Everything You Need to Know About Boilers: Types, Sizes, Maintenance, and Longevity

Boilers are the heart of the heating system. While not all boilers are built the same, they all provide the same fundamental functionality. This guide will cover everything you need to know about boilers, including the many different types of boilers, their sizes, maintenance, and more.

Boilers are the heart of the heating system. While not all boilers are built the same, they all provide the same fundamental functionality. That is, they take cold water from the mains and heat it up. Then, that hot water is pumped to taps, appliances like showers, and radiators and other heating elements. While boilers are relatively straightforward devices, there are many options. And then, there’s a lot to consider with regards to servicing them, repairing, replacing, etc. 

This guide will cover everything you need to know about boilers, including the many different types of boilers, their sizes, maintenance, and more.

What Are The Different Types Of Boiler?

Boilers are the driving force behind the central heating system. All boilers are built to do the same thing: to provide heating.

Most of the UK’s boilers are gas boilers that run off gas from the main grid. 

A small percentage are LPG or oil boilers, which are popular for off-grid homes and rural properties not connected to the main gas network. Then, there are electric boilers which perform the same functions as other boilers but use electricity as an energy source. 

Boilers, fundamentally, use an energy source, such as gas, to heat water. Hot water comes out of hot water taps and pumps through radiators to heat the home. 

Despite all gas boilers working similarly and performing virtually the same core functions, there are three different main types:

  • Combi boilers
  • Standard/conventional/traditional/heat-only boilers (they go by all of those names)
  • System boilers 

The type of boiler installed in a home depends on the size of the property and the date the boiler was built and installed.

Let’s take a closer look at the different types of boilers:

Combi boilers

Baxi Combi Boiler

Combi boilers are relatively modern and are most likely to be fitted in flats and small houses. They feature a single unit connected to the hot water supply and radiators. The boiler unit takes cold water directly from the mains, heats it, and pumps it to radiators, taps, showers, etc. 

The main advantage of combi boilers is their ability to save space. There is just one small unit, typically mounted in a cupboard, and no water tanks. Providing direct hot water to a small number of radiators and taps makes sense for smaller properties with lower hot water demands.

When hot water demands are higher, e.g. in a 3-bed house with two bathrooms and many radiators, combi boilers might struggle - but this is changing. Modern combi boilers work well for most 3-bed houses, but it’s still wise to weigh them up against system boilers if you have two bathrooms or more (e.g. two bathrooms and an en-suite). 

How do you know if you have a combi boiler? 

You have a combi boiler if: 

  • You have one boiler unit that isn’t connected to any other cylinders. 
  • If you live in a flat with your own boiler, it’s almost certainly a combi boiler. 
  • There are five copper pipes coming from the bottom. 

Pros and cons of combi boilers 


  • Combi boilers are compact and can be fitted into a cupboard. There’s no need for an extra water cylinder. 
  • Combi boilers are self-contained units that are easy to use. 
  • Most combi boilers are modern and highly efficient. 
  • New combi boilers are powerful enough to heat 3-bed homes with two bathrooms. 


  • Combi boilers rely on reasonable mains water pressure. 
  • Don’t work with power showers. 
  • Struggle with multiple bathrooms and high heating demands. 

Standard/conventional/traditional/regular/heat-only boilers 

These boilers go by multiple names, which all amount to the same thing. They’re typically fitted in houses with a loft and are probably the most common boiler in pre-90s houses where the boiler hasn’t been recently replaced. 

Conventional boilers have hot water tanks, which are typically installed in the airing cupboard. They also have a central heating feed tank and a hot water feed tank in the attic, which feeds the boiler and hot water tank by gravity. The tanks in the loft are fed from the main supply. 

Conventional heating setups are typically ‘open vented’ and are open to the atmosphere in the loft, where the feed and expansion tanks are fitted. This means they’re somewhat susceptible to debris, oxidation and sludge build-up. 

So, essentially, the mains supply feeds the tanks in the attic, which supplies the entire home with hot water and heating by passing it to the boiler and hot water tank. This setup ensures good pressure as the system is fed by gravity and pumps and doesn’t rely solely on the mains pressure. In addition, the hot water cylinder ensures a steady supply of hot water to multiple taps and showers too. 

How do you know if you have a conventional boiler? 

You have a conventional boiler if: 

  • You have an airing cupboard with a hot water tank and an attic with one or two additional tanks. 
  • The boiler is usually installed downstairs, in a cupboard, utility room or garage. It will also probably have a pilot light. 
  • Your hot water runs out if people in your house use lots of hot water, e.g. by taking multiple showers in quick succession. 

Pros and cons of conventional boilers 


  • Provides a solid supply of hot water and heating at good pressure. 
  • Some older conventional boilers are known to be very durable. 
  • Work well under high hot water demands. 
  • Generally easy to service. 


  • Can be more costly to run as water is heated even when it isn’t used (providing the heating is switched on). 
  • Tend to be older boilers. 
  • More parts to go wrong, and internal components can corrode and oxidise. 

System Boilers

System boilers are somewhat of a halfway or hybrid between a combi and a conventional boiler. Like conventional boilers, they use a hot water cylinder to keep up with higher water demands. 

But, unlike conventional boilers, system boilers are installed in a closed system that isn’t fed by a tank in the loft. As a result, they’re pretty compact but deliver on multiple taps, showers and large radiator systems. 

System boilers are hostage to mains water pressure like combi boilers, but that isn’t generally an issue in most parts of the UK. 

System boilers rate as excellent all-rounders that suit modern radiator systems that don’t mind being placed under pressure. They’re not always suited to a straight swap with a conventional boiler unless the home’s radiators (and possibly even the plumbing) are also upgraded. That’s because older radiators don’t always like the higher pressure supplied by combi and system boilers. 

Overall, system boilers are modern like combi boilers but cater to larger homes with three bathrooms (e.g. two bathrooms and an en-suite) or more. 

How do you know if you have a system boiler? 

You have a system boiler if: 

  • You have a hot water tank but don’t have a feed tank in the attic. 
  • Your boiler is nice and modern with digital controls. 
  • There are three copper pipes coming from the bottom. 

Pros and cons of system boilers 


  • Excellent hot water provision for larger homes. 
  • Modern and efficient. 
  • High pressure if the mains is high pressure. 
  • Perfect for modern radiator systems. 


  • Higher up-front costs. 
  • Not ideal for older plumbing and radiators. 
  • Rely on mains water pressure. 

How to pick a boiler type

It’s important to remember that all boilers perform the same core functions. No single boiler type is necessarily better than the others. 

Combi boilers are tough to beat for smaller homes with 1 to 3 inhabitants, whereas conventional boilers are still popular for larger homes despite system boilers taking over in recent years. 

What’s more important is the efficiency of the boiler and the output rating. It doesn’t matter what boiler you own if it’s not powerful enough for your home!

If you’re replacing your boiler, the replacement will likely be a combi or system boiler unless the radiators and plumbing are relatively old and wouldn’t tolerate the higher pressure of a modern boiler. In this situation, upgrading the entire heating system may be advantageous. 

All boilers have to be fitted and registered by Gas Safe engineers, and they’ll advise you on what boiler is best for you. 

Types of boiler FAQs

What type of boiler is best?

All modern boilers are condensing boilers. The three main types of boilers, system, combi and conventional, are definitely more similar than they are different. 

Each one excels in different setups. For the most part, new boiler installations tend to involve combi or system boilers, as these make the most sense for modern radiator systems. In addition, Combi boilers have become much more powerful in recent years and can adequately heat larger homes. 

Add in a hot water tank for a system boiler, and you’ve got a robust, modern heating system. Conventional systems are fine and are often worth maintaining, but there are a few scenarios where it’s worth going from a combi or system boiler setup to a conventional setup. 

What is the most reliable boiler?

Top brands like Worcester Bosch, Vaillant, Ideal, Baxi and many others are there on merit and nearly always deliver reliable products. Heating engineers will have their own favourites and will advise you of the best option for your home. Overall, competition is tight, and there’s no single “most reliable” boiler brand or model. 

What are the 3 types of boilers?

The three main types of gas boilers are system boilers, combi boilers and conventional boilers, which are also called traditional, standard and heat-only boilers. These boilers are fundamentally similar and provide the same core functions: distributing hot water to taps and radiators.

So, let’s move onto what size boilers you need for different flats and houses. 

What Size Boiler Do I Need For My House?

Picking the perfect boiler for a home can be a minefield. 

It goes without saying that larger properties with higher heating and hot water demands require larger boilers, but that’s not all there is to it. Picking a boiler suitable for a property requires an evaluation of insulation, the number of radiators and average hot water demands at least. 

Our heating engineers will help you, but it’s handy to have some idea of what type of boiler you’ll need and how powerful it needs to be for your home. 

‍This is a guide to boiler size and power ratings. 

How do you calculate boiler size?

Boilers have different power outputs typically measured in kilowatts (kW). 

Kilowatts are a measure of power and essentially gauge how much energy a boiler can output as heat. 

Every boiler on the market has a kW rating, but you can’t compare one type of boiler to another based on the kW value only. For example, combi boilers have considerably higher kW ratings than system or conventional boilers as they provide hot water on demand.

System boilers and conventional boilers gradually heat the water in a hot water tank and thus have lower ratings despite being able to provide the same net power. To learn about the different types of boilers, check out our guide here.

Combi boilers often have two ratings, one for hot water and one for heating. 

kW is not the only thing that matters! 

What size boiler do I need for my house?

So, what size boiler should you consider for different size homes and hot water demands? 

It depends on the following factors: 

1: Number of radiators 

room with  radiators below 3 windows

Flats might have just 6 radiators, whereas a large house can have over 30. The more radiators, the more powerful the boiler has to be. An underpowered boiler won’t be able to heat all radiators efficiently.

2: Insulation and energy performance 

Model house wrapped in Scarf

Highly efficient homes with great insulation are cheaper and easier to heat up. A well-insulated home requires a lower-rated boiler, decreasing heating bills and energy usage. 

Conversely, a poorly insulated home which loses a lot of heat will require a larger boiler to adequately heat the atmosphere. 

3: Number of bathrooms


Higher hot water demands require more powerful boilers. Combi boilers especially lack the teeth to provide over two bathrooms with regular hot water in larger households. The hot water tanks provided by system and conventional boilers help meet higher hot water demands. 

What size combi boiler does my home need?

Above: Combi boiler provide both heating and hot water 

First off, let’s discuss combi boilers. Combi boilers provide heating and hot water from a single unit without needing a separate hot water tank. That means they need more power and are rated higher in kW than equivalent system and conventional boilers.

The average combi boiler size required for different homes is below. Note that, even though combi boilers extend up to 43kW and above, it’s usually better to install a system or conventional boiler in homes that large. 

Bedrooms Radiators Bathrooms Power rating
1 to 2 0 to 10 1 24 to 27kW
3 to 4 10+ 02-Mar 28 to 34kW
More than 4 20+ More than 3 35 to 43kW

What size system or conventional boiler do I need?

Above: Conventional boiler 

System and regular boilers are broadly analogous in terms of power. They don’t require nearly as much power as combi boilers. That doesn’t necessarily mean they’re cheaper to run, though, as they warm up hot water on an ongoing basis rather than on demand like a combi boiler. 

The average system or regular boiler size required for different homes is below. It’s worth mentioning that smaller homes or flats with fewer than 10 radiators are well-provided by combi boilers. System and conventional boilers are at their best when installed in larger homes. 

Plus, they both require additional space to mount the hot water tank (and space in the loft for the conventional boiler’s cold water feed).

Bedrooms Radiators Bathrooms Power rating
1 to 2 0 to 10 1 9 to 18kW
3 to 4 10+ 02-Mar 18 to 26kW
More than 4 20+ More than 3 27 to 40kW

What size boiler by house type

Boilers vary from small heat-only boilers rated 9kW to massive 150kW+ boilers for very large homes and commercial buildings.

A heating engineer will help you choose the best boiler for your home, as there are many options available, and specialist knowledge is indispensable. Plus, boilers must be fitted by Gas Safe engineers anyway, so gaining their opinion on what boiler you need prior to installation makes sense. 

Here is a blow-by-blow of boiler size by house type: 

What size boiler do I need for a flat?

Flats are almost invariably heated by combi boilers these days unless they’re very large with 3 bedrooms and two bathrooms, in which case a system boiler could be advantageous. 

Above: Most flats benefit from small combi boilers 

The majority of flats in the UK have one or two bedrooms and are ideally provided by a small or standard-size combi boiler. 

Combi boilers have higher kW ratings as they have to heat water on demand. That doesn’t mean they’re less efficient or more expensive to run, as they don’t have to maintain hot water in a separate tank, like system or conventional heat-only boilers. As such, there are few combi boilers rated below 20kW, if any. 

‍Verdict: To heat a flat with a combi boiler, you’ll need a boiler rated around 24 to 27kW at minimum. If you want a system boiler instead (e.g. for a very large flat), this drops to around 12 to 15kW. 

What size boiler do I need for a 1 bedroom house?

1 bedroom houses are ideally powered by compact combi boilers, but small system boilers are also an option. 

Since 1 bedroom flats are likely occupied by single people or couples, hot water demands are low, and combi boilers are more than powerful enough. 

‍Verdict: To heat a 1-bedroom home with a combi boiler, you’ll need a boiler rated around 24 to 27kW at minimum. If you want a system boiler instead, this drops to around 14 to 18kW. 

What size boiler do I need for a 2 bedroom house?

Things remain steady for homes with 2 bedrooms. 2 bedroom houses could be occupied by as many as four people, e.g. two parents and two children sharing a bedroom, or even two couples. 

In this scenario, system boiler could be a good shout to cater to higher water demands. In most cases, however, a compact combi boiler will suffice. 

‍Verdict: To heat a 2-bedroom home with a combi boiler, you’ll need a boiler rated around 25 to 27kW at minimum. 30kW will be more than enough if the home has two couples. 

System boilers could be a good choice if hot water demands are high, in which case a 15 to 20kW boiler would be easily sufficient. 

What size boiler do I need for a 3 bedroom house?

You can still heat 3-bedroom homes with combi boilers, but we’re now getting into system and conventional boiler territory. 

Above: Homes with 3 or more bedrooms and two or more bathrooms may benefit from hot water tanks

Most 3-bedroom or more houses built pre-2000s come with conventional boilers and have an airing cupboard with a hot water tank and feed tanks in the loft. Combi boilers rated up to 34kW can replace them, but system boilers might be more appropriate. 

Verdict: To heat a 3-bedroom home, both combi, system and conventional boilers have their pros and cons. For combi boilers, look for something around the 28 to 34kW mark. For system or conventional boilers, 18 to 24kW should suffice. 

What size boiler do I need for a 4 bedroom house?

4 bedroom homes often have over 20 radiators and three bathrooms. If two or more people are using bathrooms at once, system and conventional boilers definitely have an advantage over combi boilers. You can get large combi boilers rated up to 45kW, but if more than three or four people live in the house, system and conventional boilers are better choices. 

‍Verdict: To heat a 4-bedroom home, combi boilers rated between 30 and 45kW are generally able to deal with most heating and hot water demands. But a 27 to 40kW conventional or system boiler will likely handle high demands more efficiently. 

What size boiler do I need for a 5 bedroom house?

It’s often said that combi boilers are unsuitable for large houses, but it depends on hot water demands.

For example, a family of five living in a large 5-bedroom home might have very high hot water demands (e.g. 3 or more simultaneous showers). In this case, it’s definitely better to look at conventional and system boilers. 

Generally speaking, 5-bedroom houses are thorough in system and conventional boiler territory, but some powerful modern combi boilers can handle it. 

‍Verdict: To heat a 5-bedroom home, look for a conventional or system boiler rated above 40kW for best results. 

Ask for advice

Working out the appropriate boiler size for different houses is not an exact science. Different boilers excel at different things, and some old pieces of advice like ‘combi boilers are not suitable for homes with more than one bathroom’ are redundant. 

Ask us for advice. We'll take your home layout and size, the number of radiators and average hot water and demands and provide you with the best options from top brands like Viessman, Worcester Bosch, Ideal, Baxi, etc. 

There are many choices when it comes to new boilers, but the standard is very high, and there isn’t much splitting some of the main brands. One thing is for sure: there is a boiler out there for you, discover our boiler quote page for more info.

Let’s take a look at how long boilers tend to last. 

What is the Average Life of a Boiler?

Boilers are tough bits of kit that we count on to work day-in, day-out.

Boilers have changed dramatically in the last 30 years or so, but many old workhorses from the 1980s or before are still going strong. Truth be told, boilers should last many, many years, especially if they’re well-maintained.

So, what is the average life of a boiler?

What is the average life of a gas boiler?

There are three types of gas boilers; system boilers, conventional (or traditional) boilers, and combi boilers.

Boiler controls

Above: Gas boiler controls

All of these boilers use gas to heat water and account for the vast majority of boilers in the UK. Gas is cheaper than electricity, which makes it the best energy choice for heating most homes.

The average life of gas boilers depends on when the boiler was manufactured, its make, use factors (e.g., how much it’s used), and environmental conditions (e.g., water hardness).

In general, most boilers last for 10 to 15 years at least. However, regular services will boost life expectancy to 15 to 30 years or longer.

Statistics show that around 5% of all boilers break down over 10 years. Many boilers last much longer than 10 to 15 years, however.

For example, the workhorse boilers fitted in homes in the 70s to 90s are extremely durable if well-maintained, and some won’t break for decades. But, that doesn’t tell the whole story, as older boilers generally become less efficient over time.

Old boilers are less efficient

While an old boiler might be virtually indestructible, it might become costly to run. In fact, old boilers manufactured over 20 years ago might be just 50 to 60% efficient compared to 90%+ for boilers manufactured in the last 5 years or so.

If you own an old boiler (15 years or older), then ask a Gas Safe engineer whether it’s worth you replacing it. Replacing your boiler might save you money in the long run.

Above: Boiler with a corroded heat exchanger

What is the average life of a combi boiler?

Combi boilers were first introduced to the UK in the 1970s. Early models struggled with the UK climate, hard water, and other factors. It wasn’t until the late 80s, 90s, and beyond that combi boiler units became popular.

Combi boilers provide hot water and heating from the same unit. Water is heated when it’s required and there’s no cold water feed in the loft or tank in the loft. Combi boilers take water straight from the mains.

Boler control panel

Above: Control panel on typical combi boiler

Most combi boilers last for 10 years at an absolute minimum but will last much longer with regular servicing (preferably annual). From a technical perspective, high-quality combi boilers should last 30 years or longer under standard conditions.

  • Combi boilers typically last for 10 to 15 years.
  • Regular servicing improves lifespan and performance.
  • Rigorous use or hard water may decrease lifespan.‍

What is the average life of an oil boiler?

Oil boilers are a popular choice for properties isolated from the mains gas grid.

Oil boilers are as long-lasting as gas boilers. In fact, one study suggests that oil boilers last 3 years longer than gas boilers (15 years instead of 12 years). However, oil boilers often feature shorter warranties than modern gas boilers.

  • Oil boilers typically last for 15 years.
  • Like gas boilers, oil boilers require regular servicing.

Like gas boilers, oil boilers require regular servicing to keep them working smoothly.

How often should a boiler be replaced?

Many older boilers have remained in homes for over 30 or 40 years and are still going strong.

Generally speaking, boilers last for 15 years or so before serious repairs are required to keep them working efficiently. While a boiler may keep working, it’ll probably become increasingly inefficient over time, leading to higher heating bills and lower heating efficiency.

Engineer fixing boiler

Above: Boiler repair

Whether or not you should replace a boiler depends on the following things:

  • Heating performance, are your radiators heating up properly?
  • Noise; old boilers are often noisy and make banging or hissing noise.
  • The pilot light glows orange (get this checked ASAP).
  • Heating bills seem to rise beyond what’s expected.
  • Boiler cuts out regularly.

Summary: What is the average life of a boiler?

Boilers typically last for at least 10 to 15 years, but can last for much longer if serviced regularly.

It’s pretty tough to estimate when a boiler needs to be replaced. For example, a boiler may suddenly fail, but it will usually grow less efficient, resulting in costlier energy bills.

Choosing when to replace a boiler is a strategic decision, so give us a call to discuss your new boiler.

When to replace your boiler FAQ

Should I replace my 10-year-old boiler?

If your new boiler is 10 years old, the warranty has probably ended already. In this case, you might be able to extend the warranty by contacting the manufacturer or seller.

Replacing a 10-year-old boiler is probably unnecessary unless it’s showing signs of serious issues that can’t be rectified by repairing the boiler. On balance, it’s unlikely that anyone would need to replace a boiler that’s just 10 years old.

Can a boiler last 25 years?

Boilers can last for 25 years, but they’ll likely need to be repaired a couple of times within that period. Many old boilers last for longer than 25 years, but servicing becomes increasingly important as the years go by.

If your boiler is 25 years old, ask a Gas Safe engineer how efficient it is. Your older boiler may be costing you a chunk in heating bills, and swapping it out for something newer might reduce your bills.

Can a boiler last 30 years?

Boilers can last for 30 years or longer if they’re well-maintained. The problem is that older boilers might be inefficient compared to newer ones, making them costlier to run. If you’ve got an old boiler, ask a Gas Safe engineer to check it and recommend whether or not you should change or upgrade it.

If you are thinking of upgrading your boiler, give us a call to discuss your options.

How Do You Extend a Boiler’s Life?

Boilers are built to last, but that certainly doesn’t mean they’ll last forever. So an efficient, good-functioning boiler is more important now than ever as they help keep energy bills as low as possible. 

Boilers are built to last, but that certainly doesn’t mean they’ll last forever. So an efficient, good-functioning boiler is more important now than ever as they help keep energy bills as low as possible. 

As a rule of thumb, boilers last 10 to 15 years, but this can be boosted to over 30 years with regular servicing. Many of the traditional workhorse boilers of the early 90s are still going strong today, but you can be certain they won’t last too much longer if they’re not serviced. 

So, how long do boilers last?

How to Increase the life of your boiler

A boiler’s average lifespan is 10 to 15 years doesn’t mean it can’t last longer. The following will help you lengthen your boiler’s life and prevent breakdowns: 

1: Annual service

You guessed it…an annual service will prolong your boiler’s life. Annual servicing is required to keep new boilers in warranty too, which is important to protect your boiler in case of catastrophic failure. 

Black spanner tightening nut on boiler pipes

Above: Boiler servicing is cheaper than repairs or a new boiler

Annual services are relatively inexpensive, costing £50 to £100 or so. The cost of a service has to be compared against: 

  1. The cost of repairing a broken boiler
  2. The cost of replacing a boiler 
  3. The cost of heating a home with a poorly maintained, inefficient boiler

So, you can’t take the boiler service fee at face value, and it’s certainly not money wasted.

2: Powerflush 

Powerflushes use chemical liquid cleaners to clean and flush the entire central heating system, including the boiler, radiator and pipes. 

Powerflushes are great if you need to clean your radiators and the boiler, but should only be carried out by trained heating engineers. 

Above: Powerflushing in action 

Older systems may not be suitable for powerflushes as they place some strain on the pipework and boiler, which may cause more harm than good. However, if you’ve got an extremely sludgy or dirty heating system, then an engineer might recommend a powerflush. 

3: Replacing magnetic filters

Magnetic filters use magnetism to attract metallic debris and filter it from the heating system. Not all boilers have them as standard, but they can usually be installed. A boiler service should include a check of magnetic filters, where applicable.

4: Fix issues as soon as you can 

If you notice issues with your heating system, such as poor or worsening heating performance, unusual sounds emanating from the boiler or leaks, then get them checked out as soon as you can. 

Fixing issues quickly is nearly always cheaper than waiting until small problems develop into full-blown faults. 

Call a heating engineer immediately if you notice an orange or blue-orange pilot light rather than a blue pilot light (older conventional boilers only) or a leak from the boiler or the pipework joining the boiler. 

If you smell gas and suspect a gas leak, evacuate the immediate area and call your supplier’s emergency line. 

How to take care of your boiler

Boilers require care, maintenance and TLC like any other household appliance. That extends to the radiators and the rest of the heating system. Any weak component might replace additional strain on the boiler, so it’s important to take care of radiators and other components too. 

Bleed radiators 

Radiators require periodical bleeding to keep them working efficiently. If you notice a decline in heating performance then bleeding should be the first thing you do. Furthermore, it’s easy to bleed radiators yourself without specialist tools or skills. 

Above: Bleeding radiators 

Radiators become blocked with air that prevents them from working well, increasing boiler pressure, increasing strain on the boiler. Boiler pressure should be maintained at 1 to 2-bar. Only closed or pressurised open boilers have pressure metres.

This is a simple task that anyone can undertake, so long as they have a bleed key. To bleed a radiator:

  1. Switch the heating off and allow to cool.
  2. Place cloth beneath the radiator.
  3. Use a bleed key to gradually release air from the bleed valve. Note, the goal of bleeding is to release air and not water.
  4. When no more air comes out, tighten the valve.
  5. Check boiler pressure; it should be in the green zone, usually between 1 and 2 bar.

Adjust pressure 

If bleeding your radiator fails to depressurise the boiler to safe level, you might need to check the expansion vessel. In this case, it’s best to call a heating engineer to check the expansion vessel and filling loop. 

Drain and balance system 

Sometimes, it’s necessary to balance or drain the entire heating system. This is a fairly long-winded job that you should only consider if you’ve bled your radiators and had a recent service. 

Servicing grey heating system

Above: Draining heating systems 

A poorly balanced heating system may result in some radiators heating up long before others or not heating up at all. Here is a basic step-by-step to balancing a heating system: 

  1. First, make a rough diagram of your radiators and their location in your home.
  2. Switch your central heating off and allow your radiators to cool.
  3. Open up every valve on every radiator (both the manual valve/TRV and lockshield valves).
  4. Switch your heat on and find which radiators heat up first, in order from start to finish.
  5. Turn the heating off, and allow all radiators to cool.
  6. Switch the heating back on and close the lockshield valve on the fastest radiator. Open it around ¼ of a turn.
  7. Use a thermometer to check the temperature difference between the valve pipework leading to the valve and leading out of the valve. Adjust the lockshield until the temperature difference is approx 12C.
  8. Repeat the process with other radiators.

Summary: How long do boilers last

Boilers should last for 10 to 15 years, providing they’re regularly serviced. Regular servicing is extremely important for any boiler but is especially important for brand new boilers, to keep them in warranty and for older boilers, to prevent them from breaking. 

There are various ways to keep your boiler working properly, like bleeding your radiators and keeping an eye on boiler pressure. 

If you notice a change in heating performance or suspect a fault, get it checked out. It’s much better to nip issues in the bud than to let them develop!

Next, we’re going to assess when a boiler needs replacing and what to do in that situation. 

When Does A Boiler Need Replacing?

Boilers are stalwart devices that should soldier on for many years of faithful service. But, eventually, boilers do reach the point where continued repairs are unrealistic and replacing them is the best course of action.

Boilers are stalwart devices that should soldier on for many years of faithful service. 

But, eventually, boilers do reach the point where continued repairs are unrealistic and replacing them is the best course of action. Boilers typically last 10 to 15 years at least, but it’s not unheard of for old workhorse boilers to reach the ripe old age of 30 years or so. 

Regular professional servicing will extend the life of a boiler, and as long as they’re working efficiently, there’s little reason to replace them in a hurry. However, that isn’t always the case, and it’s certainly possible for a 10 to the 15-year-old boiler to go downhill quickly. 

A problematic boiler older than around ten years may be worth replacing. It depends on the boiler and the type of fault(s) it’s developed. 

Signs your boiler needs replacing

It’s not always easy to tell when a boiler needs replacing. There are a few telltale signs to watch out for, however. Here are signs your boiler needs replacing: 

Higher bills

Old boilers are almost unequivocally less efficient. An older boiler might be as low as G-rated, or 60% efficiency, which is far from the 90%+ efficiency of modern boilers. New boilers must be 86% efficient for gas and 85% for oil to meet Building Regulations. 

It’s often said that boilers older than 10 to 15 years are generally inefficient and costly to run, but this does vary, so it’s worth asking a heating engineer at your next scheduled service. 

Woman wearing grey cardigan bent over piles of bil

Above: Bills higher than you expect? It could be your boiler 

While it’s tricky to tell if your bills are rising because we’ve come to expect them to rise anyway, do try and compare your bills to see if they’re increasing ahead of energy price rises. 

  • Higher bills may indicate worsening boiler performance.
  • If your bills climb over winter without a change in your gas tariff, it could be the boiler.
  • New boilers have to be at least 85% efficient, but many are 90%+ efficient.
  • A new boiler will gradually pay for itself compared to running an old, costly boiler. 

Strange smells from the boiler

Boilers should not produce any smells whatsoever. So, if your boiler starts producing a sulphurous, eggy or otherwise unusual smell, then evacuate the building and call the National Grid Emergency Line at 0800 111 999. 

Other strange smells, whether burning smells, hot smells or something else, should also be checked out immediately. When the smell of gas is accompanied, call the emergency line and evacuate the building. 

This might accompany a change to your boiler’s pilot light. If you notice the pilot light turning orange, check to ensure your carbon monoxide alarms are working properly and call an engineer as soon as possible. 

  • Boilers should be odourless.
  • Most gas smells are an emergency - don’t hesitate to call the emergency gas line if you suspect a leak.
  • Changes to the pilot light might accompany smells.
  • Never ignore a smelly boiler!

Changes to the pilot light 

Older boilers have a pilot light which lights the main burner. Newer boilers have electric ignition and no pilot light. 

blue boiler pilot light

Above: Some boilers have a pilot light visible through a front panel 

It’s firstly important to say that if you smell gas, you shouldn’t go near the boiler to check the pilot light. Instead, shut off your gas at the mains supply, evacuate the immediate area and call the gas emergency line. 

Pilot lights should burn wholly blue. If they burn orange or have an orange tinge, then shut off the boiler and call a Gas Safe Engineer.

If this is the case, the boiler might emit dangerous carbon monoxide, so check with a meter and keep away from the boiler until you receive advice. Remember, carbon monoxide is odourless and can be fatal. 

  • Changes to the pilot light are signs of existing or imminent faults.
  • Not all boilers have pilot lights - some have electric ignition.
  • Never ignore changes to the pilot light.
  • If a strange smell accompanies a change to the pilot light, evacuate the premises and call the emergency gas line. Luckily, this is rare. 

Frequent issues

If your boiler shuts off randomly, displays frequent error messages, stops working shortly after firing up or heats slowly, then it might be worth replacing. 

Boilers shouldn’t need to be repaired yearly - a regular service should keep them working smoothly. If you find yourself calling out the engineer to fix your boiler regularly, then ask if it’s worth replacing instead. 

  • Frequent boiler issues may indicate poor overall function.
  • It’s sometimes cheaper to replace the boiler than keep repairing it.
  • If regular servicing doesn't keep a boiler working smoothly, there might be other issues.
  • Ask a heating engineer for advice. 

Low performance

We’ve already mentioned how higher bills are a red flag for many boilers, but you can also gauge boiler health by observing how well your radiators heat up. 

Above: Poor boiler performance might indicate issues 

It’s worth mentioning that inefficient heating systems may be caused by many things like bad boilers, sludgy radiators and issues with feeding and water tanks. Ask a heating engineer for advice if you notice a general drop in central heating performance. 

  • Poor overall heating performance might indicate numerous faults.
  • A replacement could be a good choice if your boiler function declines over winter.
  • Poor performance, accompanied by other issues such as leaks, warrants immediate evaluation by a heating engineer.
  • Ask a heating engineer for advice. 

Parts problem 

Older boilers may work well enough to keep them going year after year, but it’s possible for some more leftfield or unusual models to develop parts problems that make them harder to service or repair. 

Like with older cars, spare parts can also become expensive and time-consuming to source. In this situation, you might have no choice but to replace the entire boiler. 

  • Old boilers become expensive to repair or service.
  • Sometimes, sourcing new parts for a boiler might become impossible.
  • In this situation, your heating engineer might warn you that you’ll need a new boiler soon.
  • Replacing the boiler is a sound economic decision in such cases. 

Noisy system

If you notice new sounds coming from your boiler, whether it be a hiss, banging noise or some other noise, then it’s best to call a heating engineer as soon as possible. 

Above: Very noisy boilers should be assessed as soon as possible 

These sorts of issues are often easy to rectify by cleaning the boiler and/or replacing heavily worn parts but might signify the start of a wider problem that warrants boiler replacement. 

  • Heating systems produce some noise, but some noises shouldn’t be ignored.
  • New or worsening noises warrant an immediate evaluation by a heating engineer.
  • If you hear noises accompanied by the smell of gas, then evacuate the premises and call the emergency gas line.
  • Noisy systems may be fault-ridden, or it could be a simple issue. 


You should address a leaking boiler as soon as possible. If left unchecked, a leak might cause a potentially disastrous electrical short. In addition, electrical shorts might require the boiler’s entire electronic system to be replaced, which might warrant a new boiler. 

  • Leaking boilers can cause catastrophic short circuits.
  • In some cases, this could pose a fire risk.
  • If you notice any water coming from your boiler unit, call a heating engineer as soon as possible.
  • Never ignore a boiler leak. 

When does a boiler need replacing? 

Some of these situations are an emergency, such as smelling gas or noticing a pilot light issue. 

If you smell gas coming from near your boiler, don’t hesitate to call the emergency gas line.

In many of these situations, it’s sensible to consider replacing the boiler. New boilers are highly efficient and can save money in the long term. They can pay for themselves over time. 

Get in touch today with one of our heating engineers for advice. They will help you weigh up the long-term benefits of replacing a boiler vs repairing and servicing your existing one.

‍Can you service your own boiler?

It’s a fair question, especially for those with DIY, plumbing, or electrical experience. 

Most manufacturers and authorities recommend annual boiler servicing, which is also required to keep most boilers under warranty. 

Most wouldn’t consider servicing their own boiler, but those with relevant experience might wonder if they’re legally allowed to tinker with their boiler or gas appliances. 

Answering the question, “can you service your own boiler?” is not as straightforward as many assume. 

Can you service your own Gas boiler?

First and foremost, the law specifies a boiler service as “gas work”. Conducting “gas work” is only allowed if you’re a) a Gas Safe engineer or b) “competent” under gas regulations. 

So what is gas work? Does a boiler service always count as gas work? Here’s what isn’t considered gas work, according to Gas Safe

  • Removing or replacing any knob, button, or dial that is designed to be removed by the consumer.
  • Making alterations to the ‘decorative case,’ which is the exterior case that can be removed without being unscrewed.
  • Work and maintenance specified in a boiler’s user manual are intended for the consumer. 
Engineer fixing a boiler

Above: You can remove a decorative case, but anything more typically counts as gas work

You’re free to do any of the above, regardless of skill or experience level. 

A proper service, however, falls under “gas work”, as we can see below. 

  • Maintaining, servicing, permanently adjusting, disconnecting, repairing, altering, renewing fittings or purging them of gas or air.
  • If a fitting is not fully movable, changing its position is gas work. 
  • Removing the fitting.
  • Any work involving seals or gas-related components like the burner, combustion chamber, gas valve, etc. 
  • Removing some casings is considered gas work, even if the casing seems decorative.
Engineer fixing boiler

 Above: Removing some covers counts as gas work

In short, a boiler service always falls under “gas work” by the above definitions. 

Therefore, doing any of the above is highly unadvised unless you’re a Gas Safe engineer but still may not be illegal if you’re “competent”. But what does that mean? 

Who is competent to work with gas?

The Gas Safety (Installation and Use) Regulations 1998 (GSIUR) as amended. Approved Code of Practice and guidance states that gas work should only be undertaken by:

  • “(a) by a person who has successfully completed an industry-recognised training course followed by assessment of competence. Training that leads to assessment of competence in safe gas work should be recognised by the industry’s standards setting body; or
  • (b) in the case of a currently or previously registered person, where they have proved competence through a certification scheme; or 
  • (c) for those working at premises that fall outside the scope of the Regulations (see regulation 2(4) and associated guidance), by a person who has successfully completed an appropriate full training course followed by assessment of competence.”
Testing a boiler

Above: Boiler services require specialist diagnostic equipment

If you do not fall under any of the above categories, then do not risk servicing your boiler under any circumstances. ‍

In essence, if you’re not a Gas Safe engineer, then do not attempt to service your boiler yourself. Previous gas experience may not suffice to make you “competent”. 

Is it illegal to service your own boiler?

If you’re not a gas professional or a Gas Safe engineer, then servicing your own boiler is illegal under The Gas Safety (Installation and Use) Regulations 1998

Fitting or removing gas boilers and other gas-related appliances also counts under the act. 

Can you service your own oil boiler?

The Oil Firing Technical Association (OFTEC) is the oil boiler equivalent of Gas Safe. It doesn’t seem like an engineer needs to be OFTEC registered to service an oil boiler, but this would be highly recommended. 

There is no clear guidance on whether you’re legally able to service your oil boiler, as the rules are not as clear as for oil boilers. However, common sense dictates that no one who isn’t highly qualified in oil boilers and related fields should attempt to service one. 

Overall, it’s safest to assume you should contact an OFTEC engineer for your oil boiler service. 

Why do you need a Gas Safe engineer?

Gas Safe engineers are qualified in gas and gas appliances and are listed on the Gas Safe Register. They can legally service, install, remove, repair, and maintain boilers and gas appliances without legal limitations. 

There are a few situations where a Gas Safe engineer is legally required: 

  • Installing or removing boilers or gas appliances.
  • Conducting an annual service boiler for any rented residential property.
  • Servicing the boiler in a residential home or commercial property and receiving payment.
  • Building Regulations require engineers to notify the Local Authority within 30 days of the installation of a new gas heat-producing appliance.

The dangers of servicing your own boiler

Gas is potentially dangerous. Each year in the UK, there are some 31 potentially deadly gas explosions. Servicing boilers regularly is important to prevent such issues, but gas companies and manufacturers also know that some people simply can’t afford yearly services. 

In many cases, only a short check is required to ensure a boiler is working safely, and this shouldn’t be too expensive. If you suspect a gas leak, ring your gas company’s emergency line and evacuate your home until you get a response. Turn off the gas emergency control valve if you can. 

Any DIY services or repairs is ill-advised, even when someone falls under the definition of “competent” enough to conduct gas work.

Gas Safe engineers exist for a reason, and the strong advice is to never mess with boilers or gas appliances without discussing things with the correct professional. 

You might void your warranty 

Another consideration is that many manufacturers require yearly boiler services by Gas Safe engineers to maintain a warranty. Therefore, if you fail to service your boiler using a Gas Safe engineer, you might void your warranty. 

If your boiler is under warranty, a Gas Safe engineer typically needs to log a yearly service with the manufacturer for as long as the warranty lasts. 

Contact our Gas Safe engineers to service your boiler.

In short, it’s not safe to service your own boiler if you’re not a Gas Safe engineer. The Gas Safe Register exists for a reason, and the stakes are too high. Gas is potentially dangerous - give it a wide berth unless you’re Gas Safe!

I Still Have Questions, What Do I Do?

If you still don’t have the answers you need, then contact SES or book our services through the Clik 2 Fix portal.

Our friendly customer service team and expert engineers are on hand to help 24/7, 365 days a year.

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