Frequently asked questions about gas boilers 

Who can install a gas boiler? 

By law, all gas boilers should be installed by a Gas Safe registered engineer. The Gas Safe Register contains details of all gas engineers who are qualified and approved to work safely on gas appliances.

By law, all boiler repairs and servicing also must be conducted by a Gas Safe registered engineer.

What is the difference between a condensing boiler and a non condensing boiler? 

By law, all boilers sold in the UK must now be condensing boilers. The difference between a condensing boiler and a non-condensing boiler is in the way they utilise waste heat. In a non-condensing boiler, waste heat is exactly that, waste heat. Whilst in a condensing boiler, the exhaust gases in the flue are put through a heat exchanger and the latent heat is extracted, rather than being released into the atmosphere. This makes condensing boilers at least 25% more efficient than non-condensing boilers and allows new Viessmann boilers to achieve efficiencies of up to 98%. Thanks to the efficiency, savings from a non-condensing boiler can be up to £310 compared to a condensing boiler, according to the Energy Saving Trust. 

How often should I get my boiler serviced?

For the best possible performance and long service life from your boiler, we recommend that your boiler is serviced annually. However, if you are a landlord servicing your boiler annually is not only recommended, it is a legal safety requirement.

By law, boiler servicing must be conducted by a Gas Safe registered engineer. 

Why should I get my boiler serviced? 

If you are a landlord, it is a legal requirement that you get the boilers in your properties serviced annually.

If you are a homeowner, it is not a legal requirement, however it is recommended that you get your boiler serviced annually.

By law, boiler servicing must be conducted by a Gas Safe registered engineer.

As part of the service, the engineer will inspect and test all key components in your appliance. By servicing your boiler 

By servicing your boiler annually, you will be ensuring that it works at maximum efficiency and minimising future breakdowns. 

How do condensing boilers work? 

Condensing boilers work by ensuring that not only the heat from the combustion of fuel oil or gas is utilised but also the heat which would otherwise escape unused up the chimney. Condensing boilers do this by extracting almost all of the latent heat in flue gases and converting this into additional heating energy. This is done in Viessmann condensing boilers with the use of a stainless steel Inox-Radial heat exchangers. These cool the flue gases before they are routed to the stack, with the process of doing this deliberately condensing the water vapour contained within them and transferring the additional heat into the system. 

What is a combi boiler? 

A combination boiler, more commonly referred to as a combi boiler, combines a high-efficiency water heater and a central heating boiler in a single compact unit

What are the benefits of a combi boiler? 

There are many benefits of a combi boiler, these include: 

  • Compact and space-saving designs, ideally suited to smaller properties
  • Only one unit is needed for both heating and hot water
  • No need for a separate pump, hot water cylinder or cold water storage tank
  • Hot water is delivered to your taps and shower at mains pressure
  • Greater reliability due to not constantly switching on and off, which means less wear and tear on components 

What is a storage combi boiler? 

A storage combi boiler combines the features of a system boiler and a combi boiler in one package, equipping them to provide efficient heating and provide hot water in large volumes.

With a storage combi boiler, there is no need for a separate domestic hot water (DHW) cylinder, meaning they are easier to install and they are an ideal solution for properties with limited space.

Storage combi boilers are ideal for households with a high hot water demand.

Will I need a separate hot water cylinder with a combi boiler? 

With a combi boiler, there is no need for a separate hot water cylinder. This is because a combi boiler combines a hot water boiler and a central heating boiler within one unit, and provides hot water on demand.

Do all types of shower work with a combination boiler? 

Most standard types of shower will work with a combination boiler. However, there are certain types of pump assisted power showers which can only be installed with tank fed systems. 

How are gas boilers rated? 

Gas boilers are rated using the SEDBUK scheme. SEDBUK stands for 'Seasonal Efficiency of a Domestic Boiler in the UK', and was developed by boiler manufacturers and the government to provide a fair comparison of boiler efficiency.

Much the same as other domestic appliances, SEDBUK ratings are shown on labels on the front of the appliance and are rated from A-G. 

What size combi boiler do I need?

When buying a combi boiler, bigger is not necessarily better. Rather than looking to buy a boiler with the highest output, you should match the size of the boiler to the output of your home. If you buy a boiler which is too small, you will find that you are not able to heat your home adequately or produce enough hot water. Whilst if you buy a boiler which is too big, you will find that you are producing more heat than required, which will increase your bills, waste energy and produce unnecessary emissions. 

The below sizing guide will ensure you buy the right size boiler for your home: 

  • 24-27 kW: small 1 - 2 bedroom house or flat, with up to 10 radiators and 1 bathroom
  • 28-34 kW: medium 3 - 4 bedroom house, with up to 15 radiators and 1-2 bathrooms
  • 35-42 kW: 4 or more bedroom large house, with up to 20 radiators and 2 or more bathrooms