What Is a Conventional Boiler? Complete Guide to Conventional (Heat-Only) Boilers

A conventional boiler (also known as a regular, traditional, or heat-only boiler) is a type of boiler that heats water for a home’s radiators while keeping tap water hot inside a separate cylinder. A conventional boiler requires a hot water cylinder, as well as separate cold water storage tanks for central heating and domestic hot water.

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Conventional boilers differ system and combi boilers in the way they get their water supply system is configured. Conventional boilers are different from system boilers because they require storage tanks for cold water whilst system boiler don’t. Regular boilers differ from combis because they heat and store water in a cylinder whilst combis supply hot water on demand. The presence of cold and hot water storage facilities in a home is an accurate indicator that your boiler is of the conventional type.

Conventional boilers’ reliance on several storage tanks to operate the heating and hot water systems means they need plenty of room on your property. Most homes with regular boilers have a dedicated space (typically in the loft) for the cold water feeder tanks. In contrast, system boilers need s single hot water cylinder, and thus require less space than their conventional counterparts. Combi boilers do not need any water storage facilities as they heat water on demand, and thus have the smallest footprint of all boiler types.

Regular boilers are not as efficient as modern combi boilers, as they require more energy to heat water and maintain a separate tank of hot water. However, they are still an efficient option if used correctly, and the modern models generally incorporate energy-saving features.

A new traditional boiler costs between between £1,500 and £3,000+ excluding installation, and anywhere from £2,000 to £5,000 fitted. The aforementioned figures assume that the new conventional boiler is replacing an existing boiler of the same type, and switching boiler types generally costs more. For example, changing from a conventional to a system boiler costs around £2,500-£5,000, whilst converting a regular boiler to a combi costs £2,700-£5,000.

Getting a new conventional boiler is worth it if you’ve got an old conventional boiler in your home already. A one-for-one swap of the same boiler type is typically the least expensive boiler replacement option. However, you may consider converting to a system boiler if your home could use more space, as such a conversion would get rid of the cold water tanks. Getting a combi boiler to replace your conventional boiler is a suitable option if your family’s hot water use is modest and you don’t need a large store of hot water.

How does a conventional boiler work?

A conventional boiler works by heating water for radiators and towel rails, and separately heating domestic water in a hot water cylinder. A conventional boiler relies on two cold water feeder tanks for its water supply (one for central heat and one for taps and showers) as it doesn’t take in water directly from the mains. Below is a diagram that illustrates how a conventional boiler works.

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How does a conventional boiler heat radiators?

A conventional boiler heats radiators following the five steps below.

The boiler’s burner ignites and heats water via the heat exchanger.
The heated water then flows through a network of pipes, radiators, and towel rails, distributing heat throughout the home.
The cool return water flows back to the boiler after passing through the home’s heating system, where it gets heated again as needed.
A cold water feeder tank (located in the loft) maintains pressure in the central heating system.
The water feeder tank gets refilled as needed from the mains.

How does a conventional boiler heat domestic hot water?

A conventional boiler heats domestic hot water via the four steps below.

The water that the boiler heats circulates through a separate circuit from the one used for heating radiators.
This separate circuit passes through a sealed coil within the hot water storage cylinder, where the coil transfers heat to the domestic, clean, potable water.
The hot water cylinder releases its contents in response to calls for domestic hot water from the home’s taps and showers.
The clean domestic hot water and the dirty central heating water never mix throughout this process.
The hot water cylinder gets topped up as required from a dedicated water feeder tank in the loft.
The domestic water feeder tank gets its water supply from the mains.

What is the difference between a conventional boiler and a system boiler?

There are three differences between a conventional boiler and a system boiler. The first difference is in the mechanism through which the heating and hot water systems get replenished with water. A conventional boiler relies on cold water feeder tanks that store and release water as needed. In contrast, a system boiler’s hot water cylinder gets refilled directly from the mains. The second difference is in the fact that a conventional boiler system is open-vented whilst a system boiler works with a sealed heating system. The third difference between a conventional and a system boiler is size. System boilers are generally smaller than their regular boiler counterparts, largely because regular boilers require feeder tanks and the associated pipework, whereas system boilers do not.

What is the difference between a conventional boiler and a combi boiler?

There are three differences between a conventional boiler and a combi boiler. Firstly, a conventional boiler heats water that’s stored in a cylinder, whilst a combi boiler heats water directly from the mains. Second, combi boilers take up less room than conventional boilers, as they do not require hot and cold water storage facilities. This compactness makes combis a better option for smaller properties than regular boilers. Third, combi boilers are generally more energy-efficient than conventional boilers, as they do not spend any energy on heating an entire cylinder of water. However, regular boilers are more suitable than combis for large homes with a high hot water demand, as a hot water cylinder makes a large volume of water available for use, whilst a combi boiler is limited to the mains flow rate.

Which is cheaper to run, a combi or a conventional boiler?

A combi boiler is generally cheaper to run than a conventional boiler for two reasons. Firstly, combis only use fuel to heat as much water as needed when it is needed. Meanwhile, regular boilers burn fuel to keep a large volume of water hot for use at any given time. Secondly, conventional boilers are less efficient than combi boilers if the hot water cylinder is not well-insulated. A poorly insulated hot water cylinder allows the water to cool down quickly, so the boiler must use more fuel to keep the water hot enough for use.

How do I know if I have a conventional boiler?

Below are the four ways to find out if you have a conventional boiler.

Check your hot water cylinder: You likely have a conventional boiler if you have a hot water cylinder in your loft or airing cupboard. The cylinder is usually large and cylindrical, and may have insulation wrapped around it.
Look for a feed and expansion tank: Conventional boilers require a feed and expansion tank, which is typically located in the loft. This tank is used to maintain the water level in the central heating system and to relieve pressure as water heats up.
Check the controls: Conventional boilers often have a separate control panel, which allows you to adjust the temperature of the hot water and heating separately. The controls may be located on the boiler itself, or on a separate panel on the wall.
Check the size of the boiler: Conventional boilers tend to be larger than other types of boiler, as they require additional components such as a hot water cylinder and feed and expansion tank. If your boiler is particularly large and you have a hot water cylinder, it’s likely that you have a conventional boiler.

What sizes do conventional boilers come in?

Conventional boilers come in sizes that range from 9kW to 70 kW to suit different property sizes and varying hot water demands. The size of the boiler required depends on the number of bathrooms and radiators, the size of the property, and the average hot water demand. The higher the quantity of radiators and bathrooms, the larger the property, and the higher the hot water usage, the larger the boiler’s output needs to be.

The four most common size ranges of conventional boilers are as follows.

9-18 kW: Up to 10 radiators
18-26 kW: 10-15 radiators
26-40 kW: 15-20 radiators
40-70 kW: 20 radiators and up

Note that the size-to-radiator correlation above is only an estimate. Getting an accurate boiler size requires a calculation based on various property parameters. The easiest way to calculate the correct boiler size is to use an online calculator, such as the one provided by Heatable. Heatable’s boiler size calculator requires you to answer anonymous questions about your property before recommending a range of suitably sized boiler models along with fixed price quotes. You can access Heatable’s online boiler calculator here.

Do conventional boilers require a lot of space?

Yes, conventional boilers can require a lot of space. The boiler itself is usually small enough to fit in an airing cupboard or even in a kitchen cupboard. However, conventional boilers require hot water cylinders and two cold water storage tanks to operate, and these storage facilities take up a significant amount of space and a dedicated area within a home. How much space a conventional boiler takes up depends on the boiler’s model as well as the size of the water storage tanks and cylinder.

Are conventional boilers efficient?

Conventional boilers are efficient if they’ve been manufactured in the past several years, but are less efficient than combi and system boilers.

The efficiency of a conventional boiler depends on the following three conditions.

Boiler’s age: Older regular boilers are generally less efficient than their newer counterparts. Conventional boilers that are over 20 years old are particularly inefficient, and upgrading your 20-year-old regular boiler to a new one could save you up to £840 per year, according to the Energy Saving Trust.
Boiler’s condition: Well-maintained conventional boilers are able to operate at the efficiency level to which they were designed. In contrast, poorly maintained boilers see their energy efficiency erode as various parts degrade due to unchecked wear and tear.
Property size: A boiler must be sized correctly for the property it is serving. A boiler that’s too large is less energy-efficient as it produces more heat than a property requires.

However, even modern, well-maintained, and properly sized regular boilers are generally less efficient than combination boilers. Heat-only boilers are less efficient than combis for two reasons. Firstly, heat-only boilers must burn more fuel to keep the water inside the hot water cylinder at the right temperature, whereas combi boilers heat mains water as needed. Secondly, a heat-only boiler must have a higher flow temperature than a combi, which prevents it from operating in its most efficient condensing mode. Regular boilers must have a higher flow temperature to ensure that the water inside the hot water cylinder remains above 60C, thus averting the risk of legionella bacteria from growing in the water. Combis do not pose a legionella risk because they get water from the mains.

How much does a new conventional boiler cost?

A new conventional boiler costs between £1,500 to £3,000+, excluding installation. The price you pay for a new conventional boiler varies depending on the make and model of the boiler, the boiler size, the boiler fuel, and any additional accessories you choose to purchase.

How much does it cost to install a new conventional boiler?

It costs between £500 and £2,000 to install a new conventional boiler, so total costs, including boiler and installation, range between £2,000 and £5,000. Conventional boiler installation costs depend on the following five factors.

Boiler brand and model: Some brands and models are cheaper to install than others. This difference in installation cost stems from the boiler’s weight, dimensions, and design. Boilers that are lighter, smaller, and easier to handle typically cost less to install.
Size of the boiler: Conventional boilers with a higher capacity cost more than those with a lower power output.
Complexity of the installation: Conventional boilers cost less to install when the hot water cylinder, water feeder tanks, and pipework do not require repairs or replacement. On the other hand, installations that affect these other components are costlier.
Your region: Certain parts of the UK have more expensive labour than others. For example, you’ll pay more to install a conventional boiler in London than you would in Cardiff.
Your installer: Well-known national installers (like British Gas) generally charge more than local heating trades or online installation companies such as Heatable.

How much does it cost to convert a conventional boiler to a system boiler?

It costs between £2,500 and £5,000 to convert a conventional boiler to a system boiler, including the new boiler costs. The following five conditions influence the cost fo a conventional-to-system boiler conversion on top of the installation factors discussed above.

New boiler make and model: A new system boiler may cost anywhere between £500 for a basic system boiler and going up to £2,500 or more for a high-end model.
New boiler location: Changing the location of the boiler during a conventional-to-system conversion may add up to £500 to the total fitting costs.
New hot water cylinder: Chances are that you’ll need a new hot water cylinder, especially if you’re upgrading from an old conventional boilers. New hot water cylinder costs range from £500 to over £2,500.
Feeder tank removal: The tanks’ location and ease of access determine how long it will take to remove the tanks and how many crew are required for the job.
Pipework upgrades: Upgrades to outdated pipework may be mandatory if the conventional boiler you’re removing is quite old.

How much does it cost to convert from a conventional to a combi boiler?

It costs between £2,700 to £5,000 to convert from a conventional to a combi boiler. The cost of converting from a regular boiler to a combi depends on the size, make, and model of the new combi, the cost of removing the hot and cold water tanks, and pipework upgrades.

It’s important to ensure that a combi boiler is suitable for your property if you’re considering a conversion. Combis generally lack the capacity to serve large volumes of hot water at once, as they’re limited by the mains water flow rates. So, getting a replacement conventional boiler or a system boiler is a more effective solution if your household consumes a lot of hot water.

How to find the best deal on a new conventional boiler?

To find the best deal on a new conventional boiler, follow the 3 steps listed below.

Research: Research different brands and models of conventional boilers. Look for reviews, ratings, and customer feedback to get an idea of the quality, efficiency, and reliability to find the best conventional boiler for your home.
Get at least three quotes: Contact heating engineers to get quotes on the cost of buying and fitting a new conventional boiler. Be sure to ask for a breakdown of costs, so you can compare quotes accurately. Heatable is a good starting point, as the company gives you fixed-price quotes for a range of suitable boiler models on your screen in only 90 seconds. You’re able to use Heatable’s quotes as a benchmark as you solicit quotes from local heating contractors.
Compare the quotes: Compare the quotes you’ve received, including the price, installation cost, warranties, and any extras the installers offer.

By following the 3 steps above, you’re able to find the best deal on a new conventional boiler and ensure a safe and efficient installation.

Is getting a new conventional boiler worth it?

Getting a new conventional boiler is worth it if you have an old, inefficient conventional boiler. Replacing an old, inefficient conventional boiler with a new, efficient model of the same type is a cost-effective means of improving your home’s energy efficiency. Your heating system is already configured to work with a heat-only boiler, so a one-for-one swap is the least expensive way to get a new boiler.

However, getting a new conventional boiler is not worth it if you currently have a different boiler type. Conventional boilers work well for large, thirsty households. However, system boilers are able to accommodate high hot water demand without needing cold water storage facilities the way regular boilers do. System boilers are a more cost-effective and space-saving option to consider if you haven’t got the infrastructure to support a conventional boiler already. Likewise, converting from a combi to a regular boiler makes little sense, as you must spend money and find space to accommodate the water storage facilities a regular boiler requires.

What are the pros of conventional boilers?

Below are the four pros of conventional boilers.

Ability to handle high demand: Conventional boilers are able to provide a large amount of hot water, which makes them well-suited for households with high hot water demand.
Easy to repair: Conventional boilers are relatively simple in design and are often easier to repair than the more complex combi boilers.
Compatibility with solar thermal systems: Conventional boilers are compatible with solar thermal systems, which may be used to heat the water in the cylinder.
Compatible with immersion heaters: The hot water cylinders that conventional boilers heat are compatible with electric immersion heaters. These electric heaters are a back-up hot water source if the gas boiler breaks down or there’s an interruption to the gas supply.

What are the cons of conventional boilers?

Some potential cons of conventional boilers include:

Needs lots of space: Conventional boilers require hot and cold water tanks, which take up a significant amount of space in a home. Accommodating these tanks is an issue in smaller properties.
Installation costs: Installing a conventional boiler is often more expensive than other types of boilers, since there are additional components required to store cold and hot water.
Energy efficiency: Conventional boilers are generally less energy-efficient than combi boilers.

FAQs about conventional boilers

Below are some of the most commonly asked questions about conventional boilers.

Can you still get conventional boilers?

Yes, you can still get conventional boilers. Newer boiler types, such as combi and system boilers, have become more popular in recent years. However, many homes still use conventional boilers because they’re more suitable for larger properties with high hot water demand. Many boiler manufacturers still produce conventional boilers to accommodate this demand.

Should I replace my conventional boiler with a combi?

Whether you should replace your conventional boiler with a combi boiler depends on the 4 factors below.

Hot water demand: Combi boilers perform best in homes with modest hot water demand, as their ability to supply hot water is limited by the pressure in the mains. It may be worth replacing a conventional boiler with a combi if your hot water demand is relatively low and you have a small to medium-sized property. On the other hand, a new conventional boiler may be a better fit if you have a large property and run multiple taps or showers at once.
Space: Combi boilers take up less space than conventional boilers because they don’t require hot or cold water storage tanks. A combi boiler is thus a more suitable option than a regular boiler if space is a concern in your home.
Cost: Replacing a conventional boiler with a combi costs about £500 more than doing a straight swap for a new conventional boiler. However, you may be able to offset the added costs if your hot water use is modest, as a combi boiler is able to operate more efficiently than a conventional boiler when hot water demand is low.
Water pressure: Combi boilers require a minimum water pressure to function properly. You may need to spend more money to have a water pump installed if you want to use a combi boiler and your mains pressure is low.

How long does it take to change a conventional boiler to a combi?

It takes 2-3 days to change a conventional boiler to a combi. The time it takes to convert from a conventional boiler to a combi boiler varies depending on the complexity of the conversion. The conventional-to-combi conversion process typically involves removing the old boiler, installing the new combi boiler, upgrading the gas and water pipework, and removing the redundant tanks and cylinders. The location of the boiler, accessibility of the pipework, and the need for any additional electrical or gas work are factors that may add to the cost of the conversion.

Is a conventional boiler the same as a system boiler?

No, a conventional boiler is not the same as a system boiler. There are two differences between conventional and system boilers. Firstly, conventional boilers require cold water storage tanks, whilst a system boiler relies on the mains for its water supply. Secondly, excess water pressure is released to a feed-and-expansion tank loft with conventional boilers, whilst system boilers have a sealed expansion vessel to handle excess pressure.

How long does a conventional boiler last?

A conventional boiler lasts between 10 and 15 years. However, some regular boilers may last even longer if they receive proper maintenance and regular service. Regular servicing and maintenance are vital in extending the boiler’s lifespan and preserving its energy-efficiency.

However, an old boiler that’s still in operation isn’t always worth keeping. Below are the two reasons you should consider replacing a very old conventional boiler, even if it still runs.

Your old boiler is not energy-efficient: Older boilers were manufactured to lower energy-efficiency standards, and a decade’s worth of wear and tear reduces a boiler’s ability to operate efficiently. Replacing an old, inefficient boiler with a new one saves you money on energy bills.
Your old boiler may be breaking down often: Aging boilers typically require frequent repairs. A replacement boiler may be a better investment than neverending repairs if the repair bills start going up into hundreds of pounds.

Heatable offers an easy way to explore your options for replacing an old conventional boiler. All you need to do is supply a few anonymous details about your property and Heatable will recommend a range of boilers that are appropriate for your property (along with fixed-price quotes).