Homeowners Professionals

Boiler replacement & installation guide

Viessmann answers your questions replacement & installation

Why do boilers leak?

A leaking boiler can be quite a serious problem and the repairs should only ever be conducted by a Gas Safe registered engineer.

The reasons for a leaking boiler can vary, though it usually indicates that an internal component such as a seal or valve is broken. It is important to get a leaking boiler looked at by a Gas Safe registered engineer as soon as possible, as the leak could cause corrosion or rust or lead electrical components within the boiler to short circuit.

Why is my boiler losing pressure?

There are a number of reasons why your boiler could be losing pressure. The most common reason for this is usually due to a leak somewhere in the heating system.

To identify a drop in pressure, check the water pressure gauge found on the front of the boiler. This should be set around the 1 bar. The position set when the boiler was installed is often shown by a red indicator needle.

If you have noticed repeated drops in your boiler pressure, make sure you check the gauge regularly. A drop in pressure usually indicates that there is a problem with your system.

As with any other repairs to your boiler, any repairs relating to pressure should only ever be conducted by a Gas Safe registered engineer. 

How do I know when to replace my boiler? 

It will be time to replace your boiler when the parts eventually wear out or they are uneconomical to replace. If your current boiler is a non-condensing boiler, switching to a modern condensing boiler also makes sense. With a Viessmann condensing boiler, you will find that it achieves up to 98% efficiency and you will be using less energy and saving money. This is because, unlike older boilers, condensing boilers use the heat from exhaust gases which otherwise would be released into the atmosphere.

How do I reset the Viessmann Vitodens 100?  

To  reset a fault  on your Viessmann Vitodens 100 boiler, you should turn the rotary selector “tr” for less than 2 seconds to “U Reset”, then back to the control range.

This will put the boiler into relay test mode if no fault is present.  

What is the most efficient type of boiler Viessmann offer?  

All Viessmann gas and  oil boilers  are condensing boilers. Condensing boilers work by ensuring that not only the heat from the combustion of gas or fuel oil is utilised but also any heat which otherwise would escape unused up the chimney. Condensing boilers do this by extracting almost all of the latent heat in flue gases and converting it into additional heating energy. This enables our boilers to be able to achieve efficiencies of up to 98%  

How do I know what size boiler to choose for my home?  

Boiler size is measured in terms of heat output. In our domestic boiler range, heat outputs range from 5 to 35 kW. If the boiler is too small, it will not be capable of heating your home properly.

Beware of the once-common practice of oversizing boilers by about 30% - with today’s technologies this is unnecessary, and if the boiler’s too big it will waste energy and increase your gas bills.

To match your boiler size to the size of your property, you should instead work out the heat loss your home experiences. This done by making a heat loss calculation which takes into account your property’s floor area, insulation and hot water requirements. Any Viessmann Trained Installer will be happy to work this out for you.

The only exception to this rule is with combi boilers, as a combi boiler will often be over-sized so that it can deliver a high volume of hot water, e.g. for showering.


What type of boiler do I need? 

The type of boiler you need will depend on the size of your property, plus your demand for hot water. Specifically find out which boiler you need here - Which Boiler Do I Need?

What are the letter ratings on boilers? 

The letter ratings on Viessmann boilers relate to the energy efficiency ratings set as part of the UK government’s Energy-related Products (ErP) initiative. Rated from G to A, G-rated boilers refer to those that are less than 70% efficient, whilst A-rated boilers refer to those that are over 90% efficient. 

Where in my house is it safe to install a boiler? 

On the whole, a boiler can be installed safely in most rooms in your home. However, there are a few things which you should consider: 

  • If wall-mounted, the boiler should be fixed to wall strong enough to take its weight when full of water
  • You should consider where the condensate pipe will be fitted, we do not recommend this being outside as it could freeze during cold weather and cause the boiler to stop working
  • The flue must be able to pass to the outside of your property, either through an outside wall or through the roof of your property
  • Your boiler can be fitted in a cupboard or airing cupboard
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