Gas Boilers

vitodens 200-w
Gas Boilers

What size boiler do I need?

A new boiler should be seen as a necessary home investment, and this is why it’s so important that you make the right choice. When it’s regularly serviced and properly maintained, a boiler should last up to around 15 years, and will be solely responsible for delivering heat and hot water to your home.

When choosing a boiler, size is really important. Too big and it might use too much energy to heat water you’re not going to use and too small, you might find yourself running out of hot water or left with a cool house that cannot heat up properly.

So what factors do you need to consider before choosing a boiler size?

How is boiler size measured?

Before we look at what size boiler you should choose, it’s important to understand how boiler size is measured.

Boilers are sized in kilowatts (kW) rather than physical dimensions. A kilowatt is a unit that is used to measure how much energy the boiler can output as heat. Generally speaking, the more heat and hot water you require, the higher the output you will need, although other factors also play a part in choosing the right size.

This means that boiler size is essentially based on the number of radiators, the number of bathrooms and the number of bedrooms. 

What size boiler should I get?

You can use our handy table below to determine which size boiler is right for your home, depending on the number of radiators, bedrooms and bathrooms.

No of radiatorsNo of bedroomsNo of bathroomsBoiler size
0-5119-18 kW
5-102224-27 kW
10-203328-34 kW
20+4+4+35-43 kW

If your home has more than four bedrooms and three bathrooms, then a combi boiler likely won’t be the best option. Instead, you may need a boiler that is better equipped to deal with high water demand, such as a system or regular boiler. 

Should I choose an oversized boiler?

In the past, homeowners may have been advised to choose a larger boiler to compensate for heat that is lost naturally, such as through the roof, walls and windows of your home. But modern advances in boiler technology means that they’re more efficient than ever, and choosing a bigger boiler than you need isn’t recommended. It could result in wasted energy and is likely to leave you paying more than you need to for your gas bill.

Calculating your heating and hot water usage

When choosing which boiler size is right for your home, it can be important to consider your heating and hot water demands, as well as other factors, such as heat loss and water pressure.

Hot water demand

Most UK households use a combi boiler, which serves both heating and hot water on demand from the mains water supply. They are practical, convenient and cost effective to run. Where there is just one bathroom, they are often quite sufficient to meet the needs of the household as far as hot water is concerned.

If, however, you have a large or growing family, or your home has more than one bathroom, a combi boiler may struggle to provide sufficient hot water. This is because the water is heated as it is used, rather than stored in a tank.

If you are likely to be using more hot water, you will certainly want to consider whether a conventional or system boiler that includes a hot water tank is the right choice.

Ultimately, which type of system you choose will have an impact on the size of boiler you need.

A combi boiler is sized to provide hot water on demand and you will see its capability expressed in terms of how much water it can heat per minute to a defined temperature rise, e.g. 12 litres per minute at 35°C rise.

The higher the flow rate you require, for filling baths more quickly or to have a more powerful shower, for example, the larger your boiler should be. This means that combis are typically oversized in order to meet heating demands, making them less efficient than other types of boilers, such as system and conventional boilers, when heating the home.

System boilers and conventional boilers preheat hot water to be stored in a water cylinder and therefore, hot water flow rate is not as critical. The heat load for an average home can vary from around 10 kW to 20 kW, which means that system boilers and conventional boilers can be sized smaller than combi boilers. They will therefore be more efficient when heating the home, but they require more space. 

Number of radiators

A key factor that will have an effect on the size of boiler you need is the number of radiators. If you have just a few, for example in a small flat, then choosing the smallest suitable boiler will offer you the greatest level of economy and ensure you avoid paying more than you need to for energy.

The average three or four-bedroom house usually has about ten radiators. For this reason, it would require a 24-30 kW combi boiler. A larger house that has more radiators and more bathrooms would need a larger 30-35 kW boiler.

If you have a large house with as many as twenty radiators, a large 35-43 kW combi boiler may be needed to provide sufficient heat with the best economy, or you would be better to change to a system boiler, which could be sized more appropriately.

Other factors to consider

Heat loss

In the past, the physical size of your house was often used as a factor in considering the right size of the boiler. Whilst this still plays a part, a much better way to choose the most effective and efficient boiler size is to have an expert calculate your home’s heat loss.

Heat loss calculations take into account a range of factors including floor area, number of radiators, number and type of windows, insulation and other aspects that will cause heat to be lost from your home.

A heating expert will then use this information to work out how much heat your house will lose on the coldest day of the year, allowing them to advise you on the optimum boiler size.

Type of fuel

The type of fuel available in your property can also affect the size of the boiler you choose, as boilers for different fuels can vary in terms of efficiency.

Most UK homes use gas boilers which offer the greatest level of efficiency, while houses that are not connected to a mains gas supply often choose to use an oil-fired or LPG (liquefied petroleum gas) system instead.

The type of fuel your boiler uses could impact the size you need, and it is recommended that you seek advice from a professional in order to make the best possible investment.

Water pressure

Although this is not usually a concern for newer houses on a mains water supply, some older homes may experience poor water pressure, and this can have an impact on the type and size of boiler you choose.

A combi boiler must be big enough to cope with the amount of water passing through it to be heated – the higher the flow rate, the bigger the boiler should be.

Whilst most manufacturers provide information on the maximum flow rate for their boilers, getting a professional to assist you could make the process easier, ensuring you choose the most economical boiler for your home and helping you to save money in the long term.

Gas Boilers

What is a heat exchanger in a boiler?

So what exactly is a heat exchanger

and how does it work in your boiler?

A heat exchanger does precisely what its name describes – it allows heat to be exchanged between two fluids or substances, usually water or gas, without letting the substances mix together. You may take it for granted that your boiler is able to heat water up and then deliver it around your home, either through your radiators or to your taps. You might not, however, have thought about how it achieves this. The heat exchanger is the key. Without this device, your boiler wouldn’t be able to warm up your water.

What is the purpose of a Heat Exchanger?

Heat exchangers can be found in all sorts of appliances and industries. They’re used to recycle heat in the waste gases at power plants and to heat swimming pools. They can also work in the opposite way and cool something down. Refrigerators and air-conditioners contain heat exchangers which draw the heat away from the room or compartment to keep it cool.

Heat exchangers are most commonly found in boilers. They allow your boiler to heat the water that is then pumped through your radiators to warm your home. The heat exchanger is a long, coiled pipe. This cooler pipe or heat exchanger surrounds the combustion chamber, where gas is burned to release its energy. The water in the heat exchanger is warmed by the hot combustion gas and then pumped round your home thereby transferring the energy from the gas to the rooms where it is needed. 


Condensing gas boilers

Condensing gas combi boilers used to have two heat exchangers, as it was thought that this was more efficient. To begin with, the water was heated in the primary exchanger. As hot waste gases were produced, the water that returned from its circuit around the radiators was pushed into the secondary heat exchanger, which only used waste gas to heat the water. This second heat exchanger recovered the “latent heat of vaporisation” which adds almost 10 % to the boiler efficiency.

What boilers have stainless steel heat exchangers?

Without a heat exchanger, your boiler wouldn’t be able to heat the water that moves around your radiators and comes out of your taps. This makes a heat exchanger a really important part of your boiler. If it breaks, you won’t have any heating or hot water. Therefore, it’s imperative that it’s made out of a strong material that is long-lasting. Stainless steel is one of the best materials for this purpose. There are many benefits of using stainless steel for the heat exchanger. First, stainless steel offers impressive resistance against corrosion.

Viessmann boilers only contain one heat exchanger. The patented stainless steel design means that it is able to capture the heat from combustion and the latent heat of vaporisation in one pass of the single heatexchanger. This is due to the exchanger’s larger surface area and the position of the burner, which sits centrally within the cylindrical heat exchanger and the patented design. As the water moves through your radiators, it’ll slowly begins to cool down. Your boiler is able to detect the temperature of the water and modulates the gas burning rate to deliver the correct amount of heat.

gas boilers by Viessmann
Combi Boilers

Combi Boilers

System Boilers

System Boilers

Open Vent Boilers

Open Vent Boilers

gas boilers by Viessmann
Gas Boilers

Why choose a Viessmann Boiler ?

Everything you need to know about Viessmann Boilers

If you are upgrading or replacing your boiler, you will know that there are many different boilers built by a wide range of manufacturers. Viessmann boilers are one of the more popular choices for gas boiler replacement, but why would you choose a Viessmann boiler in favour of any other manufacturer?

All About the Viessmann Company

The Viessmann Group was founded over 100 years ago in Germany by Johann Viessmann and has been a family-owned business since then, with four generations leading the company. Since 2018, the Chairman of the Executive Board is Martin Viessmann, supported by co-CEOs Maximillian Viessmann and Joachim Janssen.


With more than a century of experience behind the company, you can be certain that the technology within Viessmann boilers is of a high standard, and the quality of German engineering is respected around the world. Viessmann have a great reputation for producing boilers, but also produce other energy efficient heating solutions including heat pumps, hot water cylinders, and solar thermal solutions.

Are Viessmann good boilers?

gas boilers by Viessmann

With their German origins, the question is not just if Viessmann produces good boilers, but more, “Are Viessmann Boilers any good in Ireland?”

Fortunately, the boilers produced by the company are designed to work perfectly in the Irish market, and they make a great solution for anyone looking for a new boiler. They are designed to be highly efficient which will help to save money on your energy bills, in addition to reducing the overall amount of energy you use. This efficiency will have a positive impact on the environment, especially if you are replacing a much older boiler.

Viessmann boilers are built for compatibility with other components within your heating system including renewable and alternative energy sources. They are a great choice for people who have an eye on the future, and they will not quickly become outdated.

In fact, they are packed with features that enable integration with modern home environments. This includes integration with smart homes, so you can speak to your digital assistant to control the heating or use an app from a remote location to switch the heating on (or off) before you get home.

Anything you would want a boiler to do (or to connect with), the Viessmann boiler range can do.

Are Viessmann boilers reliable?

During summer, you may not be concerned about the reliability of your boiler as much as you would in winter. There is less call for hot water and heating in the warmer months but have no fear – Viessmann boilers are reliable all year round.

Viessmann boilers have been awarded “Which?” Best Buy status for several years running, giving an indication of both their reliability and value for money. In multiple comparisons and tests, the Viessmann boilers consistently perform as well or better than their closest rivals, with a higher level of reliability.

The boilers also come with long guarantees, so should a problem be experienced, it can be fixed without any additional cost to the owner.

Are Viessmann boilers efficient?

The more efficient your boiler is, the less it will cost to run. If a boiler runs at 50% efficiency, half of the energy used to run it is wasted. Running at 100% efficiency is impossible as small losses will be made even in the best boilers, but modern boilers often run at higher than 90% efficiency.

Viessmann gas boilers are at the top of the list when it comes to efficiency with some of the boilers reaching 97% efficiency. This can give an energy rating of A, A+, or even A++, indicating that the boiler is super-efficient. The less energy that is required to heat your home and water, the lower your overall energy bills will be.

Which Viessmann boiler do I need?

Viessmann boilers are suitable for replacing many existing boilers and the exact boiler you need may vary. The Which? Survey recommended the following boilers as their Best Buys:

The most appropriate boiler for your home may be one of the above boilers or a different model depending on your needs and requirements. Important factors to consider include the size of your home, the number of bathrooms, the number of radiators, and the number of people living there.

Your heating and hot water usage patterns may also affect the choice – the most reliable way to determine which Viessmann boiler you need is to speak to the professionals at Precision Heating. We can advise you, using our years of experience, on the most suitable boiler for your home.

How much is a Viessmann combi boiler?

This will depend on which model of Viessmann combi boiler you choose. The price will vary as larger homes may need a more powerful boiler, which will cost more than a less powerful boiler. Generally, the price for a combi boiler is between €800 and €3000, so it is important to choose the right boiler for your property.

An overpowered boiler will provide no benefit, while an underpowered boiler will not heat your home effectively.

Why Choose a Viessmann Boiler?

Independent tests confirm that Viessmann boilers are extremely efficient and reliable, capable of saving money on your energy bills for many years to come. With long guarantees, you can be certain the boilers will keep working in the long term, and their flexibility and options for integrating with smart home functions make them a great choice for any home.

They are built from high-quality materials with technology that far exceeds all legal requirements for energy efficiency, providing real value for money.

When you choose a Viessmann boiler, you can be confident that your home will have energy-efficient heating and hot water whenever you need it.