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Heat Only Boiler Replacement Guide


Heat only boilers are known by a few other familiar names. They are also called conventional boilers, traditional boilers, or regular boilers. Like every other boiler type in the market, the heat only boilers are designed to serve a specific niche.

They are not as common as the system or combi boilers, mainly because they take up so much space, but there are thousands of homes in the UK that could benefit from their design and functionality. If you’re considering installing a heat-only boiler in your home, this guide is everything you need to help you make the right decision and help you with all the vital information you need to complete the process successfully.

What is a Heat Only Boiler?

A heat-only boiler has a unique set up that comprises of the boiler, a hot water cylinder often placed in an airing cabinet, and cold-water storage, which is usually placed in the loft.

Properties that use heat only boiler have the central heating system linked to both the hot water and cylinder and storage tank.

The boiler is better suited for large homes and businesses that require substantial amounts of hot water and have multiple taps and showers that might be running at the same time.

Heat only boilers are capable of producing vast amounts of hot water with ease to keep up with demand with the help of the hot water cylinder, which holds the hot water.

How Does a Heat Only Boiler Work?

The heat only boiler relies on two tanks to function correctly. The first tank is the cold water tank. This fills up with water from the mains supply. This water is heated up and supplied to the taps and showerheads around the house.

The second tank is also referred to as an expansion tank or a feed tank. It brings in cold water to maintain the level of the central heating system. Because of the heat, some of the water in the radiators might evaporate or leak. The work of the feed tank is to replace the lost water.

The feed tank can also be used to hold hot water in case the boiler heats too much water.

Here is a short step by step guide on how all the components work together to keep your home comfortable.

  • Water from the mains supply fills the tank in the loft with cold water. The water flows down to the hot water tank where it is held.
  • When you turn on the shower or tap, the water from the expansion tank feeds water to the boiler. The boiler ignites, and the heat exchanger inside the boiler heats up the water.
  • A pump inside the boiler then pumps the water to the hot water cylinder where it is held until the heating comes on. When it does, the water is circulated to the radiators.

When Should You Install a Heat Only Boiler?

It’s more important to interpret the situations around your home since these can help you identify which type of boiler would flourish in your environment and meet your needs.

Occasionally, heat only boilers are installed in newly built homes. However, when replacing, you can only replace one heat only boiler with another. This is because the boiler is better suited for the way the system is set up. Replacing it with another type of boiler could require extensive work and reinstallation of the pipework.

A heat only boiler is ideal when you’re looking at a vented system where you have water tanks in the loft. This type of boiler will also flourish in areas that have low incoming water pressure.

For a heat only boiler, the best scenarios are when you need plenty of hot water on multiple taps at the same time, where you have a vented system and in areas where there are concerns about water pressure.

Advantages of a Heat Only Boiler

Plenty of hot water

The most obvious benefit of having a heat only boiler is the amount of hot water the boiler is able to produce. With a heat only boiler, all your hot water needs are well taken care of.

Run multiple taps

You can run multiple taps and showers around the house because the hot water comes from a hot water cylinder. Running several taps and showers around the house will not affect the temperature or pressure of the water if you’re using a heat only boiler.

It can work with the old heating system

If you’re looking to replace the boiler but don’t have the budget to do the pipework, a conventional boiler can be your saviour. It is often preferred for older properties with an old heating system because they integrate and work with the old heating system easily. In most cases, only small changes are required. This can save you a lot of money in pipework installation.

Compatible with solar thermals

Home users who are keen on saving money on their heating will be glad to hear that modern heat only boilers are compatible with solar thermals.

Disadvantages of a Heat Only Boiler


If you want to have a regular boiler in your home, you must be willing to part with space. You will need space in the loft and in the airing cabinet for the two tanks. This is why heat only boilers are not considered safe for small homes, flats, or apartments.

It takes patience

With a heat only boiler, the production of hot water is not instant. You have to wait for the tank to heat up before you can use it, which can take a few minutes.

Heat Loss

Over time, the water stored in the hot water cylinder will lose heat, which reduces the efficiency of the boiler. To curb this, the cylinder needs to be insulated, which is an additional cost even though it is not significant.


Heat only boilers are both complex and expensive to install. They have numerous different parts and pipework, which requires skill and patience to install successfully.

How Much Does A Heat Only Boiler Cost?

Combi boilers and installation costs can be as little as £1,750. When it comes to heat only boilers, the price tag is not that affordable. Not only does the boiler itself come with more components, but also, it requires more materials for installation compared to other types of boilers.

One of the tanks alone will cost a few hundred pounds together with the installation cost. Multiply this cost by two for the other tank and you start getting a grim idea of what you should expect to spend to get a new heat only boiler and get it working.

Ideally, the cost of installing the boiler starts from around £2,500. You could pay more depending on the state of your central heating, among other factors. You can GET A FREE BOILER INSTALLATION QUOTE to give you a clear idea of how much you’re likely to spend to get the boiler running.

As for the boiler, the prices are different based on:

  • Output – Like combi and system boilers, regular boilers also come with various output ratings. The higher the output rating, the more the boiler will cost. However, that will also mean you get more hot water and better performance from the boiler.
  • Efficiency – Efficiency is a critical factor when choosing a boiler. While the initial cost of a highly efficient heat only boiler might be higher, in the long run, it will save you even more money in the long run in the form of lower energy consumption.
  • Brand – The brand-name also has lots to say about the price tag of the boiler. Some brands focus on building premium boilers, while others focus more on practicality. Regardless of brand loyalty, the most important thing is to make sure you pick a boiler that is of good quality and safe for use.

Based on the above parameters, you can spend as little as £500 on your heat only boiler and the prices can go up to £2,000 for some models.

What are the Running Costs of a Heat Only Boiler?

It’s difficult to give an exact figure on how much you should expect to spend when using a heat only boiler.

The cost of running the boiler will depend on the type of fuel that is powering the boiler. Gas is the most affordable, followed by LPG and following closely behind is oil

You also need to consider the size of the house, the number of radiators and how frequently you use hot water. All these factors will determine how much you pay to run your heat only boiler.

Heat only boilers will cost a little more per year than combi boilers. They have a huge tank that requires to be kept warm and some of the heat escapes which mean the system might be a little less efficient.

To help curb heat loss, you should make sure the hot water tank is insulated. It doesn’t cost much, and it will save you a lot of money on your energy bills.

What is the Best Heat Only Boiler

Glow Worm Flexicom 35HX

The 35HX is beloved for a variety of reasons. The first is its high output capacity of 35kW, which makes it an ideal pick for large homes and small businesses. It is incredibly efficient at an approximate 89.2% Grade A ErP rating, ensuring you get value for your money each time you use the boiler.

The Flexicom 35HX is a gas-powered condensing boiler which guarantees affordable running costs as well as the best safety features on the market.

If you have a bias to the Flexicom range of boilers from Glow Worm, there are other varieties in the same range with different outputs that you can consider.

Baxi Solo HE Heat Only Boiler

Baxi is an extremely popular and reputable heating brand. They have been in the heating world for many years. It’s no surprise that one of their boilers also happens to be one of the best in the heat only boiler fields.

The Baxi Solo HE is a gas-powered, condensing, and fan assisted boiler with an SAP efficiency rating of 88.8%. The boiler has a 30.18kW output rating ensuring you can get plenty of hot water in your home.

The Solo HE range from Baxi is expansive, so you have different boilers to pick from depending on your preferred output. It’s, however, essential to note that some of the boilers might be available but are no longer in production.

Vaillant ecoTEC Plus 438

The Vaillant ecoTEC 438 is an extremely efficient boiler with an 89.2% efficient rating SAP. If you’re keen on reducing the long-term operation costs related to heating in your home, this boiler can help you save up to £200.

This is a high output boiler that tips the scales at 38kW. It works silently with a push-button intuitive programming features as well as a 5-year standard warranty to cover your purchase for labour and parts.

Vaillant has plenty of other decent heat only boilers that will give you good value for money. The boilers offer state of the art features and different outputs. You’re assured of finding something that will work with them.

Other brands like Worcester Bosch, Viessmann, and Ideal are also worth the mention since they also have an impressive collection of heat only boilers tailored for different situations.

What is the Difference Between a Heat Only and System or Combi Boiler?

Combi, system, and regular boilers are similar in many ways. They are also quite different. The differences depend on the type of boiler you are using.

Unlike combi boilers, regular boilers have tanks. In fact, they have two. The water in combi boilers is heated and directly supplied around the house while the water in regular boilers is heated then stored to be supplied when the need arises.

System boilers also have a storage tank. However, they don’t have a cold water storage or a feed tank. The regular boiler relies on the cold water and feed tanks to supply the heat exchanger with water for heating while the system boiler draws its water directly from the mains.

Another difference between the three types of boilers is the space requirements. Combis are the most conservative with regards to space followed by system boilers, and the regular boilers will take up the most space during installation.

Combis are compact units that come with everything neatly organised in one unit. System boilers come in two components – the heating unit and the hot water cylinder, while heat only boilers come in threes. They have the heater, a cold water storage tank and a hot water cylinder. That is why they require more space for installation.

Is a Heat Only Boiler Better than a System or Combi Boiler?

There’s no competition among the three boilers. In fact, each of them is designed to suit different households with different needs. Each of the boilers comes in various outputs, which is a reflection of its ability to produce hot water.

Combi boilers are designed for small homes with little to average demand for hot water. Combi boilers are perfect for homes with limited space and can’t afford to house the boiler.

System boilers are for homes with multiple bathrooms and require substantial amounts of hot water. To keep up with the high demand, system boilers have a hot water cylinder that is regularly topped up to make sure you have enough hot water for heating and use around the house.

Regular boilers are for large properties and small commercial businesses with a high appetite for hot water. The boilers are capable of producing incredible amounts of hot water with the help of tanks that supply and store the hot water. Regular boilers are also ideal for homes in areas where there are water pressure challenges.

The best of the three boilers depends on the challenges you are facing and your requirements. Combi boilers are best when you want a compact boiler to produce enough hot water for one or two people. System boilers are ideal when you don’t want to affect the flow rate of water in your taps and you have multiple bathrooms and regular boilers are perfect when you have water pressure problems, or you need hot water for multiple people in the home.

Get Free Quotes for a Replacement Today!

Once you have selected your preferred heat only boiler, you will need to have it installed either by a Gas Safe, or OFTEC registered engineer. The boiler installation cost will vary and you must shop around to get the most competitive boiler replacement quotes.

Compare the Boiler Market that can help you get started on your quest by providing you with accurate quotes from installers in your area. By following a few simple steps, we can have a qualified boiler installer in your area give you a call and provide you with a competitive, free, and no-obligation quote.

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