How to install a boiler in 2025: step-by-step guide

Installing a gas boiler is an efficient solution to ensure thermal comfort at home, especially during the colder months. While the process is relatively straightforward, it demands precision and technical know-how. That’s why it’s essential for the installation to be performed by a certified professional who can ensure compliance with the Spanish Regulation on Thermal Installations in Buildings (RITE).

Working with an expert not only guarantees proper installation but also helps you pick the most suitable unit for your home’s features and energy needs. In addition, a well-executed installation is key to ensuring the system's safety, energy performance, and durability.

A properly installed boiler doesn’t just improve your home’s efficiency but also helps you save energy and care for the environment. So, if you’re thinking about upgrading your heating system, make sure you do it with full technical and legal guarantees.

How to install a condensing gas boiler

Installing a condensing boiler is an efficient and increasingly common choice for households. Its setup is simpler and more affordable than other systems, such as oil boilers, mainly because it's lighter and can be mounted directly on the wall. This means that, in many cases, a single technician can handle the installation, saving on time and costs.

The installation process includes:

• Removing the old boiler and taking it to a recycling point.
• Installing the new boiler in the same spot or a previously selected location.
• Connecting it to the plumbing, gas, and electrical systems, preferably using soldering, though flexible metal hoses can also be used.
• Installing the thermostat so you can programme the heating to suit your needs.
• Connecting it to the drain, which is essential for evacuating condensate.
• Installing the flue outlet, adapted to fit the features of the space.
• Issuing the installation certificate in compliance with current regulations.

Is a drain required to install a condensing boiler?

Yes. All condensing boilers generate condensate that must be drained, typically through a connection to the wastewater system. These liquids are slightly acidic, so in older installations with metal pipes such as lead, you might need to install neutralising filters.

In newer homes, where pipes are usually made of PVC, no extra filters are needed since the material is resistant to the acidity of the condensate.

Where should the drain be located?

Ideally, the drain should be close to the boiler to minimise pipe length and ensure the proper slope for drainage. If the drain is far away, additional work may be needed to connect it, which increases installation costs.

In cases where an adequate slope cannot be guaranteed, a pump can be installed to pump the condensate away. However, this also involves higher costs and more maintenance in the long run.

What regulations govern the flue outlet?

The flue outlet installation must comply with the Spanish Regulation on Thermal Installations in Buildings (RITE). In newly constructed buildings, regulations require the flue outlet to go through the roof.

In older buildings where this option isn’t feasible, alternatives such as venting through the façade or an internal courtyard are allowed, as long as the installation uses a sealed low-NOx (nitrogen oxide) boiler or a condensing boiler with a nominal output under 70 kW.

What other requirements do the regulations include?

• In new buildings, boilers must be condensing models equipped with a modulating thermostat or an outdoor temperature sensor.
• When replacing an existing boiler, if the flue outlet runs to the façade, a condensing or low-NOx boiler must be installed.
• Atmospheric boilers have been banned since 2010.
• Since 2015, the European ErP (Energy-related Products) regulation requires all boilers to meet a minimum energy efficiency standard, which in practice means installing condensing models.

What types of pipes are used for the flue outlet?

By structure

• Coaxial pipe: made up of two concentric pipes. The outer pipe draws in oxygen for combustion, while the inner pipe expels the gases. It’s the most common choice thanks to its efficiency and compact size.
• Twin pipe: two separate ducts, one for air intake and one for exhaust. This type is used when the flue outlet is located far from the boiler, though it takes up more space.

By location

• Horizontal: common in older buildings, with an outlet through the façade. The pipe must be slightly inclined toward the boiler so any condensate can flow back to the drain.
• Vertical: more frequent in new buildings, with an outlet through the roof. It can connect to a shunt, a central duct that combines outlets from multiple homes.

Because condensing boilers emit low-temperature gases, pipes made of polypropylene—a durable and affordable material—can be used.

Why is it important to hire a professional?

Installing a condensing boiler involves complying with technical, legal, and safety requirements. That’s why it’s essential to have the work done by a certified installer who can ensure proper operation and home safety.

Understanding how the installation works will help you make informed decisions, choose the right model, and identify the factors affect cost and maintenance.

Are you thinking about upgrading your heating system? At Endesa, we’ll help you find the most efficient solution for your home, with personalised advice and tariffs tailored to your energy consumption. Because improving your comfort can also be a way to save.