Think about your household’s daily routine and how frequently hot water is used.

If you know that multiple bathrooms are used simultaneously, a new conventional boiler might be preferable. This can offer improved energy efficiency whilst maintaining a reserve supply of hot water.

However, you may benefit from a combi boiler if you live in a smaller property or only have one or two bathrooms that aren’t constantly in use. This can offer you 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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