Contact us

New heating systems in older buildings

Photo: © ArTono /

New heating systems in older buildings – efficient and professional modernisation

Sooner or later, anyone who lives in an older building will ask themselves how they can most effectively modernise their old heating system. Heating system modernisation can also be interesting for owners of newer buildings. In addition to the specifications of the German Buildings Energy Act (GEG), there are numerous reasons to install a new heating system in an older building. But many homeowners feel deterred by the legal regulations and the high investment costs. In our guide, we answer the fundamental questions that are relevant to the topic of heating system modernisation in older buildings.

When is a home considered an older building?

Many a homeowner is initially unsure whether their property even counts as an older building. It's difficult to provide a catch-all definition of an older building because there are considerable differences between building ages in terms of building fabric, materials and floor plans. In order to make our recommendations as helpful as possible for system owners, we base the question of what constitutes an older building on practical concerns. We also include modern existing buildings among older buildings. So, if you're planning to modernise your heating system and do not live in a new build, our guide will also give you a handy overview.

What regulations apply to old oil and gas heating systems?

Since the first German Energy Saving Ordinance (EnEV) of 2002, there has been an obligation to replace boilers that were put into operation before 1  October  1978. This Ordinance was repeatedly amended until 2015 and finally became part of the GEG, so that today all oil and gas boilers over 30  years old are affected.  

Anyone thinking about renewing the heating system in an older building still has free choice of all heating technologies and fuels until 2026. From 2026, new installations of heating systems with oil will only be possible as hybrid systems, i.e. with the integration of renewable energies. This is the goal of the German government's climate package, which was adopted in autumn 2019. There are exceptions for existing buildings, however, such as when a gas connection is not possible.

In addition to these obligations, you should discuss aspects such as fire safety in the boiler room with your heating contractor and the responsible flue gas inspector before purchasing a Viessmann heating system. Furthermore, safety must be taken into account when installing a tank, or the boreholes for the geothermal probes in the case of a brine/water heat pump.

What costs do you need to consider when modernising heating systems in older buildings?

Since older buildings differ greatly depending on their age and history, varying types and amounts of work are required. In order to sensibly accommodate and operate a new heating system in an older building, the premises, flue system, chimney and thermal insulation may need to be updated as well. Exact details can only be provided by a heating installer after a thorough inspection.

Advice and support

It is worth analysing both funding offers according to need and possibility. Your contractor or a certified energy advisor will provide you with detailed information about grants and subsidy programmes.

Which new heating system is suitable for your older building?

There is no blanket answer to this question, as many factors play a role. In principle, there are numerous Viessmann heating systems that you can choose from when it comes to modernising heating systems in older buildings. The focus is on innovation and efficiency – with climate policy changes, new heating systems must reduce the use of fossil fuels. There are a number of ways to go about this.

Not every system is suitable for every existing building. Depending on the technology involved, there are a number of different requirements that must be satisfied. It is therefore all the more important that you clarify a few things for yourself beforehand, and define your selection criteria. Below, you will learn what you should look for before making your decision and which heating systems are best used in an older building.

The first question is which energy source you would like to use for heating in the future or how important sustainability and independence from power supply utilities are to you. In addition, acquisition and running costs will almost certainly play a role when deciding on a new heating system.

The following heating systems may be considered for a general switch to renewable energy sources:

If you're still using a constant-temperature boiler for oil or gas, you can, in the case of oil heating, replace it with a high performance oil condensing boiler. If you operate a gas heating system, we recommend installing an efficient gas condensing boiler. It costs about the same as a low temperature gas boiler, but runs much more economically.    

Technical requirements

Individual and technical criteria must be taken into account before modernising a heating system. The following aspects must always be verified:

How large is the installation room and is there sufficient space for energy storage (e.g. do the pellet heaters, pellet store and conveyor device require a certain installation space)?

Is the existing flue system sufficient for modern heating systems? Remember that new condensing boilers work with lower flue gas temperatures. Appropriate measures must be taken to protect the pipes and the chimney from damage.

In older buildings in particular, there are no standardised dimensions for windows or doorways to access the boiler room. Are the paths and the rooms large enough for the installation of the new heating system? Our professional trade partners would be happy to advise you on these topics.

What should you pay attention to beforehand?

You should consider the following factors:

  • Heat load calculation by a specialist heating contractor Energy status of the building and need for modernisation
  • Previous energy source and fuel of choice
  • Space requirements for the new heating system
  • Structural conditions of the building
  • Change with regard to heat demand
  • Previous and new heat distribution (radiators and area heating systems)

Fossil heating systems for older buildings

With Vitodens and Vitoladens, Viessmann offers efficient heating solutions for fossil fuels. Viessmann condensing boilers are highly efficient because they utilise the heat hidden in the flue gases. In addition, they are characterised by a favourable price-performance ratio. A Vitodens or Vitoladens can also be supplemented with a Viessmann Vitosol solar thermal system. This takes over a large part of the domestic hot water heating and can serve as a central heating backup, especially during spring/autumn. This also reduces the use of fossil fuels, as the solar thermal system uses free solar energy to provide heat.

Renewable energies for older buildings

In addition to the combination with Vitosol, you can also rely entirely on renewable energies in an older building – one way is with Vitocal heat pumps or Viessmann biomass heating systems. Heat pumps use free environmental energy from the air or ground. When using an air source heat pump in particular, the corresponding modernisation of the energy systems in the older building has to be taken into account. The heat demand of the building should be as low as possible so that the system can be operated efficiently.  

In addition, we also offer an option for carbon neutral heating with boilers fired by logs, pellets and woodchips. Viessmann Vitoligno products in particular offer an excellent alternative to existing oil condensing boilers. With these, the old oil tank can be used for pellet storage. A micro CHP unit with fuel cell can also be used under certain conditions (only in the course of extensive energy modernisation measures) as a means of significantly reducing heat demand in the building.

FAQ – frequently asked questions about planning, installation and operation

(Bild: © SN-Photography /

What do you need to consider when planning a new heating system for an older building? What must the premises be like so that the heating system installer can install the new heating system? And what steps need to be taken after installation?

You will find answers and useful empirical values on the topic of heating system installation in older buildings here.

A modern condensing boiler requires an adapted flue system that is designed for lower flue gas temperatures. In addition, it must be ensured that the condensate that is created in the flue system is collected and drained off. A standard drain connection near the boiler is sufficient for this.

The boiler room must be accessible from a corridor through a fire safety door. There must no other rooms directly adjacent to the boiler room. The heating contractor will clarify further details during a site inspection.

Many doors in older buildings are not yet standardised to the common 80  centimetre width. Make sure that your doors have the width needed to allow the system to be installed. For compact gas systems, kitchen or bathroom doors should be wide enough, depending on where the gas boiler is installed.

In older buildings, pay attention to the height of the boiler when tilted, i.e. the diagonal of the appliance and not just the height. This is because the heating system installer will need to tilt the boiler for transport and installation. The height when tilted is always higher than the build height. This is best clarified during the on-site appointment with the heating system contractor.

In addition to the heating system, you might have also modernised doors, windows or the faççade to ensure sufficient thermal insulation. This leads to a change in air circulation and humidity in the older building, as natural air exchange is reduced. To prevent mould growth, fresh air must be exchanged and regular ventilation must be ensured. Decentralised mechanical ventilation is also an option –– you can find out more about this in our guide to mechanical ventilation.

How can we help?

Tap on the button below to WhatsApp us at +65 8028 0468 during office hours (Mondays – Fridays, 9am – 6pm), or select Contact Form below to provide us with details of your enquiry. We will get back to you as soon as possible.

Contact us